The Linux Commands Handbook
Author: Flavio Copes
Date: 0101-01-01T00:00:00+00:00
Keywords: Linux; Ubuntu; Windows; Commands; CLI; Client-Server Computing; Server
Description: This PDF was created for free by UbuntuFree.com - Visit our site for more downloads like software and guides we recommend and yes, more Linux eBooks!
Publisher: UbuntuFree.com

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Table of Contents

Preface

Introduction to Linux and shells
man
ls
cd
pwd
mkdir
rmdir
mv
cp
open
touch
find
ln
gzip
gunzip
tar
alias
cat
less
tail
wc
grep
sort

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uniq
diff
echo
chown
chmod
umask
du
df
basename
dirname
ps
top
kill
killall
jobs
bg
fg
type
which
nohup
xargs
vim
emacs
nano
whoami
who
su
sudo

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passwd
ping
traceroute
clear
history
export
crontab
uname
env
printenv

Conclusion

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Preface

The Linux Commands Handbook follows the 80/20 rule: learn in 20% of the time the 80% of a topic.

I find this approach gives a well-rounded overview.

This book does not try to cover everything under the sun related to Linux and its commands. It focuses on the small core commands that you will use the 80% or 90% of the time, trying to simplify the usage of the more complex ones.

All those commands work on Linux, macOS, WSL, and anywhere you have a UNIX environment.

I hope the contents of this book will help you achieve what you want: get comfortable with Linux.

This book is written by Flavio. I publish programming tutorials every day on my website
flaviocopes.com.

You can reach me on Twitter @flaviocopes.

Enjoy!

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www.dbooks.org Introduction to Linux and
shells

Linux is an operating system, like macOS or Windows.

It is also the most popular Open Source and free, as in freedom, operating system.

It powers the vast majority of the servers that compose the Internet. It's the base upon which everything is built upon. But not just that. Android is based on (a modified version of) Linux.

The Linux "core" (called kernel) was born in 1991 in Finland, and it went a really long way from its humble beginnings. It went on to be the kernel of the GNU Operating System, creating the duo GNU/Linux.

There's one thing about Linux that corporations like Microsoft and Apple, or Google, will never be able to offer: the freedom to do whatever you want with your computer.

They're actually going in the opposite direction, building walled gardens, especially on the mobile side.

Linux is the ultimate freedom.

It is developed by volunteers, some paid by companies that rely on it, some independently, but there's no single commercial company that can dictate what goes into Linux, or the project priorities.

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Linux can also be used as your day to day computer. I use macOS because I really enjoy the applications, the design and I also used to be an iOS and Mac apps developer, but before using it I used Linux as my main computer Operating System.

No one can dictate which apps you can run, or "call home" with apps that track you, your position, and more.

Linux is also special because there's not just "one Linux", like it happens on Windows or macOS. Instead, we have distributions.

A "distro" is made by a company or organization and packages the Linux core with additional programs and tooling.

For example you have Debian, Red Hat, and Ubuntu, probably the most popular.

Many, many more exist. You can create your own distribution, too. But most likely you'll use a popular one, one that has lots of users and a community of people around it, so you can do what you need to do without losing too much time reinventing the wheel and figuring out answers to common problems.

Some desktop computers and laptops ship with Linux preinstalled. Or you can install it on your Windows-based computer, or on a Mac.

But you don't need to disrupt your existing computer just to get an idea of how Linux works.

I don't have a Linux computer.

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www.dbooks.org If you use a Mac you need to know that under the hood macOS is a UNIX Operating System, and it shares a lot of the same ideas and software that a GNU/Linux system uses, because GNU/Linux is a free alternative to UNIX.

UNIX is an umbrella term that groups many
operating systems used in big corporations and
institutions, starting from the 70's

The macOS terminal gives you access to the same exact commands I'll describe in the rest of this handbook.

Microsoft has an official Windows Subsystem for Linux which you can (and should!) install on Windows. This will give you the ability to run Linux in a very easy way on your PC.

But the vast majority of the time you will run a Linux computer in the cloud via a VPS (Virtual Private Server) like DigitalOcean.

A shell is a command interpreter that exposes to the user an interface to work with the underlying operating system.

It allows you to execute operations using text and commands, and it provides users advanced features like being able to create scripts.

This is important: shells let you perform things in a more optimized way than a GUI (Graphical User Interface) could ever possibly let you do. Command line tools can offer many different configuration options without being too complex to use.

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There are many different kind of shells. This post focuses on Unix shells, the ones that you will find commonly on Linux and macOS computers.

Many different kind of shells were created for those systems over time, and a few of them dominate the space: Bash, Csh, Zsh, Fish and many more!

All shells originate from the Bourne Shell, called  sh . "Bourne" because its creator was Steve Bourne.

Bash means Bourne-again shell.  sh  was proprietary and not open source, and Bash was created in 1989 to create a free alternative for the GNU project and the Free Software Foundation. Since projects had to pay to use the Bourne shell, Bash became very popular.

If you use a Mac, try opening your Mac terminal. That by default is running ZSH. (or, pre-Catalina, Bash)

You can set up your system to run any kind of shell, for example I use the Fish shell.

Each single shell has its own unique features and advanced usage, but they all share a common functionality: they can let you execute programs, and they can be programmed.

In the rest of this handbook we'll see in detail the most common commands you will use.

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man

The first command I want to introduce is a command that will help you understand all the other commands.

Every time I don't know how to use a command, I type  man   to get the manual:

This is a man (from manual) page. Man pages are an essential tool to learn, as a developer. They contain so much information that sometimes it's almost too much.

The above screenshot is just 1 of 14 screens of explanation for the  ls  command.

Most of the times when I'm in need to learn a command quickly I use this site called tldr pages:

https://tldr.sh/. It's a command you can install, then you run it like this:  tldr  , which gives you a very quick overview of a command, with some handy examples of common usage scenarios:

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This is not a substitute for  man  , but a handy tool to avoid losing yourself in the huge amount of information present in a man page. Then you can use the man page to explore all the different options and parameters you can use on a command.

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ls

Inside a folder you can list all the files that the folder contains using the  ls  command:

ls

If you add a folder name or path, it will print that folder contents:

ls /bin

ls  accepts a lot of options. One of my favorite options combinations is  -al  . Try it:

ls -al /bin

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compared to the plain  ls  , this returns much more information.

