chmod command

chmod stands for change mode — used to set permissions (read, write, execute) on files and directories.

chmod 755 file.sh

• Sets permissions using numeric mode

Permission Basics

• r → read (4)
• w → write (2)
• x → execute (1)

Combined

• 7 = rwx
• 5 = r-x
• 4 = r--

Examples

1. Make file executable:

chmod 755 script.sh

2. Read-only file:

chmod 444 file.txt

3. Private file:

chmod 600 secret.txt

Symbolic Mode

1. Add execute:

chmod +x script.sh

2. Remove write:

chmod -w file.txt

3. Set specific:

chmod u+x file.sh

Why

• Controls who can access or modify files
• Critical for security and scripts

Pro tip :)

• chmod +x is the fastest way to make scripts runnable.
• Permissions define power — use chmod wisely.

whoami command

whoami shows the current logged-in user.

whoami

• Outputs your username

Why

• Verify current user
• Useful with sudo and permissions
• Helps in scripts & debugging

Pro tip :)

• After using sudo, whoami tells you if you're running as root.
• Always know who you are in the system.

chown command

chown stands for change owner — used to change the owner and group of files or directories.

chown user file.txt

• Changes owner of file.txt

Examples

1. Change owner + group:

chown user:group file.txt

2. Change recursively

chown -R user:group folder/

3. Using sudo:

sudo chown user file.txt

Why

• Controls who owns and manages files
• Important for servers, permissions, deployments

Pro tip :)

• Use chown -R carefully — it affects all files inside a directory.
• Ownership defines control — chown lets you set it.

exec command

exec is used to replace the current shell process with another command.

• No new process is created — it takes over the current one

exec bash

• Replaces current shell with a new bash session

Examples

1. Run script in same process:

exec ./script.sh

2. Redirect output permanently:

exec > output.txt

Why

• Avoids creating extra processes
• Useful in scripts, containers, process management

Pro tip :)

• After exec, there’s no going back — the original process is gone.
• exec doesn’t run a command — it becomes it.

grep command

grep is used to search for patterns (text) inside files or command output.

grep "error" file.txt

• Finds lines containing "error"

Uses

1. Case-insensitive search:

grep -i "error" file.txt

2. Search recursively:

grep -r "error" .

3. Show line numbers:

grep -n "error" file.txt

Why

• Core tool for logs, debugging, filtering data
• Works perfectly with pipes

Pro tip :)

• Combine with pipes for power:

ls | grep ".js"

history command

history shows a list of previously executed commands in your terminal.

history

• Displays command history with line numbers

Uses

1. Run a previous command:

!123

2. Search history:

history | grep "git"

3. Clear history:

history -c

Why

• Saves time (reuse commands)
• Helps debugging and tracking actions

Pro tip :)

• Press Ctrl + R for instant reverse search — fastest way to find past commands.
• Your past commands are assets — use history to leverage them.

regex

Regex (Regular Expression) is used to search, match, and manipulate text patterns.

Examples

1. Match exact word:

grep "error" file.txt

2. Match any digit:

grep "[0-9]" file.txt

3. Match words starting with “a”:

grep "^a" file.txt

Most used/common

• . → any character
• ('*') → 0 or more
• ('+') → 1 or more
• ^ → start of line
• $ → end of line
• [] → character set

Why

• Powerful text filtering
• Used in:
  • grep
  • validation
  • parsing logs/data

Pro tip :)

• Regex looks complex, but mastering a few patterns covers most real-world use cases.
• Regex lets you search smarter, not harder.