if / elif / else

Basic syntax.

if command or condition; then
    process
elif command or condition; then
    process
else
    process
fi

Using [ ] (equivalent to the test command).

if [ condition ]; then

Using [[ ]] (Bash extension — recommended).

if [[ condition ]]; then

Using logical operators.

if [ condition1 ] && [ condition2 ]; then
if [[ condition1 && condition2 ]]; then

Condition	Description
[ -e file ]	True if the file or directory exists.
[ -f file ]	True if a regular file exists.
[ -d directory ]	True if the directory exists.
[ -z "$var" ]	True if the variable is an empty string.
[ -n "$var" ]	True if the variable is not empty.
[ "$a" = "$b" ]	True if the strings are equal (== also works).
[ "$a" != "$b" ]	True if the strings are not equal.
[ $a -eq $b ]	True if the integers are equal.
[ $a -ne $b ]	True if the integers are not equal.
[ $a -lt $b ]	True if a is less than b.
[ $a -gt $b ]	True if a is greater than b.
[[ $str =~ regex ]]	True if the string matches the regular expression ([[ ]] only).

Use -f to check whether a file exists and print a message based on the result.

check_file.sh

#!/bin/bash
file="config.json"
if [ -f "$file" ]; then
    echo "$file was found"
else
    echo "$file was not found"
fi

bash check_file.sh
config.json was not found

You can also enter an if statement directly in the terminal. After pressing Enter following then, a > prompt appears — this signals that input is still expected. Typing fi executes the statement.

if [ -f "config.json" ]; then
    echo "Found"
else
    echo "Not found"
fi
Not found

Use multiple branches to compare numbers. -ge means "greater than or equal to".

grade.sh

score=85
if [ $score -ge 90 ]; then
    echo "A (Excellent)"
elif [ $score -ge 80 ]; then
    echo "B (Good)"
elif [ $score -ge 70 ]; then
    echo "C (Pass)"
else
    echo "D (Fail)"
fi

bash grade.sh
B (Good)

Use [[ ]] for string comparisons. It is also safe when you forget to quote variables.

check_env.sh

env="production"
if [[ "$env" = "production" ]]; then
    echo "Running in production"
fi

bash check_env.sh
Running in production

Inside [[ ]], you can use && and || for logical operations.

check_age.sh

age=25
if [[ $age -ge 18 && $age -lt 65 ]]; then
    echo "Working-age adult"
fi

bash check_age.sh
Working-age adult

Use the =~ operator inside [[ ]] to match against a regular expression.

check_email.sh

email="user@example.com"
if [[ "$email" =~ ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ]]; then
    echo "Valid email address"
fi

bash check_email.sh
Valid email address

You can use a command's exit status directly as the condition. grep -q returns status 0 (true) on a match.

check_error.sh

# Check if app.log contains any errors
if grep -q "ERROR" app.log; then
    echo "Errors were found"
fi

The difference between [ ] and [[ ]] matters. [ ] is equivalent to the POSIX test command — forgetting to quote variables can cause word splitting and unexpected errors. [[ ]] is a Bash extension that supports regex matching (=~), pattern matching, and the logical operators && ||, and it handles unquoted variables safely. For modern Bash scripts, [[ ]] is recommended.

For a full list of condition operators, see test / [ ] / [[ ]]. For multi-value branching, case is a convenient alternative.