Common Commands with examples

Using these commands in various scenarios to become proficient in basic command-line tasks, file management, and system administration.

ls - List directory contents

The ls command is used to list files and directories in a specified location. It has many options to customize the output.

Examples:

• ls -l: Long listing format, showing permissions, owner, size, and modification date
• ls -a: Show all files, including hidden ones (those starting with a dot)
• ls -R: Recursively list subdirectories
• ls -lh: Use human-readable file sizes (e.g., 1K, 234M, 2G)
• ls /etc: List contents of the /etc directory

cd - Change directory

cd is used to navigate the file system. It can use absolute or relative paths.

Examples:

• cd /home/user: Change to the /home/user directory (absolute path)
• cd ..: Move up one directory level
• cd -: Return to the previous directory
• cd ~: Change to the user's home directory
• cd Documents/Projects: Change to a subdirectory (relative path)

pwd - Print working directory

pwd displays the full path of the current working directory.

Examples:

• pwd: Outputs something like /home/user/Documents
• pwd -P: Print the physical directory, without any symbolic links

mkdir - Create a new directory

mkdir is used to create one or more directories.

Examples:

• mkdir newdir: Create a directory named "newdir"
• mkdir -p parent/child/grandchild: Create nested directories
• mkdir dir1 dir2 dir3: Create multiple directories at once
• mkdir -m 755 secured_dir: Create a directory with specific permissions

touch - Create an empty file or update file timestamps

touch can create empty files or update the access and modification times of existing files.

Examples:

• touch newfile.txt: Create a new empty file or update timestamps if it exists
• touch -t 202502191200 file.txt: Set specific timestamp (YYYYMMDDhhmm)
• touch -r reference_file target_file: Use timestamps from reference_file

cp - Copy files and directories

cp is used to copy files and directories from one location to another.

Examples:

• cp file.txt /home/user/backup/: Copy a file to another directory
• cp -r dir1 dir2: Copy a directory and its contents recursively
• cp -i *.txt /destination/: Copy all .txt files, prompting before overwrite
• cp -p source dest: Preserve file attributes (mode, ownership, timestamps)

mv - Move or rename files and directories

mv can move files/directories or rename them.

Examples:

• mv file.txt newname.txt: Rename a file
• mv /home/user/doc.txt /tmp/: Move a file to another directory
• mv dir1 dir2: Rename a directory (if dir2 doesn't exist) or move dir1 into dir2
• mv -i *.jpg /home/user/Pictures/: Move all .jpg files, prompting before overwrite

rm - Remove files or directories

rm is used to delete files and directories. Use with caution!

Examples:

• rm file.txt: Delete a file
• rm -r directory/: Recursively remove a directory and its contents
• rm -f unwanted_file: Force removal without prompting
• rm -i *.log: Remove .log files with confirmation for each

rmdir - Remove empty directories

rmdir is used to remove empty directories.

Examples:

• rmdir empty_folder: Remove an empty directory
• rmdir -p parent/child/grandchild: Remove nested empty directories

shutdown - Shut down or restart the system

The shutdown command is used to safely bring down or restart a Linux system, allowing for scheduled shutdowns and custom messages to users.

Examples:

• shutdown now: Shut down the system immediately
• shutdown -r +10: Reboot the system in 10 minutes
• shutdown -h 23:30: Schedule a shutdown at 11:30 PM
• shutdown -c: Cancel a scheduled shutdown
• shutdown +15 "System maintenance in 15 minutes": Schedule a shutdown with a custom message
• shutdown -P now: Power off the system immediately

halt - Stop all CPU functions

The halt command stops all CPU functions and brings the system to a halt state. It's similar to shutdown but may not power off the system on all hardware.

Examples:

• halt: Halt the system
• halt -p: Halt the system and power off
• halt -f: Force halt without calling shutdown
• halt -w: Don't actually halt, just write a wtmp record

reboot - Restart the system

The reboot command restarts the system, similar to shutdown -r but more direct.

Examples:

• reboot: Reboot the system
• reboot -f: Force an immediate reboot without calling shutdown
• reboot -w: Don't actually reboot, just write a wtmp record
• reboot --no-wall: Don't send wall message before rebooting

poweroff - Shut down and power off the system

The poweroff command shuts down the system and turns off the power. It's equivalent to shutdown -P.

Examples:

• poweroff: Shut down and power off the system
• poweroff -f: Force power off without calling shutdown
• poweroff -w: Don't actually power off, just write a wtmp record
• poweroff --no-wall: Don't send wall message before powering off

set: Displays all shell variables and functions.

Example: set | grep PATH

• unset: Removes an environment variable15.

Example: unset GNUPLOT_DRIVER_DIR

• export: Sets an environment variable for the current shell and all its child processes36.

Example: export VAR="value"

• env: Displays all environment variables6.

Example: env

• echo: Prints the value of a specific environment variable3.

Example: echo $VAR

• Quotes are used when setting variables containing spaces or special characters6:

Example: export GREETING="Hello, World!"

To set a temporary environment variable:

• VAR="value"
• To make it persistent, add it to ~/.bashrc or /etc/environment47.

To unset a variable, use:

• unset VAR
• export VAR=

history - Command to view and manipulate command history

The history command allows users to view, search, and manipulate the list of commands previously executed in the shell.

Examples:

• history: Display the entire command history
• history 10: Show the last 10 commands
• history | grep ssh: Search for all ssh-related commands in history
• !42: Execute the command at line 42 in the history
• !-3: Execute the third-to-last command
• !!: Execute the last command again

.bash_history - File storing command history

.bash_history is a hidden file in the user's home directory that stores the command history across sessions.

Examples:

• cat ~/.bash_history: View the contents of the history file
• echo "export HISTSIZE=10000" >> ~/.bashrc: Increase history size
• echo "export HISTCONTROL=ignoredups" >> ~/.bashrc: Avoid duplicate entries
• echo "export HISTIGNORE='ls:bg:fg:history'" >> ~/.bashrc: Ignore certain commands

Additional history-related features:

• Ctrl+R: Reverse search through command history
• !string: Execute the most recent command starting with "string"
• history -c: Clear the current session history
• history -d 50: Delete line 50 from history
• export HISTTIMEFORMAT="%F %T ": Add timestamps to history entries

bash

Bourne Again Shell, the default command-line interpreter for most Linux distributions

Examples:

• bash script.sh: Execute a bash script
• bash -c "echo Hello, World": Run a bash command directly
• bash --version: Display bash version information

. (dot)

Represents the current directory in file paths

Examples:

• ls .: List contents of the current directory
• cp file.txt .: Copy file.txt to the current directory
• ./script.sh: Execute a script in the current directory

Relative path

Specifies the location of a file or directory relative to the current working directory

Examples:

• cd documents: Change to the documents subdirectory
• ../images/photo.jpg: Reference a file in a sibling directory
• ./config.ini: Reference a file in the current directory

Absolute path

Specifies the complete path to a file or directory from the root of the file system

Examples:

• /home/user/documents/report.pdf: Full path to a file
• /etc/ssh/sshd_config: Path to a system configuration file
• /var/log/syslog: Path to a system log file