You have, from left to right:

the file permissions (and if your system supports ACLs, you get an ACL flag as well)

the number of links to that file
the owner of the file
the group of the file
the file size in bytes
the file modified datetime
the file name

This set of data is generated by the  l  option. The  a  option instead also shows the hidden files.

Hidden files are files that start with a dot (  . ).

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cd

Once you have a folder, you can move into it using the  cd  command.  cd  means change directory. You invoke it specifying a folder to move into. You can specify a folder name, or an entire path.

Example:

mkdir fruits
cd fruits

Now you are into the  fruits  folder.

You can use the  ..  special path to indicate the parent folder:

cd .. #back to the home folder

The # character indicates the start of the comment, which lasts for the entire line after it's found.

You can use it to form a path:

mkdir fruits
mkdir cars
cd fruits
cd ../cars

There is another special path indicator which is  .  , and indicates the current folder.

You can also use absolute paths, which start from the root folder  / :

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cd /etc

This command works on Linux, macOS, WSL,
and anywhere you have a UNIX environment

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pwd

Whenever you feel lost in the filesystem, call the  pwd  command to know where you are:

pwd

It will print the current folder path.

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mkdir

You create folders using the  mkdir  command:

mkdir fruits

You can create multiple folders with one command:

mkdir dogs cars

You can also create multiple nested folders by adding the  -p  option:

mkdir -p fruits/apples

Options in UNIX commands commonly take this form. You add them right after the command name, and they change how the command behaves. You can often combine multiple options, too.

You can find which options a command supports by typing  man   . Try now with  man mkdir  for example (press the  q  key to esc the man page). Man pages are the amazing built-in help for UNIX.

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rmdir

Just as you can create a folder using  mkdir , you can delete a folder using  rmdir :

mkdir fruits
rmdir fruits

You can also delete multiple folders at once:

mkdir fruits cars
rmdir fruits cars

The folder you delete must be empty.

To delete folders with files in them, we'll use the more generic  rm  command which deletes files and folders, using the  -rf  options:

rm -rf fruits cars

Be careful as this command does not ask for confirmation and it will immediately remove anything you ask it to remove.

There is no bin when removing files from the command line, and recovering lost files can be hard.

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mv

Once you have a file, you can move it around using the  mv  command. You specify the file current path, and its new path:

touch pear
mv pear new_pear

The  pear  file is now moved to  new_pear . This is how you rename files and folders.

If the last parameter is a folder, the file located at the first parameter path is going to be moved into that folder. In this case, you can specify a list of files and they will all be moved in the folder path identified by the last parameter:

touch pear
touch apple
mkdir fruits
mv pear apple fruits #pear and apple moved to the fr

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cp

You can copy a file using the  cp  command:

touch test
cp apple another_apple

To copy folders you need to add the  -r  option to recursively copy the whole folder contents:

mkdir fruits
cp -r fruits cars

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open

The  open  command lets you open a file using this syntax:

open

You can also open a directory, which on macOS opens the Finder app with the current directory open:

open

I use it all the time to open the current directory:

open .

The special   .  symbol points to the current
directory, as  ..  points to the parent directory

The same command can also be be used to run an application:

open

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touch

You can create an empty file using the   touch  command:

touch apple

If the file already exists, it opens the file in write mode, and the timestamp of the file is updated.

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find

The  find  command can be used to find files or folders matching a particular search pattern. It searches recursively.

Let's learn it by example.

Find all the files under the current tree that have the  .js  extension and print the relative path of each file matching:

find . -name '*.js'

It's important to use quotes around special characters like  *  to avoid the shell interpreting them.

Find directories under the current tree matching the name "src":

find . -type d -name src

Use  -type f  to search only files, or  -type l  to only search symbolic links.

-name  is case sensitive. use  -iname  to perform a case-insensitive search.

You can search under multiple root trees:

find folder1 folder2 -name filename.txt

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find . -type d -name node_modules -or -name public

You can also exclude a path, using  -not -path :

find . -type d -name '*.md' -not -path 'node_modules

You can search files that have more than 100 characters (bytes) in them:

find . -type f -size +100c

Search files bigger than 100KB but smaller than 1MB:

find . -type f -size +100k -size -1M

Search files edited more than 3 days ago
find . -type f -mtime +3

Search files edited in the last 24 hours
find . -type f -mtime -1

You can delete all the files matching a search by adding the  -delete  option. This deletes all the files edited in the last 24 hours:

find . -type f -mtime -1 -delete

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You can execute a command on each result of the search. In this example we run  cat  to print the file content:

find . -type f -exec cat {} \;

notice the terminating  \; .  {}  is filled with the file name at execution time.

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ln

The  ln  command is part of the Linux file system commands.

It's used to create links. What is a link? It's like a pointer to another file. A file that points to another file. You might be familiar with Windows shortcuts. They're similar.

We have 2 types of links: hard links and soft links.

Hard links

Hard links are rarely used. They have a few limitations: you can't link to directories, and you can't link to external filesystems (disks).

A hard link is created using
ln

For example, say you have a file called recipes.txt. You can create a hard link to it using:

ln recipes.txt newrecipes.txt

The new hard link you created is indistinguishable from a regular file:

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Now any time you edit any of those files, the content will be updated for both.

If you delete the original file, the link will still contain the original file content, as that's not removed until there is one hard link pointing to it.

Soft links

Soft links are different. They are more powerful as you can link to other filesystems and to directories, but when the original is removed, the link will be broken.

You create soft links using the  -s  option of  ln :

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ln -s

For example, say you have a file called recipes.txt. You can create a soft link to it using:

ln -s recipes.txt newrecipes.txt

In this case you can see there's a special  l  flag when you list the file using  ls -al , and the file name has a  @  at the end, and it's colored differently if you have colors enabled:

Now if you delete the original file, the links will be broken, and the shell will tell you "No such file or directory" if you try to access it:

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gzip

You can compress a file using the gzip compression
protocol named LZ77 using the  gzip  command.

