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Basic Linux Commands

The following file people.txt is used throughout this tutorial:

firstName       lastName        age     city       ID   department
Thomas          Shelby          30      Rio        400  IT
Omega           Night           45      Ontario    600  Design
Wood            Tinker          54      Lisbon     N/A  IT
Giorgos         Georgiou        35      London     300  Data
Timmy           Turner          32      Berlin     N/A  Engineering

man​

This command shows manual of other commands. Simply run man <command> to show the manual.

sudo​

The sudo command allows you to run programs with the security previleges of another user (by default, as the superuser).

pwd (Print Working Directory)​

This command prints an absolute path of the current working directory.

ls (List current directory)​

This command lists all files and folders in the current directory. This command also accepts flags that may show different results.

  • List all files including hidden files.

    ls -a

  • List all files and their info

    ls -l

  • You can even combine flags

    `ls -la

cd (Change Directory)​

This command is used for changing the current working directory.

mv (Move)​

This command moves files from a directory to another. This command can also be used to change a file or directory's name

cp (Copy)​

This command copies files from a directory to another.

rm (Remove)​

This command removes files or directories. Please note that removed objects are not stored in recycle bin and gone for good.

deleting a file

rm file.txt

recursive deletion (used for deleting a directory)

rm -r file.txt

touch (Create new file)​

This command creates a new file.

touch file.txt

mkdir (Make Directory)​

This command creates a new directory.

mkdir dir

echo​

This command displays a line of text.

echo 'hello world'

cat (Print file content)​

This command concatenate files and prints to stdout.

cat people.txt

head & tail​

This command outputs the first and last part of the file respectively.

Print first 2 lines of the file

cat people.txt | head 2

Print last 2 lines of the file

cat people.txt | tail 2

Piping (|)​

The Unix/Linux systems allow stdout of a command to be connected to stdin of another command. You can make it do so by using the pipe character |.

cat people.txt | head 2

grep​

This command prints lines that match the given pattern.

Prints lines that contain 'IT'.

cat people.txt | grep IT

find​

This command searches for files in the directory hierarchy.

  • Find files that matches the given pattern

    find -name PATTERN

which​

WHICH is used for locating the executable file associated wit the given command.

which python

alias​

ALIAS allows you to create shortcuts for repetitive long commands.

Syntax

alias [option] [name]='[value]'

Example: Make an alias​

alias c='clear'

Example: List all aliases​

$alias

-='cd -'
...=../..
....=../../..
.....=../../../..
......=../../../../..
1='cd -1'
2='cd -2'
3='cd -3'
4='cd -4'
5='cd -5'
6='cd -6'
7='cd -7'
8='cd -8'
9='cd -9'
_='sudo '
afind='ack -il'
diff='diff --color'
egrep='egrep --color=auto --exclude-dir={.bzr,CVS,.git,.hg,.svn,.idea,.tox}'
fgrep='fgrep --color=auto --exclude-dir={.bzr,CVS,.git,.hg,.svn,.idea,.tox}'
g=git
ga='git add'
gaa='git add --all'
gam='git am'
gama='git am --abort'
...

Make alias permanent​

Alias lives in the active shell session. When the shell is closed, the alias is also gone. To make aliases permanent, you can add them to .bashrc.

export​

This command is shell dependent. This may not work on other shells rather than Bash.

Export is Bash shell built-in commands. It marks an environment variables to be exported to child-processes.

Example: View all export variables​

export

Example: Set PATH environment variable​

export PATH=$PATH:/your/path

cut​

push & pop​

chmod (Change mode)​

CHMOD controls who can access files, search directories, and run scripts.

When listing files in Linux, you may see the following output.

drwxrwxr-x 4 siraphob siraphob 4096 Jun 18 22:22 backend
drwxrwxr-x 6 siraphob siraphob 4096 Jun 18 22:22 developers-handbook
drwxrwxr-x 3 siraphob siraphob 4096 Jun 18 22:22 frontend
drwxrwxr-x 9 siraphob siraphob 4096 Jun 18 22:22 infra
drwxrwxr-x 2 siraphob siraphob 4096 Jun 18 22:22 internship-2022
-rw-rw-r-- 1 siraphob siraphob 2164 Jun 18 22:22 README.md
drwxrwxr-x 2 siraphob siraphob 4096 Jun 18 22:22 soilcrete
drwxrwxr-x 4 siraphob siraphob 4096 Jun 18 22:22 training
drwxrwxr-x 4 siraphob siraphob 4096 Jun 18 22:22 uploads
drwxrwxr-x 3 siraphob siraphob 4096 Jun 18 22:22 uxui

The 10 characters of each file indicates its type and permissions.

