08-02 — Shell Scripting: Conditionals

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Conditionals — if / elif / else

An if statement runs code only when a condition is true.

#!/bin/bash

if [ $1 -gt 10 ]; then
    echo "Greater than 10"
elif [ $1 -eq 10 ]; then
    echo "Equal to 10"
else
    echo "Less than 10"
fi

Structure: - if [ condition ]; then — start - elif [ condition ]; then — else-if (optional, can have many) - else — runs if nothing above matched (optional) - fi — ends the if block (fi is if backwards)

The [ ] is the test command. Always put spaces inside the brackets: [ $A -gt 10 ] is correct. [$A -gt 10] will fail.

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Comparison Operators

For numbers

Operator	Meaning
-eq	equal to
-ne	not equal to
-lt	less than
-le	less than or equal to
-gt	greater than
-ge	greater than or equal to

if [ $AGE -ge 18 ]; then
    echo "Adult"
fi

For strings

Operator	Meaning
=	strings are equal
!=	strings are not equal
-z "$var"	string is empty
-n "$var"	string is not empty

if [ "$NAME" = "root" ]; then
    echo "You are root"
fi

if [ -z "$INPUT" ]; then
    echo "Nothing was entered"
fi

Always quote string variables: "$NAME" not $NAME. If the variable is empty or contains spaces, an unquoted variable will break the condition.

For files

Operator	Meaning
-f file	file exists and is a regular file
-d file	file exists and is a directory
-e file	file or directory exists
-r file	file exists and is readable
-x file	file exists and is executable
-s file	file exists and is not empty

if [ -f /etc/passwd ]; then
    echo "The file exists"
fi

if [ -d /home ]; then
    echo "The directory exists"
fi

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Combining Conditions

Use && (AND) and || (OR) to combine conditions:

if [ $AGE -ge 18 ] && [ $AGE -le 65 ]; then
    echo "Working age"
fi

if [ ! -f "$FILE" ]; then
    echo "File does not exist"
fi

! negates a condition — [ ! -f "$FILE" ] means "file does NOT exist".

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Practical Example — Script That Validates Input

#!/bin/bash

if [ $# -ne 1 ]; then
    echo "Usage: $0 <filename>"
    exit 1
fi

FILE=$1

if [ ! -e "$FILE" ]; then
    echo "Error: '$FILE' does not exist."
    exit 1
fi

if [ -f "$FILE" ]; then
    echo "'$FILE' is a regular file."
    echo "Size: $(du -sh "$FILE" | cut -f1)"
elif [ -d "$FILE" ]; then
    echo "'$FILE' is a directory."
    echo "Contains: $(ls "$FILE" | wc -l) items"
fi

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Navigation: ← 08-01 — Shell Scripting: Basics | Notes Index | Next → 08-03 — Shell Scripting: Loops Exercise: Exercise 08-02