Here's the simplest usage:

gzip filename

This will compress the file, and append a   .gz  extension to it. The original file is deleted. To prevent this, you can use the   -c  option and use output redirection to write the output to the  filename.gz  file:

gzip -c filename > filename.gz

The  -c  option specifies that output will go to the
standard output stream, leaving the original file
intact

Or you can use the  -k  option:

gzip -k filename

There are various levels of compression. The more the compression, the longer it will take to compress (and decompress). Levels range from 1 (fastest, worst compression) to 9 (slowest, better compression), and the default is 6.

You can choose a specific level with the  -  option:

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gzip -1 filename

You can compress multiple files by listing them:

gzip filename1 filename2

You can compress all the files in a directory, recursively, using the  -r  option:

gzip -r a_folder

The  -v  option prints the compression percentage information. Here's an example of it being used along with the  -k  (keep) option:

gzip  can also be used to decompress a file, using the  -d  option:

gzip -d filename.gz

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gunzip

The  gunzip  command is basically equivalent to the  gzip  command, except the  -d  option is always enabled by default.

The command can be invoked in this way:

gunzip filename.gz

This will gunzip and will remove the  .gz  extension, putting the result in the  filename  file. If that file exists, it will overwrite that.

You can extract to a different filename using output redirection using the  -c  option:

gunzip -c filename.gz > anotherfilename

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tar

The  tar  command is used to create an archive, grouping multiple files in a single file.

Its name comes from the past and means tape archive. Back when archives were stored on tapes.

This command creates an archive named  archive.tar  with the content of  file1  and  file2 :

tar -cf archive.tar file1 file2

The  c  option stands for create. The  f  option is
used to write to file the archive.

To extract files from an archive in the current folder, use:

tar -xf archive.tar

the  x  option stands for extract
and to extract them to a specific directory, use:

tar -xf archive.tar -C directory

You can also just list the files contained in an archive:

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tar  is often used to create a compressed archive, gzipping the archive.

This is done using the  z  option:

tar -czf archive.tar.gz file1 file2

This is just like creating a tar archive, and then running  gzip  on it.

To unarchive a gzipped archive, you can use  gunzip , or  gzip -d , and then unarchive it, but  tar -xf  will recognize it's a gzipped archive, and do it for you:

tar -xf archive.tar.gz

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alias

It's common to always run a program with a set of options you like using.

For example, take the  ls  command. By default it prints very little information:

while using the  -al  option it will print something more useful, including the file modification date, the size, the owner, and the permissions, also listing hidden files (files starting with a  . :

You can create a new command, for example I like to call it  ll , that is an alias to  ls -al .

You do it in this way:

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alias ll='ls -al'

Once you do, you can call  ll  just like it was a regular UNIX command:

Now calling  alias  without any option will list the aliases defined:

The alias will work until the terminal session is closed.

To make it permanent, you need to add it to the shell configuration, which could be   ~/.bashrc  or  ~/.profile  or  ~/.bash_profile  if you use the Bash shell, depending on the use case.

Be careful with quotes if you have variables in the command: using double quotes the variable is resolved at definition time, using single quotes it's resolved at invocation time. Those 2 are different:

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alias lsthis="ls $PWD"

alias lscurrent='ls $PWD'

$PWD refers to the current folder the shell is into. If you now navigate away to a new folder,  lscurrent  lists the files in the new folder,  lsthis  still lists the files in the folder you were when you defined the alias.

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cat

Similar to  tail  in some way, we have  cat  . Except  cat  can also add content to a file, and this makes it super powerful.

In its simplest usage,  cat  prints a file's content to the standard output:

cat file

You can print the content of multiple files:

cat file1 file2

and using the output redirection operator  >  you can concatenate the content of multiple files into a new file:

cat file1 file2 > file3

Using  >>  you can append the content of multiple files into a new file, creating it if it does not exist:

cat file1 file2 >> file3

When watching source code files it's great to see the line numbers, and you can have  cat  print them using the  -n  option:

cat -n file1

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www.dbooks.org You can only add a number to non-blank lines using  -b , or you can also remove all the multiple empty lines using  -s .

cat  is often used in combination with the pipe operator  |  to feed a file content as input to another command:  cat file1 | anothercommand .

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less

The  less  command is one I use a lot. It shows you the content stored inside a file, in a nice and interactive UI.

Usage:  less  .

Once you are inside a  less  session, you can quit by pressing  q .

You can navigate the file contents using the  up  and  down  keys, or using the  space bar  and   b  to navigate page by page. You can also jump to the end of the file pressing  G  and jump back to the start pressing  g .

You can search contents inside the file by pressing  /  and typing a word to search. This searches forward. You can search backwards using the  ?  symbol and typing a word.

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www.dbooks.org This command just visualises the file's content. You can directly open an editor by pressing  v . It will use the system editor, which in most cases is  vim .

Pressing the  F  key enters follow mode, or watch mode. When the file is changed by someone else, like from another program, you get to see the changes live. By default this is not happening, and you only see the file version at the time you opened it. You need to press   ctrl-C  to quit this mode. In this case the behaviour is similar to running the   tail -f   command.

You can open multiple files, and navigate through them using  :n  (to go to the next file) and  :p  (to go to the previous).

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tail

The best use case of tail in my opinion is when called with the  -f  option. It opens the file at the end, and watches for file changes. Any time there is new content in the file, it is printed in the window. This is great for watching log files, for example:

tail -f /var/log/system.log

To exit, press  ctrl-C .

You can print the last 10 lines in a file:

tail -n 10

You can print the whole file content starting from a specific line using  +  before the line number:

tail -n +10

tail  can do much more and as always my advice is to check  man tail .

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wc

The  wc  command gives us useful information about a file or input it receives via pipes.

echo test >> test.txt
wc test.txt

1 1 5 test.txt

Example via pipes, we can count the output of running the  ls -al  command:

ls -al | wc

6 47 284

The first column returned is the number of lines. The second is the number of words. The third is the number of bytes.

We can tell it to just count the lines:

wc -l test.txt

or just the words:

wc -w test.txt

or just the bytes:

wc -c test.txt

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Bytes in ASCII charsets equate to characters, but with non-ASCII charsets, the number of characters might differ because some characters might take multiple bytes, for example this happens in Unicode.

In this case the  -m  flag will help getting the correct value:

wc -m test.txt

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grep

The  grep  command is a very useful tool, that when you master will help you tremendously in your day to day.

