Essential Linux commands to know

Here, I gather useful Linux commands that you need know to work with a Bash terminal.

Change current directory with cd command:

cd /home

I highly recommend installing zsh instead of Bash. In zsh, you don’t need to write cd, only the name of the directory takes you there. Here, I assume you don’t use zsh.

Go one folder up, or to parent directory with ..

cd /home/sorush
# we are at /home/sorush
cd ..
# we are at /home/

And . means the current directory, it doesn’t do anything with cd command.

we can mix .. with the absolute path:

cd /home/sorush/../../home/sorush/../.
# we are at /home

Two useful shortcuts are

  • ~ : home folder, where current user’s files reside
  • - : previous folder
cd /
cd ~
# we are at /home/sorush
cd -
# we are back to /

Get current directory address using the print-working-directory command :

pwd

Create file

Create an empty file via

touch myFile.txt

If you are a vi/vim user,

vim myFile.txt

opens vim. When you save your changes, the file is created.

See file content

To print all the content of a file on screen

cat myFile.txt

To browse up and down in an interactive way, use

less myFile.txt
# or
more myFile.txt

vim and nano are two popular text editors in Bash which can also be used for viewing the content of files.

Read live logs

To see the live updates to a log file that is being written by a program, run

tail -f logFile
  • -f: follow updates

If -f is dropped, you see only the last 10 lines of the file at the moment you press Enter.

Directory

Create a directory with

mkdir nameOfDirectory

Get content of a directory with the list command

ls -al /home
  • -a: show everything
  • -l: show details.

You can drop -al to see the difference. If you drop the destination folder, it shows the content of the current directory.

To show a tree-like list of content:

tree -d -L 2
  • -d: show only directories
  • -L 2: show only 2 layers of subdirectories

On some Linux distros tree is not available by default, you have to install it.

Get the size of a directory (I read it “dush”):

du -sh ~/myFolder
  • -h: human-readable like MegaByte (M), GigaByte (G)
  • -s: to hide subdirectories.

To write the output of a command into a file, use > followed by the name of the file:

ls > myFile.txt

Nothing will be shown in the terminal.

Copy file

A file in the current folder can be copied into another folder

cp myfile.txt ~/myfolder/

A folder can be copied into another place:

cp -r ~/myfolder ~/anotherFolder 
  • -r: copy the folder and everything inside (Recursive)

Note cp command overwrites existing files without any question.

Move file

Move command is to copy a file/folder to another place and delete the original file/folder. It is equivalent to cut-paste in Windows and copy-move in Mac.

A file can be moved to a new folder via:

mv myfile.txt ~/myfolder

A folder can be moved into another folder:

mv ~/myfolder ~/anotherFolder 

Rename file

The move command is used for renaming a file

mv myfile.txt info.csv

or renaming folder:

mv ~/myfolder ~/Simulation01

So when using mv command, if the destination folder exists, cut-paste happens, otherwise, the current folder is renamed to the destination folder.

Remove file

To remove a file or directory

rm ./myFileOrFolder -rf
  • -r : recursively delete i.e. removes everything including subfolders and files in them. This is what happens when we delete a folder in Windows or Mac. This is not necessary for files.

  • -f : Use forced deletion i.e. rm will remove all the files without any confirmation, even if the files are write-protected.

Sudo

Add it before any command to be run as a superuser (admin). You will be requested for your admin password. For example, to install an application on Ubuntu you need to have sudo privilege:

sudo apt install tree

Changing and deleting sensitive system folders needs a superuser.

Search for file

You can find a file in the current directory and its subdirectories via

find . -name "*.txt"
  • *: wildcard for any txt file.

You can also put the exact name there.

History

To get a list of all the commands you’ve used so far, run

history

the result is two columns, the first column is the ID of command and the second is the command like

  316  cd /home/sorush/../../home/sorush/../.
  317  cd /
  318  cd ~
  319  pwd

To run command again, use !+ID of the command:

!319
# will run pwd

To run the last command, use !!. Here I run the last command with sudo:

sudo !!

Grep via pipes

Using grep, we can filter the outcome of a command. For example, we want to filter the outcome of history command to only show the ones that contain ssh word:

history | grep ssh

or list the files of the current directory but only show text files

ls | grep .txt

Variables

We can set a variable in Bash by

nameOfVariable=TheContentOfVariable

We store a text in a variable. If there are spaces in the content, use quotations:

nameOfVariable="The Content Of Variable"

The content is usually a path to a file or folder:

destination=/home/sorush/documents

The variable is called with $ sign. Use echo to see the content of a variable

echo $destination

We can use the variable in any Bash command:

cd $destination
cp myFile.txt $destination

Store the outcome of a command into a variable :

aVariable=`aCommand`

for example:

address='pwd'

Environment variables

If a variable is needed to be available inside every program, we use export command to make it an environment variable:

export variable=Content

For example, CMake reads CC environment variable to find C compiler path. In a terminal, we can set it as:

export CC=/usr/local/gcc-11.1.0/bin/gcc

We can see all the environment variables defined, with the command

env

A good place to define environment variables is ~/.bashrc in Ubuntu or ~/.zshrc for Z-shell.