The first characters indentifies the file type. If it's a dash (-) then it's a file. If it is the letter d then it's a directory.

The next 9 characters represents the permissions.

  • The first 3 characters show the permissions for the user who own the file. (user permissions).
  • The middle 3 characters show the permissions for the members of the file group. (group permissions).
  • The last 3 characters show the permissions for anyone not in the first two categories (other permissions).

There are 3 letters that represent permissions.

  • r is for Read permission
  • w is for Write permission
  • x is for Execute permission

df (Disk Free)​

DF is used to display the amount of available disk space for file systems.

$ df -h

Filesystem      Size  Used Avail Use% Mounted on
udev             16G     0   16G   0% /dev
tmpfs           3.1G  2.4M  3.1G   1% /run
/dev/nvme0n1p5   48G   26G   21G  56% /
tmpfs            16G   13M   16G   1% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs            16G     0   16G   0% /sys/fs/cgroup
/dev/loop0      128K  128K     0 100% /snap/bare/5
/dev/loop1       56M   56M     0 100% /snap/core18/2409
/dev/loop2      249M  249M     0 100% /snap/gnome-3-38-2004/99
/dev/loop3      255M  255M     0 100% /snap/gnome-3-38-2004/106
/dev/loop4       66M   66M     0 100% /snap/gtk-common-themes/1519
/dev/loop5       62M   62M     0 100% /snap/core20/1518
/dev/loop6       82M   82M     0 100% /snap/gtk-common-themes/1534
/dev/loop7       62M   62M     0 100% /snap/core20/1328
/dev/loop8      321M  321M     0 100% /snap/telegram-desktop/3975
/dev/nvme0n1p1   96M   46M   51M  48% /boot/efi
/dev/loop9      296M  296M     0 100% /snap/vlc/2344
/dev/loop10      55M   55M     0 100% /snap/snap-store/558
/dev/loop11      44M   44M     0 100% /snap/snapd/14978
/dev/loop12      47M   47M     0 100% /snap/snapd/16010
tmpfs           3.1G   72K  3.1G   1% /run/user/1000

du (Disk Usage)​

How much disk space are being used by these files.

$ du -hs

13G /home/siraphob

tar​

TAR is used to create a .tar.gz or .tgz archive files, also called tarballs.

  • Compress an entrie directory into an archive
tar -czvf archive.tar.gz /path/to/dir-or-file

# c - create an archive
# z - compress an archive with gzip
# v - display progress in the terminal
# f - allows you to specify the file name of the archive
  • Extract a .tar.gz archive.
tar -xvzf archive.tar.gz

# x - extract files
# v - verbose, print the file names as they are extracted
# z - the file is a `gzipped` file
# f - use the following `tar` archive for the operation

history​

HISTORY shows a list of commands used in the terminal session.

kill​

KILL is used for terminating processes manually. KILL sends a signal to a process which terminates it.

  • Show all available signals
$ kill -l
HUP INT QUIT ILL TRAP ABRT BUS FPE KILL USR1 SEGV USR2 PIPE ALRM TERM STKFLT CHLD CONT STOP TSTP TTIN TTOU URG XCPU XFSZ VTALRM PROF WINCH POLL PWR SYS
  • kill a process
kill -s <signal> <pid>

To see PID of processes, use pidof <program-name>.

ping​

PING is used to ping the target server.

$ ping google.com
PING google.com(kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e)) 56 data bytes
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=1 ttl=115 time=51.8 ms
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=2 ttl=115 time=39.8 ms
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=3 ttl=115 time=39.6 ms
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=4 ttl=115 time=39.3 ms
^C
--- google.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3005ms
rtt min/avg/max/mdev = 39.279/42.634/51.835/5.315 ms

fg (Foreground)​

FG is a commadn that moves a background process on the current shell to the foreground.

To simulate and sending a job to the background, we'll invoke the PING command.

ping google.com

After that, press Ctrl+Z to suspend the job and send it to the background.

To list all jobs, use the following command.

$ jobs -l
[1]  + 9850 suspended  ping google.com

You can invoke the background process back to the foreground by referencing them using the following command.

fg %<job-id>

In this case, it'll be

fg %1
[1]  + 9850 continued  ping google.com
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=3 ttl=115 time=75.4 ms
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=4 ttl=115 time=41.8 ms
64 bytes from kul08s10-in-x0e.1e100.net (2404:6800:4001:80e::200e): icmp_seq=5 ttl=115 time=40.8 ms

bg (Background)​

BG runs jobs in the background. Continues a suspended job in the background.