If you're wondering,   grep  stands for global
regular expression print

You can use  grep  to search in files, or combine it with pipes to filter the output of another command.

For example here's how we can find the occurences of the  document.getElementById  line in the  index.md  file:

grep document.getElementById index.md

Using the  -n  option it will show the line numbers:

grep -n document.getElementById index.md

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One very useful thing is to tell grep to print 2 lines before, and 2 lines after the matched line, to give us more context. That's done using the  -C  option, which accepts a number of lines:

grep -nC 2 document.getElementById index.md

Search is case sensitive by default. Use the  -i  flag to make it insensitive.

As mentioned, you can use grep to filter the output of another command. We can replicate the same functionality as above using:

less index.md | grep -n document.getElementById

The search string can be a regular expression, and this makes  grep  very powerful.

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www.dbooks.org Another thing you might find very useful is to invert the result, excluding the lines that match a particular string, using the  -v  option:

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sort

Suppose you have a text file which contains the names of dogs:

This list is unordered.

The  sort  command helps us sorting them by name:

Use the  r  option to reverse the order:

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Sorting by default is case sensitive, and alphabetic. Use the  --ignore-case  option to sort case insensitive, and the  -n  option to sort using a numeric order.

If the file contains duplicate lines:

You can use the  -u  option to remove them:

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sort  does not just works on files, as many UNIX commands it also works with pipes, so you can use on the output of another command, for example you can order the files returned by  ls  with:

ls | sort

sort  is very powerful and has lots more options, which you can explore calling  man sort .

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uniq

uniq  is a command useful to sort lines of text.

You can get those lines from a file, or using pipes from the output of another command:

uniq dogs.txt
ls | uniq

You need to consider this key thing:  uniq  will only detect adjacent duplicate lines.

This implies that you will most likely use it along with  sort  :

sort dogs.txt | uniq

The   sort  command has its own way to remove duplicates with the  -u  (unique) option. But  uniq  has more power.

By default it removes duplicate lines:

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You can tell it to only display duplicate lines, for example, with the  -d  option:

sort dogs.txt | uniq -d

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You can use the   -u  option to only display non-duplicate lines:

You can count the occurrences of each line with the  -c  option:

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Use the special combination:

sort dogs.txt | uniq -c | sort -nr

to then sort those lines by most frequent:

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diff

diff  is a handy command. Suppose you have 2 files, which contain almost the same information, but you can't find the difference between the two.

diff  will process the files and will tell you what's the difference.

Suppose you have 2 files:   dogs.txt  and  moredogs.txt  . The difference is that  moredogs.txt  contains one more dog name:

diff dogs.txt moredogs.txt  will tell you the second file has one more line, line 3 with the line  Vanille :

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www.dbooks.org If you invert the order of the files, it will tell you that the second file is missing line 3, whose content is  Vanille :

Using the  -y  option will compare the 2 files line by line:

The  -u  option however will be more familiar to you, because that's the same used by the Git version control system to display differences between versions:

Comparing directories works in the same way. You must use the   -r  option to compare recursively (going into subdirectories):

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In case you're interested in which files differ, rather than the content, use the  r  and  q  options:

There are many more options you can explore in the man page running  man diff :

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echo

The  echo  command does one simple job: it prints to the output the argument passed to it.

This example:

echo "hello"

will print  hello  to the terminal.

We can append the output to a file:

echo "hello" >> output.txt

We can interpolate environment variables:

echo "The path variable is $PATH"

Beware that special characters need to be escaped with a backslash  \ .  $  for example:

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www.dbooks.org This is just the start. We can do some nice things when it comes to interacting with the shell features.

We can echo the files in the current folder:

echo *

We can echo the files in the current folder that start with the letter  o :

echo o*

Any valid Bash (or any shell you are using) command and feature can be used here.

You can print your home folder path:

echo ~

You can also execute commands, and print the result to the standard output (or to file, as you saw):

echo $(ls -al)

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Note that whitespace is not preserved by default. You need to wrap the command in double quotes to do so:

You can generate a list of strings, for example ranges:

echo {1..5}

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chown

Every file/directory in an Operating System like Linux or macOS (and every UNIX systems in general) has an owner.

The owner of a file can do everything with it. It can decide the fate of that file.

The owner (and the   root  user) can change the owner to another user, too, using the   chown  command:

chown

Like this:

chown flavio test.txt

For example if you have a file that's owned by  root , you can't write to it as another user:

You can use  chown  to transfer the ownership to you:

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It's rather common to have the need to change the ownership of a directory, and recursively all the files contained, plus all the subdirectories and the files contained in them, too.

You can do so using the  -R  flag:

chown -R

Files/directories don't just have an owner, they also have a group. Through this command you can change that simultaneously while you change the owner:

chown :

Example:

chown flavio:users test.txt

You can also just change the group of a file using the  chgrp  command:

chgrp

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chmod

Every file in the Linux / macOS Operating Systems (and UNIX systems in general) has 3 permissions: Read, write, execute.

Go into a folder, and run the  ls -al  command.

The weird strings you see on each file line, like  drwxr-xr-x  , define the permissions of the file or folder.

Let's dissect it.

The first letter indicates the type of file:

-  means it's a normal file  d  means it's a directory  l  means it's a link

Then you have 3 sets of values:

The first set represents the permissions of the owner of the file

The second set represents the permissions of the members of the group the file is associated to

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The third set represents the permissions of the everyone else

Those sets are composed by 3 values.  rwx  means that specific persona has read, write and execution access. Anything that is removed is swapped with a   -  , which lets you form various combinations of values and relative permissions:  rw-  ,  r--  ,  r-x  , and so on.

You can change the permissions given to a file using the  chmod  command.

chmod  can be used in 2 ways. The first is using symbolic arguments, the second is using numeric arguments. Let's start with symbols first, which is more intuitive.

You type  chmod  followed by a space, and a letter:

a  stands for all

u  stands for user  g  stands for group  o  stands for others

Then you type either  +  or  -  to add a permission, or to remove it. Then you enter one or more permissions symbols (  r ,  w ,  x ).

All followed by the file or folder name.

