First Line of Script

• #!/bin/bash → defines the shell interpreter

Comments

• Use # for commenting

Common Elements

• Commands: echo, cp, etc.
• Statements: if, while, for
• +x → make the script executable
• Use absolute path to run: /home/userdir/myscript.bash

Variable Definition

a=hello
b=mytext

• Usage:

echo "text1 $a"
echo "text2 $b"

• Example:

a=$(hostname)
echo $a  # => myfirstlinux

Input / Output Variable

read myinputvariable
echo "name $myinputvariable"

If Else Statement

if [[ $count -eq 100 ]]; then
  echo "count is 100"
else
  echo "no"
fi

File Existence Check

if [[ -e /file.txt ]]; then
  echo "file exists"
fi

Check If a Variable Matches a Value

a=$(date | awk '{print $1}')
if [[ "$a" == "Mon" ]]; then
  echo "Today is $a"
else
  echo "Today is not Monday"
fi

Check Multiple Values

if [[ "$a" == "Monday" ]] || [[ "$a" == "Tuesday" ]]; then
  echo "It's early week"
fi

File Operation Tests

• -s → file exists and is not empty
• -f → file exists and is a regular file
• -d → directory exists
• -x → file is executable
• -w → file is writable
• -r → file is readable

Comparison Operators

• -eq → equal (numeric)
• = → equal (string)
• -ne → not equal (numeric)
• != → not equal (string)
• -lt → less than
• -le → less than or equal
• -gt → greater than
• -ge → greater than or equal
• $((...)) → arithmetic evaluation

For Loop Examples

for i in 1 2 3 4 5; do
  echo "Welcome $i times"
done

for i in eat run jump play; do
  echo "User $i"
done

for i in {1..5}; do
  touch $i
  echo "File $i created"
done

Indexed For Loop Example

i=1
for day in Mon Tue Wed Thu Fri; do
  echo "Weekday $((i++)) : $day"
done

User Listing with For + AWK

i=1
for username in $(awk -F: '{print $1}' /etc/passwd); do
  echo "Username $((i++)) : $username"
done

While Loop

c=1
while [[ $c -le 5 ]]; do
  echo "Welcome $c times"
  ((c++))
done

Case Statement

echo "Choose function"
echo "A"
echo "B"
read choices  # could also use: read -s password for silent input

case $choices in
  A) date;;
  B) ls;;
  *) echo "Invalid choice";;
esac

Ping Check with Conditional

hosts="192.168.0.1"
ping -c1 $hosts &> /dev/null

if [[ $? -eq 0 ]]; then
  echo "$hosts OK"
else
  echo "$hosts NOT OK"
fi

• alias l="ls -al" → define alias
• unalias l → remove alias

Persistent Alias Config

• User-specific: ~/.bashrc

• Global: /etc/bashrc

  • Add: alias l="ls -al" at end of file
  • Save file

Command History

• Location: ~/.bash_history
• View: history
• Recall specific: !4 → run whoami (4th command in history)

Screen (Terminal Multiplexer)

• Multi-terminal sessions in one window
• Alt+A | → split vertical
• Alt+A Shift+S → split horizontal
• Alt+A Tab → switch windows
• Alt+A C → create new session
• screen -r → reconnect to detached session
• screen -r [id] → recover session by ID

TMUX (Terminal Multiplexer)

• Ctrl+B Shift+% → split window horizontally
• Ctrl+B D → detach session
• Ctrl+B Arrow Keys → move between panes
• Ctrl+B Shift+" → split window vertically
• tmux ls → list sessions
• tmux a → resume last session
• tmux new -s name → create named session
• Ctrl+B C → new window
• Ctrl+B N → switch to next window
• tmux attach-session -t 0 → attach session ID 0
• Ctrl+B , → rename window
• tmux kill-session -t 0 → kill session ID 0

TMUX Advanced Commands

• tmux new -s name → new named session
• Ctrl+B C → new window
• Ctrl+B N → switch to next window
• tmux attach-session -t 0 → attach session 0
• Ctrl+B , → rename window
• tmux kill-session -t 0 → kill session 0

First Line of Script

• #!/bin/bash → defines the shell interpreter

Comments

• Use # for commenting

Common Elements

• Commands: echo, cp, etc.
• Statements: if, while, for
• +x → make the script executable
• Use absolute path to run: /home/userdir/myscript.bash

Variable Definition

a=hello
b=mytext

• Usage:

echo "text1 $a"
echo "text2 $b"

• Example:

a=$(hostname)
echo $a  # => myfirstlinux

Input / Output Variable

read myinputvariable
echo "name $myinputvariable"

If Else Statement

if [[ $count -eq 100 ]]; then
  echo "count is 100"
else
  echo "no"
fi

File Existence Check

if [[ -e /file.txt ]]; then
  echo "file exists"
fi

• chmod u+s xyz.sh → add SetUID (user-level special permission)
• chmod g+s xyz.sh → add SetGID (group-level special permission)
• chmod u-s xyz.sh → remove SetUID
• chmod g-s xyz.sh → remove SetGID
• find / -perm /6000 -type f → find all executables with SetUID/SetGID

⚠️ SetUID/SetGID works only for C/C++ compiled binaries, not Bash scripts.

Sticky Bit

• Protects files from deletion by non-owners
• Example: ls / → shows sticky bit as t in rwxrwxrwt

• chmod +t file/dir → assign sticky bit

  • Only the creator can delete contents of that folder, even with full permissions