Bash For Loops - Iterating Over Lists, Ranges, and Files

For Loops in Bash

For loops are one of the most versatile control flow constructs in bash. They allow you to iterate over lists, ranges, file patterns, and command output.

Basic List Iteration

The simplest form iterates over a space-separated list:

for fruit in apple banana cherry; do
  echo "I like $fruit"
done

Range Expressions

Bash supports brace expansion for numeric ranges:

# Simple range
for i in {1..10}; do
  echo "Number: $i"
done

# Range with step
for i in {0..100..5}; do
  echo "Value: $i"
done

C-Style For Loop

For more control over iteration, use C-style syntax:

for ((i=0; i<10; i++)); do
  echo "Iteration $i"
done

# Nested loops
for ((i=1; i<=3; i++)); do
  for ((j=1; j<=3; j++)); do
    echo "($i, $j)"
  done
done

Iterating Over Files

File globbing lets you process matching files:

# Process all log files
for file in *.log; do
  echo "Compressing $file"
  gzip "$file"
done

# Process files in subdirectories
for file in src/**/*.ts; do
  echo "Checking $file"
done

Command Output Iteration

Iterate over the output of a command:

# Process each line of a command's output
for user in $(cut -d: -f1 /etc/passwd); do
  echo "User: $user"
done

# Process files found by find
for file in $(find . -name "*.tmp" -type f); do
  rm "$file"
done

Practical Examples

# Batch rename files
for file in *.jpeg; do
  mv "$file" "${file%.jpeg}.jpg"
done

# Check multiple servers
for server in web{1..5}.example.com; do
  ping -c 1 -W 2 "$server" > /dev/null 2>&1 \
    && echo "$server: UP" \
    || echo "$server: DOWN"
done

# Process CSV rows
while IFS=',' read -r name email role; do
  echo "Creating account for $name ($email) as $role"
done < users.csv

Note: When iterating over filenames that may contain spaces, always quote the variable: "$file" instead of $file.