Here are some examples:

chmod a+r filename #everyone can now read
chmod a+rw filename #everyone can now read and write
chmod o-rwx filename #others (not the owner, not in

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www.dbooks.org You can apply the same permissions to multiple personas by adding multiple letters before the  + /  - :

chmod og-r filename #other and group can't read any

In case you are editing a folder, you can apply the permissions to every file contained in that folder using the  -r  (recursive) flag.

Numeric arguments are faster but I find them hard to remember when you are not using them day to day. You use a digit that represents the permissions of the persona. This number value can be a maximum of 7, and it's calculated in this way:

1  if has execution permission  2  if has write permission  4  if has read permission

This gives us 4 combinations:

0  no permissions  1  can execute

2  can write

3  can write, execute  4  can read

5  can read, execute  6  can read, write  7  can read, write and execute

We use them in pairs of 3, to set the permissions of all the 3 groups altogether:

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chmod 777 filename
chmod 755 filename
chmod 644 filename

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umask

When you create a file, you don't have to decide permissions up front. Permissions have defaults.

Those defaults can be controlled and modified using the  umask  command.

Typing  umask  with no arguments will show you the current umask, in this case  0022 :

What does  0022  mean? That's an octal value that represent the permissions.

Another common value is  0002 .

Use  umask -S  to see a human-readable notation:

In this case, the user (  u ), owner of the file, has read, write and execution permissions on files.

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Other users belonging to the same group (  g ) have read and execution permission, same as all the other users (  o ).

In the numeric notation, we typically change the last 3 digits.

Here's a list that gives a meaning to the number:

0  read, write, execute  1  read and write

2  read and execute  3  read only

4  write and execute  5  write only

6  execute only

7  no permissions

Note that this numeric notation differs from the one we use in  chmod .

We can set a new value for the mask setting the value in numeric format:

umask 002

or you can change a specific role's permission:

umask g+r

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du

The  du  command will calculate the size of a directory as a whole:

du

The  32  number here is a value expressed in bytes.

Running  du *  will calculate the size of each file individually:

You can set  du  to display values in MegaBytes using  du -m  , and GigaBytes using  du -g .

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The  -h  option will show a human-readable notation for sizes, adapting to the size:

Adding the  -a  option will print the size of each file in the directories, too:

A handy thing is to sort the directories by size:

du -h | sort -nr

and then piping to   head  to only get the first 10 results:

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df

The   df  command is used to get disk usage information.

Its basic form will print information about the volumes mounted:

Using the  -h  option (  df -h ) will show those values in a human-readable format:

You can also specify a file or directory name to get information about the specific volume it lives on:

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basename

Suppose you have a path to a file, for example  /Users/flavio/test.txt  .

Running
basename /Users/flavio/test.txt

will return the  test.txt  string:

If you run  basename  on a path string that points to a directory, you will get the last segment of the path. In this example,  /Users/flavio  is a directory:

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dirname

Suppose you have a path to a file, for example  /Users/flavio/test.txt  .

Running
dirname /Users/flavio/test.txt

will return the  /Users/flavio  string:

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ps

Your computer is running, at all times, tons of different processes.

You can inspect them all using the  ps  command:

This is the list of user-initiated processes currently running in the current session.

Here I have a few   fish  shell instances, mostly opened by VS Code inside the editor, and an instances of Hugo running the development preview of a site.

Those are just the commands assigned to the current user. To list all processes we need to pass some options to  ps .

The most common I use is  ps ax :

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The  a  option is used to also list other users
processes, not just our own.  x  shows processes
not linked to any terminal (not initiated by users
through a terminal).

As you can see, the longer commands are cut. Use the command  ps axww  to continue the command listing on a new line instead of cutting it:

We need to specify   w  2 times to apply this
setting, it's not a typo.

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www.dbooks.org You can search for a specific process combining  grep  with a pipe, like this:

ps axww | grep "Visual Studio Code"

The columns returned by  ps  represent some key information.

The first information is  PID  , the process ID. This is key when you want to reference this process in another command, for example to kill it.

Then we have  TT  that tells us the terminal id used.

Then  STAT  tells us the state of the process:

I  a process that is idle (sleeping for longer than about 20 seconds)  R  a runnable process   S  a process that is sleeping for less than about 20 seconds  T  a stopped process   U  a process in uninterruptible wait  Z  a dead process (a zombie)

If you have more than one letter, the second represents further information, which can be very technical.

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It's common to have  +  which indicates the process is in the foreground in its terminal.   s  means the
process is a session leader.

TIME  tells us how long the process has been running.

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top

A quick guide to the  top  command, used to list the processes running in real time

The  top  command is used to display dynamic real-time information about running processes in the system.

It's really handy to understand what is going on.

Its usage is simple, you just type   top  , and the terminal will be fully immersed in this new view:

The process is long-running. To quit, you can type the  q  letter or  ctrl-C .

There's a lot of information being given to us: the number of processes, how many are running or sleeping, the system load, the CPU usage, and a lot more.

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Below, the list of processes taking the most memory and CPU is constantly updated.

By default, as you can see from the  %CPU  column highlighted, they are sorted by the CPU used.

You can add a flag to sort processes by memory utilized:

top -o mem

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kill

Linux processes can receive signals and react to them.

That's one way we can interact with running programs.

The  kill  program can send a variety of signals to a program.

It's not just used to terminate a program, like the name would suggest, but that's its main job.

We use it in this way:

kill

By default, this sends the  TERM  signal to the process id specified.

We can use flags to send other signals, including:

kill -HUP
kill -INT
kill -KILL
kill -TERM
kill -CONT
kill -STOP

HUP  means hang up. It's sent automatically when a terminal window that started a process is closed before terminating the process.

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INT  means interrupt, and it sends the same signal used when we press  ctrl-C  in the terminal, which usually terminates the process.

KILL  is not sent to the process, but to the operating system kernel, which immediately stops and terminates the process.

TERM  means terminate. The process will receive it and terminate itself. It's the default signal sent by  kill  .

CONT  means continue. It can be used to resume a stopped process.

STOP  is not sent to the process, but to the operating system kernel, which immediately stops (but does not terminate) the process.

You might see numbers used instead, like  kill -1   . In this case,

1  corresponds to  HUP .  2  corresponds to  INT .  9  corresponds to  KILL  .  15  corresponds to  TERM  .   18  corresponds to   CONT  .   15  corresponds to  STOP  .

