Shell Script Conditionals: Complete Guide

Table of Contents

Introduction to Conditionals

Conditionals in shell scripts are fundamental constructs that allow scripts to make decisions based on various conditions. They enable programs to execute different code paths depending on the evaluation of expressions, file states, variable values, or command exit statuses. Shell conditionals form the backbone of logic flow control in bash scripting.

The shell provides several mechanisms for implementing conditional logic, including the if statement, case statement, and logical operators. These constructs work by evaluating test conditions that return either true (exit status 0) or false (non-zero exit status).

Basic Conditional Syntax

The if Statement Structure

The basic if statement in shell scripting follows this syntax:

`bash if [ condition ]; then # commands to execute if condition is true fi `

The extended form includes else and elif clauses:

`bash if [ condition1 ]; then # commands for condition1 elif [ condition2 ]; then # commands for condition2 else # commands if no conditions are true fi `

Command Notes

- The [ is actually a command (alias for test) - Spaces around brackets are mandatory - The then keyword can be on the same line with a semicolon or on a new line - Each if must have a corresponding fi - The elif clause allows multiple condition testing

Test Commands and Operators

File Test Operators

File test operators check various properties of files and directories:

| Operator | Description | Example | |----------|-------------|---------| | -e | File exists | [ -e /path/file ] | | -f | Regular file exists | [ -f /path/file ] | | -d | Directory exists | [ -d /path/dir ] | | -r | File is readable | [ -r /path/file ] | | -w | File is writable | [ -w /path/file ] | | -x | File is executable | [ -x /path/file ] | | -s | File exists and is not empty | [ -s /path/file ] | | -L | File is a symbolic link | [ -L /path/link ] | | -O | File is owned by current user | [ -O /path/file ] | | -G | File group matches current user | [ -G /path/file ] |

String Comparison Operators

String operators compare text values and check string properties:

| Operator | Description | Example | |----------|-------------|---------| | = or == | Strings are equal | [ "$str1" = "$str2" ] | | != | Strings are not equal | [ "$str1" != "$str2" ] | | < | String1 is less than string2 | [ "$str1" \< "$str2" ] | | > | String1 is greater than string2 | [ "$str1" \> "$str2" ] | | -z | String is empty | [ -z "$string" ] | | -n | String is not empty | [ -n "$string" ] |

Numeric Comparison Operators

Numeric operators perform mathematical comparisons:

| Operator | Description | Example | |----------|-------------|---------| | -eq | Equal to | [ $num1 -eq $num2 ] | | -ne | Not equal to | [ $num1 -ne $num2 ] | | -lt | Less than | [ $num1 -lt $num2 ] | | -le | Less than or equal | [ $num1 -le $num2 ] | | -gt | Greater than | [ $num1 -gt $num2 ] | | -ge | Greater than or equal | [ $num1 -ge $num2 ] |

Logical Operators

Logical operators combine multiple conditions:

| Operator | Description | Example | |----------|-------------|---------| | && | Logical AND | [ condition1 ] && [ condition2 ] | | \|\| | Logical OR | [ condition1 ] \|\| [ condition2 ] | | ! | Logical NOT | [ ! condition ] | | -a | AND (within test) | [ condition1 -a condition2 ] | | -o | OR (within test) | [ condition1 -o condition2 ] |

Conditional Statements

Basic if-then-else Examples

Here are practical examples demonstrating various conditional constructs:

`bash #!/bin/bash

Simple file existence check

Multiple conditions with elif

if [ $age -lt 13 ]; then echo "You are a child" elif [ $age -lt 20 ]; then echo "You are a teenager" elif [ $age -lt 60 ]; then echo "You are an adult" else echo "You are a senior" fi

String comparison

if [ "$username" = "admin" ]; then echo "Welcome administrator" elif [ -z "$username" ]; then echo "No username provided" else echo "Hello $username" fi `

Complex Conditional Logic

`bash #!/bin/bash

File permission and existence check

if [ -e "$filename" ]; then if [ -r "$filename" ] && [ -w "$filename" ]; then echo "File exists and is readable and writable" elif [ -r "$filename" ]; then echo "File exists and is readable only" else echo "File exists but no read permissions" fi else echo "File does not exist" fi

Combining multiple test conditions

if [ -n "$user_input" ] && [ $number -gt 40 ]; then echo "Both conditions are true" fi

Using logical NOT

Case Statement

The case statement provides an alternative to multiple elif conditions:

`bash #!/bin/bash

read -p "Enter a letter: " letter

case $letter in [aeiou]|[AEIOU]) echo "You entered a vowel" ;; [0-9]) echo "You entered a number" ;; [a-zA-Z]) echo "You entered a consonant" ;; *) echo "You entered a special character" ;; esac `

Case Statement Patterns

| Pattern Type | Example | Description | |--------------|---------|-------------| | Literal | "hello") | Exact string match | | Wildcard | *.txt) | Pattern matching | | Character class | [0-9]) | Range of characters | | Multiple patterns | yes\|y\|Y) | OR conditions | | Default | *) | Catch-all pattern |

Advanced Conditional Constructs

Double Bracket Syntax

The [[ construct provides enhanced conditional testing:

`bash #!/bin/bash

Pattern matching with double brackets

if [[ $string == "world" ]]; then echo "String contains 'world'" fi

Regular expression matching

Empty variable check without quotes

Arithmetic Evaluation

Using (( for arithmetic conditions:

`bash #!/bin/bash

num1=10 num2=20

Arithmetic comparison

Complex arithmetic

Increment and test

Conditional Assignment

`bash #!/bin/bash

Default value assignment

Conditional variable setting

if [ "$debug_mode" = "true" ]; then echo "Debug mode enabled" fi

Parameter expansion with conditions

Short-Circuit Evaluation

`bash #!/bin/bash

Command execution based on conditions

Create directory if it doesn't exist

Backup file if it exists

Chain multiple commands

Best Practices

Quoting Variables

Always quote variables to prevent word splitting and pathname expansion:

`bash

Correct

Incorrect - may fail with spaces in variable

Error Handling

`bash #!/bin/bash

Check command line arguments