linux commands

detailed 61 linux commands as curated off https://www.geeksforgeeks.org/linux-commands/?ref=gcse_outind

useradd

• Purpose: Creates a new user account within the system.

• Syntax: useradd [options] username

• Options:

  • -m: Create the user's home directory if it does not exist.

  • -G: Add the user to one or more supplementary groups.

  • -s: Specify the login shell for the user.

  • -d: Set the home directory.

• Example: useradd -m -G admin newuser

usermod

• Purpose: Modifies the attributes and settings of an existing user.

• Syntax: usermod [options] username

• Options:

  • -g: Change the primary group for the user.

  • -G: Add the user to additional groups.

  • -d: Change the user's home directory.

  • -e: Set an expiration date for the user account.

• Example: usermod -g users -G admin,newgroup username

userdel

• Purpose: Deletes an existing user account from the system.

• Syntax: userdel [options] username

• Options:

  • -r: Remove the user's home directory and mail spool.

  • -f: Force the deletion of the user account even if they are logged in.

• Example: userdel -r olduser

passwd

• Purpose: Changes the password for a user account in the system.

• Syntax: passwd [options] username

• Options:

  • -e: Expire the user’s password immediately.

  • -l: Lock the user's password.

  • -u: Unlock a previously locked password.

• Example: passwd -e username

su

• Purpose: Switches the current user to a different account or to the root user.

• Syntax: su [options] [username]

• Options:

  • -: Start the shell as a login shell, simulating an initial login.

• Example: su - username

groups

• Purpose: Displays the groups that a specific user is a member of.

• Syntax: groups [username]

• Example: groups john (if no username is specified, it shows the current user's groups)

id

• Purpose: Retrieves information about user and group IDs.

• Syntax: id [options] [username]

• Options:

  • -u: Display the user ID (UID) only.

  • -g: Display the group ID (GID) only.

• Example: id username

ls

• Purpose: Lists the contents of a specified directory.

• Syntax: ls [options] [directory]

• Options:

  • -l: Use a long listing format.

  • -a: Include hidden files in the listing.

• Example: ls -la /home

cd

• Purpose: Changes the current working directory in the terminal session.

• Syntax: cd [directory]

• Example: cd /home/user or cd .. (to go up one directory)

pwd

• Purpose: Displays the full path of the current working directory.

• Syntax: pwd

• Example: Output might be /home/user.

mkdir

• Purpose: Creates new directories in the filesystem.

• Syntax: mkdir [options] directory

• Options:

  • -p: Create parent directories as necessary.

• Example: mkdir -p /home/user/newfolder

rm

• Purpose: Removes files or directories from the filesystem.

• Syntax: rm [options] file

• Options:

  • -r: Recursively remove directories and their contents.

  • -f: Force removal without prompting for confirmation.

• Example: rm -rf /home/user/oldfolder

cp

• Purpose: Copies files or directories from one location to another.

• Syntax: cp [options] source destination

• Options:

  • -r: Recursively copy directories.

  • -p: Preserve file attributes when copying.

• Example: cp -r /srcdir /destdir

mv

• Purpose: Moves files or directories to a new location or renames them.

• Syntax: mv [options] source destination

• Options:

  • -i: Prompt before overwriting files.

• Example: mv oldname newname

touch

• Purpose: Creates an empty file or updates the timestamp of an existing file.

• Syntax: touch [options] file

• Example: touch newfile.txt

cat

• Purpose: Displays the contents of one or more files in the terminal.

• Syntax: cat [options] file

• Options:

  • -n: Number the output lines.

• Example: cat file.txt

find

• Purpose: Searches for files in a directory hierarchy based on specified criteria.

• Syntax: find [path] [expression]

• Example: find /home -name '*.txt'

tar

• Purpose: Archives files by creating a single file containing multiple files and directories.

• Syntax: tar [options] archive files

• Options:

  • -c: Create a new archive.

  • -f: Specify the filename of the archive.

• Example: tar -cvf archive.tar /path/to/files

gzip

• Purpose: Compresses files to save disk space.

• Syntax: gzip [options] file

• Example: gzip file.txt

gunzip

• Purpose: Decompresses files that have been compressed by gzip.

• Syntax: gunzip [options] file

• Example: gunzip file.txt.gz

zip

• Purpose: Compresses multiple files into a single zip archive.

• Syntax: zip [options] archive files

• Options:

  • -r: Recursively add files from directories.

• Example: zip archive.zip file1.txt file2.txt

chmod

• Purpose: Changes the access permissions of files or directories.

• Syntax: chmod [options] mode file

• Options:

  • u: User's permission.

  • g: Group's permission.

  • o: Others' permission.

• Example: chmod u+x script.sh

chown

• Purpose: Changes the ownership of files or directories to a specified user and/or group.

• Syntax: chown [options] user[:group] file

• Options:

  • -R: Change the ownership recursively.

• Example: chown user:group file.txt

chgrp

• Purpose: Changes the group ownership of a specified file or directory.

• Syntax: chgrp [options] group file

• Example: chgrp admin file.txt

ln

• Purpose: Creates a link (hard or symbolic) to a file.

• Syntax: ln [options] target link

• Options:

  • -s: Create a symbolic link.

• Example: ln -s /path/to/file linkname

grep

• Purpose: Searches for a specific string or pattern in files and outputs matching lines.

• Syntax: grep [options] pattern file

• Options:

  • -i: Ignore case distinctions during matching.

• Example: grep 'searchterm' file.txt

sort

• Purpose: Arranges the lines of a text file in a specified order (ascending or descending).