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killall

Similar to the  kill  command,  killall  instead of sending a signal to a specific process id will send the signal to multiple processes at once.

This is the syntax:

killall

where  name  is the name of a program. For example you can have multiple instances of the  top  program running, and  killall top  will terminate them all.

You can specify the signal, like with  kill  (and check the  kill  tutorial to read more about the specific kinds of signals we can send), for example:

killall -HUP top

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jobs

When we run a command in Linux / macOS, we can set it to run in the background using the  &  symbol after the command. For example we can run  top  in the background:

top &

This is very handy for long-running programs.

We can get back to that program using the   fg  command. This works fine if we just have one job in the background, otherwise we need to use the job number:  fg 1  ,  fg 2  and so on. To get the job number, we use the  jobs  command.

Say we run  top &  and then  top -o mem &  , so we have 2 top instances running.  jobs  will tell us this:

Now we can switch back to one of those using  fg   . To stop the program again we can hit  cmd-Z .

Running  jobs -l  will also print the process id of each job.

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bg

When a command is running you can suspend it using  ctrl-Z .

The command will immediately stop, and you get back to the shell terminal.

You can resume the execution of the command in the background, so it will keep running but it will not prevent you from doing other work in the terminal.

In this example I have 2 commands stopped:

I can run  bg 1  to resume in the background the execution of the job #1.

I could have also said  bg  without any option, as the default is to pick the job #1 in the list.

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fg

When a command is running in the background, because you started it with  &  at the end (example:  top &  or because you put it in the background with the  bg  command, you can put it to the foreground using  fg .

Running
fg

will resume to the foreground the last job that was suspended.

You can also specify which job you want to resume to the foreground passing the job number, which you can get using the  jobs  command.

Running  fg 2  will resume job #2:

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type

A command can be one of those 4 types:

an executable
a shell built-in program
a shell function
an alias

The  type  command can help figure out this, in case we want to know or we're just curious. It will tell you how the command will be interpreted.

The output will depend on the shell used. This is Bash:

This is Zsh:

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This is Fish:

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One of the most interesting things here is that for aliases it will tell you what is aliasing to. You can see the  ll  alias, in the case of Bash and Zsh, but Fish provides it by default, so it will tell you it's a built-in shell function.

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which

Suppose you have a command you can execute, because it's in the shell path, but you want to know where it is located.

You can do so using  which . The command will return the path to the command specified:

which  will only work for executables stored on disk, not aliases or built-in shell functions.

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nohup

Sometimes you have to run a long-lived process on a remote machine, and then you need to disconnect.

Or you simply want to prevent the command to be halted if there's any network issue between you and the server.

The way to make a command run even after you log out or close the session to a server is to use the  nohup  command.

Use   nohup   to let the process continue working even after you log out.

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xargs

The   xargs  command is used in a UNIX shell to convert input from standard input into arguments to a command.

In other words, through the use of  xargs  the output of a command is used as the input of another command.

Here's the syntax you will use:

command1 | xargs command2

We use a pipe (  | ) to pass the output to  xargs . That will take care of running the  command2  command, using the output of  command1  as its argument(s).

Let's do a simple example. You want to remove some specific files from a directory. Those files are listed inside a text file.

We have 3 files:  file1 ,  file2 ,  file3 .

In  todelete.txt  we have a list of files we want to delete, in this example  file1  and  file3 :

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We will channel the output of  cat todelete.txt  to the  rm  command, through  xargs .

In this way:

cat todelete.txt | xargs rm

That's the result, the files we listed are now deleted:

The way it works is that  xargs  will run  rm  2 times, one for each line returned by  cat .

This is the simplest usage of   xargs  . There are several options we can use.

One of the most useful in my opinion, especially when starting to learn  xargs , is  -p . Using this option will make   xargs  print a confirmation prompt with the action it's going to take:

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www.dbooks.org The  -n  option lets you tell  xargs  to perform one iteration at a time, so you can individually confirm them with  -p . Here we tell  xargs  to perform one iteration at a time with  -n1 :

The  -I  option is another widely used one. It allows you to get the output into a placeholder, and then you can do various things.

One of them is to run multiple commands:

command1 | xargs -I % /bin/bash -c 'command2 %; comm

You can swap the  %  symbol I used above with

anything else, it's a variable

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vim

vim  is a very popular file editor, especially among programmers. It's actively developed and frequently updated, and there's a very big community around it.

There's even a Vim conference!

vi  in modern systems is just an alias to  vim , which means  vi  i  m proved.

You start it by running  vi  on the command line.

You can specify a filename at invocation time to edit that specific file:

vi test.txt

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You have to know that Vim has 2 main modes:

command (or normal) mode insert mode

When you start the editor, you are in command mode. You can't enter text like you expect from a GUI-based editor. You have to enter insert mode. You can do this by pressing the  i  key. Once you do so, the   --INSERT --  word appear at the bottom of the editor:

Now you can start typing and filling the screen with the file contents:

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You can move around the file with the arrow keys, or using the  h  -  j  -  k  -  l  keys.  h-l  for left-right,  j-k  for down-up.

Once you are done editing you can press the  esc  key to exit insert mode, and go back to command mode.

At this point you can navigate the file, but you can't add content to it (and be careful which keys you press as they might be commands).

One thing you might want to do now is saving the file. You can do so by pressing  :  (colon), then  w .

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www.dbooks.org You can save and quit pressing  :  then  w  and  q :  :wq

You can quit without saving, pressing  :  then  q  and  ! :  :q!

You can undo and edit by going to command mode and pressing  u  . You can redo (cancel an undo) by pressing  ctrl-r .

Those are the basics of working with Vim. From here starts a rabbit hole we can't go into in this little introduction.

I will only mention those commands that will get you started editing with Vim:

pressing the   x  key deletes the character currently highlighted
pressing   A  goes at the end of the currently selected line
press  0  to go to the start of the line go to the first character of a word and press  d  followed by  w  to delete that word. If you follow it with  e  instead of  w , the white space before the next word is preserved
use a number between  d  and  w  to delete more than 1 word, for example use  d3w  to delete 3 words forward
press  d  followed by  d  to delete a whole entire line. Press  d  followed by  $  to delete the entire line from where the cursor is, until the end

To find out more about Vim I can recommend the Vim

FAQ and especially running the  vimtutor  command, which should already be installed in your system and

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will greatly help you start your  vim  explorations.

