Logical Operators

&& for Sequential Success

&& runs the command on its right only if the command on its left exited with status 0. This is a compact way to chain steps that should only proceed if the previous one worked, without writing a full if block.

Example: && for Sequential Success

#!/bin/bash
mkdir -p output_dir && echo "Directory ready, continuing"
cd output_dir && echo "Now inside output_dir: $(pwd)"

|| for Fallback on Failure

|| runs the command on its right only if the command on its left exited with a non-zero status. This is commonly used to provide a fallback action or error message when something fails.

Example: || for Fallback on Failure

#!/bin/bash
grep -q "xyz" <<< "abc def" || echo "Pattern 'xyz' was not found, using fallback"

! to Negate a Condition

! inverts the exit status of the command or test immediately following it, turning success into failure and failure into success. Inside [ ] or [[ ]], it is placed directly before the condition being negated.

Example: ! to Negate a Condition

#!/bin/bash
if [ ! -f nonexistent_file.txt ]; then
    echo "Confirmed: nonexistent_file.txt does not exist"
fi
if ! grep -q "zzz" <<< "hello world"; then
    echo "Confirmed: pattern not found"
fi

Combining && and || Together

&& and || can be chained on one line for a compact 'if success do X, else do Y' pattern, evaluated strictly left to right. Be careful with more complex combinations -- for anything beyond a simple two-way branch, a proper if/else is clearer and less error-prone.

Warning: If the middle command in an &&/|| chain can itself fail for reasons unrelated to the condition, the || fallback might run unexpectedly -- prefer if/else when the logic gets complex.

Example: Combining && and || Together

#!/bin/bash
value=10
[ "$value" -gt 5 ] && echo "Value is greater than 5" || echo "Value is 5 or less"