Shell by Example: If Statements

Shell by Example: If Statements POSIX + Bash

The `if` statement lets you execute code conditionally. It’s fundamental to controlling program flow in shell scripts. Basic if statement syntax:	Edit `#!/bin/sh if true; then echo "This always runs" fi`
	Output: This always runs

The `if` tests the exit status of a command. Exit status 0 means success (true), non-zero means failure (false).	Edit `#!/bin/sh if ls /tmp >/dev/null 2>&1; then echo "/tmp exists and is readable" fi`
	Output: /tmp exists and is readable

Use the `test` command or `[ ]` for comparisons. Note: spaces inside `[ ]` are required! Numeric comparisons: `-eq` (equal), `-ne` (not equal) `-lt` (less than), `-le` (less or equal) `-gt` (greater than), `-ge` (greater or equal)	Edit `#!/bin/sh value=10 count=5 if [ "$value" -eq 10 ]; then echo "Value is 10" fi if [ "$count" -gt 3 ]; then echo "Count is greater than 3" fi`
	Output: Value is 10 Count is greater than 3

String comparisons use `=` and `!=`.	Edit `#!/bin/sh name="Alice" if [ "$name" = "Alice" ]; then echo "Hello, Alice!" fi if [ "$name" != "Bob" ]; then echo "Hello, not Bob!" fi`
	Output: Hello, Alice! Hello, not Bob!

All `test` expressions are supported, including checking if a string is empty or non-empty.	Edit `#!/bin/sh empty="" nonempty="hello" if [ -z "$empty" ]; then echo "Variable is empty" fi if [ -n "$nonempty" ]; then echo "Variable is not empty" fi`
	Output: Variable is empty Variable is not empty

File `test` expressions are also supported.	Edit `#!/bin/sh if [ -f "/etc/passwd" ]; then echo "/etc/passwd exists and is a file" fi if [ -d "/tmp" ]; then echo "/tmp exists and is a directory" fi`
	Output: /etc/passwd exists and is a file /tmp exists and is a directory

Always quote variables in tests to handle empty values and spaces correctly.	Edit `#!/bin/sh user_input="" # this will be a syntax error if the variable is not quoted if [ $user_input = "some_value" ]; then echo "No input provided" fi`
	Output: sh: some_value: unknown operand

Bash Bash provides extended test syntax `[[ ]]` which is safer and more powerful than single brackets `[ ]`. With `[[ ]]`, quoting is often optional:	Edit `#!/bin/bash name="Alice" if [[ $name = Alice ]]; then echo "Hello, Alice!" fi`
	Output: Hello, Alice!

Bash Bash supports pattern matching with `[[ ]]` expressions. These are similar to regular expressions, but used for more simple string matching.	Edit `#!/bin/bash filename="example.txt" if [[ $filename = *.txt ]]; then echo "It's a text file" fi`
	Output: It's a text file

Bash Regex matching with `=~` can be used for more complex string matching.	Edit `#!/bin/bash email="test@example.com" # this regex is not comprehesive # and merely used for demonstration purposes if [[ $email =~ ^[a-z]+@[a-z]+\.[a-z]+$ ]]; then echo "Valid email format" fi`
	Output: Valid email format

Bash Bash also provides `(( ))` for arithmetic evaluation, which is cleaner for numeric comparisons:	Edit `#!/bin/bash count=5 if ((count > 3)); then echo "Count is greater than 3" else echo "Count is not greater than 3" fi x=40 if ((x >= 0 && x <= 100)); then echo "x is between 0 and 100" else echo "x is not between 0 and 100" fi`
	Output: Count is greater than 3 x is between 0 and 100

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