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emacs

emacs  is an awesome editor and it's historically regarded as the editor for UNIX systems. Famously  vi  vs  emacs  flame wars and heated discussions caused many unproductive hours for developers around the world.

emacs  is very powerful. Some people use it all day long as a kind of operating system

(https://news.ycombinator.com/item?id=19127258). We'll just talk about the basics here.

You can open a new emacs session simply by invoking  emacs :

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macOS users, stop a second now. If you are on

Linux there are no problems, but macOS does not
ship applications using GPLv3, and every built-in

UNIX command that has been updated to GPLv3
has not been updated. While there is a little
problem with the commands I listed up to now, in
this case using an emacs version from 2007 is not
exactly the same as using a version with 12 years
of improvements and change. This is not a
problem with Vim, which is up to date. To fix this,
run  brew install emacs  and running  emacs  will
use the new version from Homebrew (make sure
you have Homebrew installed)

You can also edit an existing file calling   emacs   :

You can start editing and once you are done, press  ctrl-x  followed by  ctrl-w . You confirm the folder:

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and Emacs tell you the file exists, asking you if it should overwrite it:

Answer  y , and you get a confirmation of success:

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You can exit Emacs pressing  ctrl-x  followed by  ctrl-c  . Or  ctrl-x  followed by  c  (keep  ctrl  pressed).

There is a lot to know about Emacs. More than I am able to write in this little introduction. I encourage you to open Emacs and press  ctrl-h   r  to open the built-in manual and  ctrl-h   t  to open the official tutorial.

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nano

nano  is a beginner friendly editor.

Run it using  nano  .

You can directly type characters into the file without worrying about modes.

You can quit without editing using  ctrl-X  . If you edited the file buffer, the editor will ask you for confirmation and you can save the edits, or discard them. The help at the bottom shows you the keyboard commands that let you work with the file:

pico  is more or less the same, although  nano  is the GNU version of  pico  which at some point in history was not open source and the  nano  clone was made to satisfy the GNU operating system license requirements.

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whoami

Type  whoami  to print the user name currently logged in to the terminal session:

Note: this is different from the   who am i

command, which prints more information

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who

The  who  command displays the users logged in to the system.

Unless you're using a server multiple people have access to, chances are you will be the only user logged in, multiple times:

Why multiple times? Because each shell opened will count as an access.

You can see the name of the terminal used, and the time/day the session was started.

The   -aH  flags will tell   who  to display more information, including the idle time and the process ID of the terminal:

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The special  who am i  command will list the current terminal session details:

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su

While you're logged in to the terminal shell with one user, you might have the need to switch to another user.

For example you're logged in as root to perform some maintenance, but then you want to switch to a user account.

You can do so with the  su  command:

su

For example:  su flavio .

If you're logged in as a user, running  su  without anything else will prompt to enter the   root  user password, as that's the default behavior.

su  will start a new shell as another user.

When you're done, typing  exit  in the shell will close that shell, and will return back to the current user's shell.

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sudo

sudo  is commonly used to run a command as root.

You must be enabled to use  sudo , and once you do, you can run commands as root by entering your user's password (not the root user password).

The permissions are highly configurable, which is great especially in a multi-user server environment, and some users can be granted access to running specific commands through  sudo .

For example you can edit a system configuration file:

sudo nano /etc/hosts

which would otherwise fail to save since you don't have the permissions for it.

You can run  sudo -i  to start a shell as root:

You can use  sudo  to run commands as any user.  root  is the default, but use the  -u  option to specify another user:

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sudo -u flavio ls /Users/flavio

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passwd

Users in Linux have a password assigned. You can change the password using the  passwd  command.

There are two situations here.

The first is when you want to change your password. In this case you type:

passwd

and an interactive prompt will ask you for the old password, then it will ask you for the new one:

When you're  root  (or have superuser privileges) you can set the username of which you want to change the password:

passwd

In this case you don't need to enter the old one.

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ping

The  ping  command pings a specific network host, on the local network or on the Internet.

You use it with the syntax   ping   where    could be a domain name, or an IP address.

Here's an example pinging  google.com :

The commands sends a request to the server, and the server returns a response.

ping  keep sending the request every second, by default, and will keep running until you stop it with  ctrl-C  , unless you pass the number of times you want to try with the  -c  option:  ping -c 2 google.com .

Once  ping  is stopped, it will print some statistics about the results: the percentage of packages lost, and statistics about the network performance.

As you can see the screen prints the host IP address, and the time that it took to get the response back.

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Not all servers support pinging, in case the requests times out:

Sometimes this is done on purpose, to "hide" the server, or just to reduce the load. The ping packets can also be filtered by firewalls.

ping  works using the ICMP protocol (Internet Control Message Protocol), a network layer protocol just like TCP or UDP.

The request sends a packet to the server with the   ECHO_REQUEST  message, and the server returns a  ECHO_REPLY  message. I won't go into details, but this is the basic concept.

Pinging a host is useful to know if the host is reachable (supposing it implements ping), and how distant it is in terms of how long it takes to get back to you. Usually the nearest the server is geographically, the less time it will take to return back to you, for simple physical laws that cause a longer distance to introduce more delay in the cables.

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traceroute

When you try to reach a host on the Internet, you go through your home router, then you reach your ISP network, which in turn goes through its own upstream network router, and so on, until you finally reach the host.

Have you ever wanted to know what are the steps that your packets go through to do that?

The  traceroute  command is made for this.

You invoke
traceroute

and it will (slowly) gather all the information while the packet travels.

In this example I tried reaching for my blog with  traceroute flaviocopes.com :

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Not every router travelled returns us information. In this case,  traceroute  prints  * * * . Otherwise, we can see the hostname, the IP address, and some performance indicator.

For every router we can see 3 samples, which means traceroute tries by default 3 times to get you a good indication of the time needed to reach it. This is why it takes this long to execute  traceroute  compared to simply doing a  ping  to that host.

You can customize this number with the  -q  option:

traceroute -q 1 flaviocopes.com

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clear

Type  clear  to clear all the previous commands that were ran in the current terminal.