Example​

  1. ping google.com
  2. Send it to background using Ctrl+Z.
  3. Continue the job in the background using bg

curl​

CURL is a command line tool that allows to transfer data across the network.

It supports lots of protocols out of the box, including HTTP, HTTPS, FTP, FTPS, SFTP, IMAP, SMTP, POP3, and many more.

  • Perform a HTTP GET request
curl https://httpbin.org/get

TIPS: use json_pp to pretty print the json output.

  • Perfom a HTTP POST request
curl -d '{"hello": "world"}' \
  -H 'Content-Type: application/json' \
  -X POST https://httpbin.org/post

rsync​

RSYNC stands for remote sync. It's a local and remote file synchronization tool. It's used for sending files from a machine to another machine.

Example 1: Sync directories​

Create two directories

mkdir dir1 dir2

Create files in dir1

touch dir1/file{1..100}

Sync files from dir1 to dir2

rsync -av dir1/ dir2

# trailing slash on dir1 indicates that to sync all contents inside dir1
# a - sync in the archive mode, this will sync files recursively and preserve symbolic links
# v - verbose mode, prints files as they're syncing

Example 2: Sync files to remote system​

rsync -a ~/dir1 username@remote_host:destination_directory
rsync -a ~/dir1 [email protected]:/home/siraphob/dir1

xargs​

XARGS builds and executes command provided through the standard input. It takes the input and converts it into a command argument for another command.

Example 1​

seq 5 | echo
1
2
3
4
5

You'll see no output because ECHO doesn't take stdin as input but command line arguments.

$ seq 5 | xargs echo
1 2 3 4 5

You can use XARGS to feed the output as the argument of the ECHO command.

Example 2: Recursive LS​

ls | xargs ls

Difference between XARGS and PIPE(|)

XARGS can be used when you need to take the output from one command and use it as an argument to another. On the other hand, piping is taking output from a command and feed into the stdin of another.

Example 3: Placeholder argument​

ls | xargs -I {} echo "hello {}"

Example 4: Execute arguments separatedly​

$ ls | xargs -t echo

echo Desktop Documents Downloads Music Pictures Programs Public qmk_firmware Scripts snap Templates Videos Workspace
Desktop Documents Downloads Music Pictures Programs Public qmk_firmware Scripts snap Templates Videos Workspace

The above command runs the ECHO only once. If we want to run ECHO separately for each directory, we could use -n <number-of-max-args> to fix the maximum number of arguments to run by XARGS.

$ ls | xargs -t -n 1 echo

echo Desktop
Desktop
echo Documents
Documents
echo Downloads
Downloads
echo Music
Music
echo Pictures
Pictures
echo Programs
Programs
echo Public
Public
echo qmk_firmware
qmk_firmware
echo Scripts
Scripts
echo snap
snap
echo Templates
Templates
echo Videos
Videos
echo Workspace
Workspace

The above command uses -I flag to specify a placeholder. In this case, we use {} as the placeholder.

awk​

AWK is a command line text manipulation scripting language.

AWK works on programs that contain rules comprised of patterns and actions. Patterns are enclosed in curly braces {}. Together, a pattern and an action form a rule. The entrie AWK program is enclosed in single quotes ''.

Example 1: Print the first column​

cat people.txt | awk '{ print $1 }'

Example 2: Print 2 columns​

cat people.txt | awk '{ print $1, $2 }'

Example 3: Skip the first line​

cat people.txt | awk '{ if (NR > 1) { print $0 } }'

Example 4: Find people working in IT​

AWK allows you to use Regex by adding a /pattern/ at the start of the rules.

cat people.txt | awk '/IT/'

sed​

This command is a stream editor for filtering and transforming text. Most common use of SED command is for substitution or for find and replace.

  • Replace power with tech

    echo "apiplus power" | sed "s/power/tech/"

  • Delete a particular line

    sed '5d' file.txt # remove line 5

Bash shortcuts​

Ctrl+c​

Sends SIGINT signal and kills the current process.

Ctrl+z​

Send SIGTSTP to suspend the current process.

Ctrl+d​

Send an EOF marker. This will close the current shell.

Ctrl+r​

Recall the last command including the specified character.

References​