Bash — Operators

Arithmetic operators

#!/bin/bash

a=10
b=3

echo "$((a + b))"   # 13
echo "$((a - b))"   # 7
echo "$((a * b))"   # 30
echo "$((a / b))"   # 3
echo "$((a % b))"   # 1

String comparison

#!/bin/bash

[ "abc" = "abc" ] && echo "Equal"
[ "abc" != "xyz" ] && echo "Not equal"
[ "abc" \< "xyz" ] && echo "Less than"
[ "xyz" \> "abc" ] && echo "Greater than"
[ -z "" ] && echo "Empty"
[ -n "abc" ] && echo "Non-empty"

File test operators

#!/bin/bash

file="/etc/passwd"

[ -f "$file" ] && echo "Regular file"
[ -d "$file" ] && echo "Directory"
[ -r "$file" ] && echo "Readable"
[ -w "$file" ] && echo "Writable"
[ -x "$file" ] && echo "Executable"
[ -s "$file" ] && echo "Non-empty"

Logical operators

#!/bin/bash

a=10
b=20

# AND
[ $a -gt 5 ] && [ $b -gt 15 ] && echo "Both true"

# OR
[ $a -gt 15 ] || [ $b -gt 15 ] && echo "At least one true"

# NOT
[ ! $a -gt 15 ] && echo "a is not greater than 15"

Combined test

#!/bin/bash

a=10
b=20

# AND with &&
[ $a -gt 5 ] && [ $b -gt 15 ] && echo "Both true"

# OR with ||
[ $a -gt 15 ] || [ $b -gt 15 ] && echo "At least one"

# NOT with !
[ ! $a -gt 15 ] && echo "Not greater"

Mini Practice

Write Bash code that:

1. Compares two strings
2. Tests if a file exists and is readable
3. Combines conditions with && and ||
4. Uses the NOT operator

Up Next

In the next lesson, you'll learn about If Else — conditional branching in Bash.