The screen will clear and you will just see the prompt at the top:

Note: this command has a handy shortcut:  ctrl-

L

Once you do that, you will lose access to scrolling to see the output of the previous commands entered.

So you might want to use  clear -x  instead, which still clears the screen, but lets you go back to see the previous work by scrolling up.

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history

Every time we run a command, that's memorized in the history.

You can display all the history using:

history

This shows the history with numbers:

You can use the syntax  !  to repeat a command stored in the history, in the above example typing  !121  will repeat the  ls -al | wc -l  command.

Typically the last 500 commands are stored in the history.

You can combine this with  grep  to find a command you ran:

history | grep docker

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To clear the history, run  history -c

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export

The  export  command is used to export variables to child processes.

What does this mean?

Suppose you have a variable TEST defined in this way:

TEST="test"

You can print its value using  echo $TEST :

But if you try defining a Bash script in a file  script.sh  with the above command:

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This is because in Bash the   TEST  variable was defined local to the shell. When executing a shell script or another command, a subshell is launched to execute it, which does not contain the current shell local variables.

To make the variable available there we need to define  TEST  not in this way:

TEST="test"

but in this way:

export TEST="test"

Try that, and running  ./script.sh  now should print "test":

Sometimes you need to append something to a variable. It's often done with the  PATH  variable. You use this syntax:

export PATH=$PATH:/new/path

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It's common to use  export  when you create new variables in this way, but also when you create variables in the   .bash_profile  or   .bashrc  configuration files with Bash, or in  .zshenv  with Zsh.

To remove a variable, use the  -n  option:

export -n TEST

Calling   export  without any option will list all the exported variables.

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crontab

Cron jobs are jobs that are scheduled to run at specific intervals. You might have a command perform something every hour, or every day, or every 2 weeks. Or on weekends. They are very powerful, especially on servers to perform maintenance and automations.

The  crontab  command is the entry point to work with cron jobs.

The first thing you can do is to explore which cron jobs are defined by you:

crontab -l

You might have none, like me:

Run
crontab -e

to edit the cron jobs, and add new ones.

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By default this opens with the default editor, which is usually  vim . I like  nano  more, you can use this line to use a different editor:

EDITOR=nano crontab -e

Now you can add one line for each cron job.

The syntax to define cron jobs is kind of scary. This is why I usually use a website to help me generate it
without errors: https://crontab-generator.org/

You pick a time interval for the cron job, and you type the command to execute.

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www.dbooks.org I chose to run a script located in  /Users/flavio/test.sh  every 12 hours. This is the crontab line I need to run:

* */12 * * * /Users/flavio/test.sh >/dev/null 2>&1

I run  crontab -e :

EDITOR=nano crontab -e

and I add that line, then I press  ctrl-X  and press  y  to save.

If all goes well, the cron job is set up:

Once this is done, you can see the list of active cron jobs by running:

crontab -l

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You can remove a cron job running  crontab -e  again, removing the line and exiting the editor:

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uname

Calling   uname  without any options will return the Operating System codename:

The  m  option shows the hardware name (  x86_64  in this example) and the  p  option prints the processor architecture name (  i386  in this example):

The  s  option prints the Operating System name.  r  prints the release,  v  prints the version:

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The  n  option prints the node network name:

The  a  option prints all the information available:

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www.dbooks.org On macOS you can also use the  sw_vers  command to print more information about the macOS Operating System. Note that this differs from the Darwin (the Kernel) version, which above is  19.6.0 .

Darwin is the name of the kernel of macOS. The
kernel is the "core" of the Operating System, while
the Operating System as a whole is called
macOS. In Linux, Linux is the kernel, GNU/Linux
would be the Operating System name, although
we all refer to it as "Linux"

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env

The  env  command can be used to pass environment variables without setting them on the outer environment (the current shell).

Suppose you want to run a Node.js app and set the  USER  variable to it.

You can run
env USER=flavio node app.js

and the  USER  environment variable will be accessible from the Node.js app via the Node   process.env  interface.

You can also run the command clearing all the environment variables already set, using the   -i  option:

env -i node app.js

In this case you will get an error saying  env: node: No such file or directory  because the  node  command is not reachable, as the  PATH  variable used by the shell to look up commands in the common paths is unset.

So you need to pass the full path to the   node  program:

env -i /usr/local/bin/node app.js

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Try with a simple  app.js  file with this content:

console.log(process.env.NAME)

console.log(process.env.PATH)

You will see the output being
undefined
undefined

You can pass an env variable:

env -i NAME=flavio node app.js

and the output will be
flavio
undefined

Removing the  -i  option will make  PATH  available again inside the program:

The  env  command can also be used to print out all the environment variables, if ran with no options:

env

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it will return a list of the environment variables set, for example:

HOME=/Users/flavio

LOGNAME=flavio

PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/L

PWD=/Users/flavio

SHELL=/usr/local/bin/fish

You can also make a variable inaccessible inside the program you run, using the  -u  option, for example this code removes the   HOME  variable from the command environment:

env -u HOME node app.js

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printenv

A quick guide to the  printenv  command, used to print the values of environment variables

In any shell there are a good number of environment variables, set either by the system, or by your own shell scripts and configuration.

You can print them all to the terminal using the  printenv  command. The output will be something like this:

HOME=/Users/flavio

LOGNAME=flavio

PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/L

PWD=/Users/flavio

SHELL=/usr/local/bin/fish

with a few more lines, usually.

You can append a variable name as a parameter, to only show that variable value:

printenv PATH

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Conclusion

Thanks a lot for reading this book.

I hope it will inspire you to know more about Linux and its capabilities.

To learn more, check out my blog flaviocopes.com.

Send any feedback, errata or opinions at
hey@flaviocopes.com

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Table of Contents

- Preface
- Introduction to Linux and shells
- man
- ls
- cd
- pwd
- mkdir
- rmdir
- mv
- cp
- open
- touch
- find
- ln
- gzip
- gunzip
- tar
- alias
- cat
- less
- tail
- wc
- grep
- sort
- uniq
- diff
- echo
- chown
- chmod
- umask
- du
- df
- basename
- dirname
- ps
- top
- kill
- killall
- jobs
- bg
- fg
- type
- which
- nohup
- xargs
- vim
- emacs
- nano
- whoami
- who