For Loops

For loops allow you to iterate over a list of items, executing commands for each one. They're essential for processing files, arrays, and sequences.

Basic For Loop Syntax

for variable in list; do
    commands
done

The variable takes each value in the list, one at a time.

Looping Over a List

#!/bin/bash
for fruit in apple banana cherry; do
    echo "I like $fruit"
done

Output:

I like apple
I like banana
I like cherry

Looping Over Files

One of the most common uses:

#!/bin/bash
# Process all .txt files
for file in *.txt; do
    echo "Processing: $file"
    wc -l "$file"
done

Handle the case when no files match:

#!/bin/bash
shopt -s nullglob    # Pattern expands to nothing if no match

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

Exercise: Loop Over Items

Use a for loop:

Using Brace Expansion

Generate sequences easily:

# Numbers
for i in {1..5}; do
    echo "Count: $i"
done

# With step
for i in {0..10..2}; do    # 0, 2, 4, 6, 8, 10
    echo "$i"
done

# Letters
for letter in {a..e}; do
    echo "$letter"
done

# Padding with zeros
for i in {01..10}; do
    echo "file$i.txt"
done

Using Command Output

Loop over command output:

#!/bin/bash
# Loop over files from find
for file in $(find . -name "*.sh"); do
    echo "Script: $file"
done

# Loop over lines (be careful with whitespace!)
for user in $(cat /etc/passwd | cut -d: -f1); do
    echo "User: $user"
done

Warning: Word splitting happens! Use while read for lines with spaces.

C-Style For Loop

Familiar syntax for programmers:

for ((initialization; condition; increment)); do
    commands
done

Examples:

#!/bin/bash
# Count from 0 to 4
for ((i=0; i<5; i++)); do
    echo "i = $i"
done

# Countdown
for ((i=10; i>=0; i--)); do
    echo "$i..."
done
echo "Liftoff!"

# Multiple variables
for ((i=0, j=10; i<5; i++, j--)); do
    echo "i=$i, j=$j"
done

Exercise: C-Style Loop

Use the C-style for loop:

Looping Over Arrays

Iterate through array elements:

#!/bin/bash
FRUITS=("apple" "banana" "cherry" "date")

# Loop over all elements
for fruit in "${FRUITS[@]}"; do
    echo "Fruit: $fruit"
done

# Loop with indices
for i in "${!FRUITS[@]}"; do
    echo "$i: ${FRUITS[$i]}"
done

Looping Over Command Line Arguments

Process all script arguments:

#!/bin/bash
# process_files.sh

for arg in "$@"; do
    echo "Processing: $arg"
done

Run with: ./process_files.sh file1.txt file2.txt file3.txt

Nested For Loops

Loops within loops:

#!/bin/bash
for i in {1..3}; do
    for j in {1..3}; do
        echo "i=$i, j=$j"
    done
done

# Create multiplication table
for i in {1..5}; do
    for j in {1..5}; do
        printf "%4d" $((i * j))
    done
    echo    # Newline
done

Loop Variable Scope

The loop variable persists after the loop:

#!/bin/bash
for i in 1 2 3; do
    echo "$i"
done

echo "After loop: $i"    # Prints: 3

Infinite For Loop

Create a loop that runs forever:

#!/bin/bash
for (( ; ; )); do
    echo "Running..."
    sleep 1
done

# Or using while
for item in $(cat); do
    # Reads from stdin until EOF
    echo "$item"
done

Practical Example: Batch Rename

#!/bin/bash
# Rename all .txt to .bak

for file in *.txt; do
    if [ -f "$file" ]; then
        newname="${file%.txt}.bak"
        echo "Renaming $file to $newname"
        mv "$file" "$newname"
    fi
done

For Loop One-Liners

Quick loops on the command line:

# Create multiple directories
for d in dir1 dir2 dir3; do mkdir -p "$d"; done

# Check multiple hosts
for host in server1 server2 server3; do ping -c 1 "$host"; done

# Process numbered files
for i in {1..10}; do touch "file$i.txt"; done

Exercise: Process Files

Loop over files in a directory:

Best Practices

1. Always quote variables: for file in "$@" not for file in $@
2. Use "${array[@]}" for arrays: Preserves elements with spaces
3. Be careful with command substitution: Word splitting can cause issues
4. Use meaningful variable names: for file in *.txt not for x in *.txt
5. Handle empty lists: Use shopt -s nullglob or check if list is empty

# Good practice
for file in *.txt; do
    [ -f "$file" ] || continue    # Skip if no match
    process "$file"
done

# Better with nullglob
shopt -s nullglob
for file in *.txt; do
    process "$file"
done
shopt -u nullglob

Key Takeaways

• for var in list; do ... done iterates over items
• Use brace expansion {1..10} for number sequences
• Use *.txt to loop over files matching a pattern
• C-style for ((i=0; i<10; i++)) for traditional loops
• Loop over arrays with "${array[@]}"
• Loop over arguments with "$@"
• Always quote variables to handle spaces correctly
• Use shopt -s nullglob when globbing might match nothing