To have an executable available in every working directory, add its folder to PATH

export PATH="/home/shared/bin:$PATH"

To have a library accessible by compilers, add its folder to LD_LIBRARY_PATH:

export LD_LIBRARY_PATH="/home/vtk6.3/lib:$LD_LIBRARY_PATH"

Alias

To create a custom command, we use alias. See some of aliases I use in my terminal:

alias hgrep="history | grep"
alias lgrep="ls | grep"
alias current='cd "/home/sorush/test program"'
alias brc="vim ~/.bashrc"
alias zrc="vim ~/.zshrc"
alias dirsize="du -sh"
alias unicomputer="ssh sorush@xxxx.xxx.xx"

So we can do this:

hgrep ssh
# equals to history | grep ssh

A good place for aliases is ~/.bashrc in Ubuntu. We can see all the aliases defined in a terminal by running

alias

System info

Get the list of disks and their free space with disk free command

df -h
  • -h: human-readable.

Get memory usage by

free -h

Get the list (ls) of CPUs (cpu) and their info with

lscpu

Get a list of running processes similar to Windows Task Manager with

top

I recommend using htop, it looks much nicer than top:

htop

But usually htop is not available, and you have to install it by yourself.

Clear & Exit

You can clear the terminal with

clear

You can press ctrl+l to do the same.

You can exit a terminal by:

exit

Stop a process

Press ctrl+c to kill a process running in the current terminal:

# takes 5 seconds to finish
sleep 5 
# press ctrl+c before it finishes

To stop an application (or process) that is running in the background or another terminal:

killall nameOfapplication

Another way is to first get the ID of the application:

pidof nameOfapplication

You can also use top or htop. Then stop it using its ID

kill idOfApplication

Extract tar.gz

tar.gz is a very popular format for compressed files in Linux. You can extract them via:

tar -xvf myFile.tar.gz
  • -x: extract
  • -v: verbosely
  • -f: file

The same works for .tar.bz2 files.

Clipboard Copy-paste

Sometimes you want to copy a text from the terminal into the Linux clipboard, and then paste it somewhere else, for example in a Word document. This is what I do in Ubuntu:

  • Copy: select a text with the mouse and press ctrl+shift+c to copy it.
  • Paste: paste the clipboard content into a terminal with ctrl+shift+v.

You can also do both of the above with right-click on a terminal.

Terminal cut-paste

Write a line in a terminal and move the cursor to the center:

  • ctrl+k: cut everything after the cursor (I say Knock)
  • ctrl+u: cut everything before the cursor (I say Up)
  • ctrl+y: paste what cut before (Yank)

Jump cursor

Jump cursor:

  • ctrl+a: to the begining of the line (Arch)
  • ctrl+e: to the end of the the line (End)

Permissions

You can change who is eligible to read/write/execute a file using chmod command. There are a lot of details there but a few useful tricks are mentioned here.

Make a file executable:

chmod +x nameOfTheFile

Make a file/folder to be read/written/executed by only you (a private file)

chmod 700 nameOfTheFile

Make a file available to every user (a shared file)

chmod 777 nameOfTheFile

Background process

Sometimes you want to run a command that takes a lot of time like copying huge files and in the meantime, you want to do other stuff. You can run the time-consuming task in the background of the terminal by adding & to it:

# A CPU sleeps 20 seconds
sleep 20 &

You can see the list of all processes in the background via

jobs
# [1]  + running    sleep 20

You can kill the process with its job id

# Note the % sign
kill %1

If you want to run a program, for example, a GUI program, from a terminal but you want them to be detached. In other words, if you close the terminal, the GUI program is still running, use nohup:

nohup paraview & 

The window of Paraview will be opened and detached from the terminal. All the messages that Paraview sends to the terminal are redirected to nohup.out file.

Sometimes you need to find a process

ps -ef | grep text_1 | grep text_2

ps -ef shows all the processes on the machine in columns of username, process id, date, process command, and so on. We can filter the results with as many grep pipes as we like. For example, text_1 can be the username, text_2 can be the process command.

A process is killed by its id (we get it from ps -ef command):

kill process_id

Download

You can download a file from the internet by knowing its download link:

wget urlOfFile

Manual

Before jumping to google to understand a Linux command, try the man command. It may explain them better:

man ls
man cp
man history

Find executables

Some executables are in the system path. You run them without you know where they reside. To know where the binary, source and manual of command are, run:

whereis ls
whereis history
whereis gcc

If you just want to know the address of the binary of a command, use

which cp
which ls
which gcc

A symbolic link or symlink is a file that refers to another file or folder. It is similar to a shortcut in Windows.

To create a link file from an existing file, run

ln -s ExistingFile LinkFile

See this example:

# create a new file
touch myFile.txt
# create a symlink to it
ln -s myFile.txt linkToMyFile.txt 

Now any change to the content of linkToMyFile.txt is applied to myFile.txt.

Note symlinks can be also created for folders.

If you create a symlink to an executable, running the symlink is the same as running the executable.

Tags ➡ Linux HPC

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