• Syntax: sort [options] file

• Options:

  • -r: Sort in reverse order.

• Example: sort -r file.txt

awk

• Purpose: A scripting language for processing and analyzing text data.

• Syntax: awk [options] 'pattern {action}' file

• Example: awk '{print $1}' file.txt

sed

• Purpose: A stream editor for basic text manipulation such as substitution, insertion, and deletion.

• Syntax: sed [options] 'expression' file

• Example: sed 's/original/replacement/' file.txt

cut

• Purpose: Extracts specific sections from lines of files based on delimiters or byte positions.

• Syntax: cut [options] file

• Options:

  • -d: Specify the delimiter.

  • -f: Specify which fields to extract.

• Example: cut -d',' -f1 file.txt

tr

• Purpose: Translates or deletes specified characters from the input data.

• Syntax: tr [options] set1 set2

• Example: tr 'a-z' 'A-Z' < file.txt

echo

• Purpose: Outputs a line of text or variable values to the terminal.

• Syntax: echo [options] text

• Example: echo 'Hello, World!'

wc

• Purpose: Counts the number of lines, words, bytes, and characters in a text file.

• Syntax: wc [options] file

• Options:

  • -l: Count lines.

  • -w: Count words.

• Example: wc -l file.txt

head

• Purpose: Displays the first N lines of a file.

• Syntax: head [options] file

• Options:

  • -n: Specify number of lines to display.

• Example: head -n 10 file.txt

tail

• Purpose: Displays the last N lines of a file.

• Syntax: tail [options] file

• Options:

  • -n: Specify number of lines to display.

• Example: tail -n 10 file.txt

ifconfig

• Purpose: Configures network interfaces, displaying their current settings.

• Syntax: ifconfig [options] interface

• Example: ifconfig eth0

ping

• Purpose: Sends ICMP echo request packets to a host to check network connectivity.

• Syntax: ping [options] host

• Example: ping google.com

ssh

• Purpose: Securely connects to a remote server for command execution.

• Syntax: ssh [options] user@host

• Example: ssh user@remotehost

scp

• Purpose: Securely transfers files between hosts over a network.

• Syntax: scp [options] source destination

• Example: scp file.txt user@remotehost:/path

wget

• Purpose: Retrieves files from the web using HTTP/HTTPS protocols.

• Syntax: wget [options] URL

• Example: wget http://example.com/file.txt

curl

• Purpose: Transfers data using various protocols, allowing uploads and downloads.

• Syntax: curl [options] URL

• Example: curl -O http://example.com/file.txt

df

• Purpose: Displays the amount of disk space used and available on mounted filesystems.

• Syntax: df [options]

• Example: df -h

du

• Purpose: Estimates and displays the disk space used by files and directories.

• Syntax: du [options] file

• Options:

  • -h: Display human-readable sizes.

• Example: du -h /home/user

mount

• Purpose: Mounts a filesystem, making it accessible at a specified directory.

• Syntax: mount [options] device directory

• Example: mount /dev/sda1 /mnt

ps

• Purpose: Displays information about running processes, including their ID and state.

• Syntax: ps [options]

• Options:

  • -aux: Show all users' processes with detailed information.

• Example: ps -aux

top

• Purpose: Provides a real-time overview of system performance and running processes.

• Syntax: top

• Example: Simply run top to see active processes.

date

• Purpose: Displays or sets the system's current date and time.

• Syntax: date [options]

• Example: date '+%Y-%m-%d %H:%M:%S'

cal

• Purpose: Outputs a calendar for the current month or a specified date.

• Syntax: cal [options]

• Example: cal 2023.

shutdown

• Purpose: Safely powers off the system, allowing active processes to terminate gracefully.

• Syntax: shutdown [options] time

• Options:

  • now: Shut down immediately.

• Example: shutdown now

reboot

• Purpose: Restarts the system, clearing active sessions and processes.

• Syntax: reboot

• Example: Simply run reboot.

uname

• Purpose: Displays system information such as the operating system and kernel version.

• Syntax: uname [options]

• Example: uname -a.

history

• Purpose: Shows the command history of the current user session.

• Syntax: history

• Example: Simply run history.

alias

• Purpose: Creates shortcuts (aliases) for commands to simplify command-line operations.

• Syntax: alias name='command'

• Example: alias ll='ls -la'

sleep

• Purpose: Pauses execution for a specified duration.

• Syntax: sleep time

• Example: sleep 10 (pauses for 10 seconds).

kill

• Purpose: Terminates running processes using their process ID (PID).

• Syntax: kill [options] PID

• Options:

  • -9: Forcefully kill the process.

• Example: kill -9 1234.

init

• Purpose: The parent process for all other processes, often used for system reboot and shutdown operations.

• Syntax: init [options]

• Example: init 6 (to reboot the system).

man

• Purpose: Displays the user manual for specific commands, providing detailed documentation.

• Syntax: man [options] command

• Example: man ls.

apropos

• Purpose: Lists commands related to a specific keyword found within manual pages.

• Syntax: apropos [keyword]

• Example: apropos copy.

info

• Purpose: Accesses detailed documentation in the info format, providing comprehensive command details.

• Syntax: info [command]

• Example: info ls.

which

• Purpose: Locates the executable file associated with a command, revealing its path.

• Syntax: which command

• Example: which python.

whereis

• Purpose: Finds the source, binary, and manual sections of a command to provide comprehensive location data.

• Syntax: whereis command

• Example: whereis gcc.