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