Shell by Example: If Statements

Shell by Example: If Statements POSIX

	`#!/bin/sh`
The `if` statement lets you execute code conditionally. It’s fundamental to controlling program flow in shell scripts.
Basic if statement syntax:	`if true; then echo "This always runs" fi`
The `if` tests the exit status of a command. Exit status 0 means success (true), non-zero means failure (false).	`if ls /tmp > /dev/null 2>&1; then echo "/tmp exists and is readable" fi`
Use the `test` command or `[ ]` for comparisons. Note: spaces inside `[ ]` are required!	`value=10 if [ "$value" -eq 10 ]; then echo "Value is 10" fi`
String comparisons use `=` and `!=`.	`name="Alice" if [ "$name" = "Alice" ]; then echo "Hello, Alice!" fi`
Check if a string is empty or non-empty.	`empty="" nonempty="hello" if [ -z "$empty" ]; then echo "Variable is empty" fi if [ -n "$nonempty" ]; then echo "Variable is not empty" fi`
Numeric comparisons: -eq (equal), -ne (not equal) -lt (less than), -le (less or equal) -gt (greater than), -ge (greater or equal)	`count=5 if [ "$count" -gt 3 ]; then echo "Count is greater than 3" fi`
File tests check file properties.	`if [ -f "/etc/passwd" ]; then echo "/etc/passwd exists and is a file" fi if [ -d "/tmp" ]; then echo "/tmp exists and is a directory" fi`
Always quote variables in tests to handle empty values and spaces correctly.	`user_input="" if [ "$user_input" = "" ]; then echo "No input provided" fi`
Bash Bash provides extended test syntax [[ ]] which is safer and more powerful than single brackets.
With [[ ]], quoting is often optional:	`if [[ $name = Alice ]]; then echo "Hello, Alice!" fi`
Pattern matching with [[ ]]:	`if [[ $filename = *.txt ]]; then echo "It's a text file" fi`
Regex matching with =~:	`if [[ $email =~ ^[a-z]+@[a-z]+\.[a-z]+$ ]]; then echo "Valid email format" fi`
Bash also provides (( )) for arithmetic evaluation, which is cleaner for numeric comparisons:	`if (( count > 3 )); then echo "Count is greater than 3" fi if (( x >= 0 && x <= 100 )); then echo "x is between 0 and 100" fi`

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