Conditionals and Loops

Control flow structures let your scripts make decisions and repeat actions. They're the building blocks of useful automation.

If Statements

Basic syntax:

if [ condition ]; then
    commands
fi

With else:

if [ condition ]; then
    commands
else
    other_commands
fi

With elif:

if [ condition1 ]; then
    commands1
elif [ condition2 ]; then
    commands2
else
    default_commands
fi

Test Conditions

String Tests

Test	Meaning
[ -z "$STR" ]	String is empty
[ -n "$STR" ]	String is not empty
[ "$A" = "$B" ]	Strings are equal
[ "$A" != "$B" ]	Strings are not equal

Numeric Tests

Test	Meaning
[ $A -eq $B ]	Equal
[ $A -ne $B ]	Not equal
[ $A -lt $B ]	Less than
[ $A -gt $B ]	Greater than
[ $A -le $B ]	Less than or equal
[ $A -ge $B ]	Greater than or equal

File Tests

Test	Meaning
[ -e FILE ]	File exists
[ -f FILE ]	Is a regular file
[ -d FILE ]	Is a directory
[ -r FILE ]	Is readable
[ -w FILE ]	Is writable
[ -x FILE ]	Is executable

If Example

#!/bin/bash

FILE="/etc/hosts"

if [ -f "$FILE" ]; then
    echo "File exists"
else
    echo "File not found"
fi

Exercise: Check File Existence

Write a conditional to check if a file exists:

For Loops

Basic For Loop

for item in item1 item2 item3; do
    echo "$item"
done

Loop Through Files

for file in *.txt; do
    echo "Processing: $file"
done

C-style For Loop

for ((i=0; i<5; i++)); do
    echo "Count: $i"
done

While Loops

COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

Reading File Line by Line

while read line; do
    echo "$line"
done < file.txt

Until Loops

Runs until condition becomes true:

COUNT=0
until [ $COUNT -ge 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

Case Statements

Multiple choice selection:

case $1 in
    start)
        echo "Starting..."
        ;;
    stop)
        echo "Stopping..."
        ;;
    *)
        echo "Usage: $0 {start|stop}"
        ;;
esac

Loop Control

Break

Exit the loop:

for i in 1 2 3 4 5; do
    if [ $i -eq 3 ]; then
        break
    fi
    echo $i
done
# Outputs: 1 2

Continue

Skip to next iteration:

for i in 1 2 3 4 5; do
    if [ $i -eq 3 ]; then
        continue
    fi
    echo $i
done
# Outputs: 1 2 4 5

Combining Conditions

# AND
if [ condition1 ] && [ condition2 ]; then
    echo "Both true"
fi

# OR
if [ condition1 ] || [ condition2 ]; then
    echo "At least one true"
fi

# NOT
if [ ! condition ]; then
    echo "Condition is false"
fi

[[ ]] vs [ ]

[[ ]] is more powerful (bash-specific):

# Pattern matching
if [[ $STRING == *"substring"* ]]; then
    echo "Contains substring"
fi

# Regex
if [[ $STRING =~ ^[0-9]+$ ]]; then
    echo "Is a number"
fi

Key Takeaways

• Use if [ condition ]; then ... fi for conditionals
• Test strings with =, numbers with -eq, -lt, etc.
• Test files with -f, -d, -e
• Use for and while for loops
• break exits loops, continue skips iterations
• case handles multiple options elegantly