How do I write a 'for' loop in Bash?

Mastering 'for' Loops in Bash: A Comprehensive Guide

Learn the various syntaxes and practical applications of 'for' loops in Bash scripting to automate repetitive tasks and process data efficiently.

The for loop is a fundamental construct in Bash scripting, allowing you to iterate over a list of items, a range of numbers, or the output of a command. Understanding its different forms is crucial for writing effective and efficient shell scripts. This article will guide you through the various ways to write for loops in Bash, complete with examples and best practices.

Basic 'for' Loop: Iterating Over a List

The most common form of the for loop in Bash iterates over a space-separated list of words. This list can be explicitly defined, come from a variable, or be the result of command substitution. Each item in the list is assigned to the loop variable in sequence, and the commands within the loop body are executed for each item.

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

Basic 'for' loop iterating over an explicit list of strings.

FILES=$(ls *.txt)
for file in $FILES;
do
  echo "Found text file: $file"
done

Iterating over the output of a command (e.g., ls).

C-Style 'for' Loop: Iterating with Numeric Ranges

Bash also supports a C-style for loop syntax, which is ideal for iterating through numeric sequences. This syntax is familiar to programmers coming from languages like C, Java, or JavaScript, and it provides explicit control over the initialization, condition, and increment of the loop variable.

for (( i=1; i<=5; i++ ));
do
  echo "Count: $i"
done

C-style 'for' loop for numeric iteration from 1 to 5.

for (( j=10; j>=0; j-=2 ));
do
  echo "Countdown by 2: $j"
done

C-style 'for' loop demonstrating a decrementing step.

flowchart TD
    A[Start Loop] --> B{"Initialize i=1"}
    B --> C{"Is i <= 5?"}
    C -- Yes --> D[Execute Loop Body]
    D --> E["Increment i (i++)"]
    E --> C
    C -- No --> F[End Loop]

Flowchart illustrating the logic of a C-style 'for' loop.

Sequence Expression 'for' Loop: Simple Numeric Ranges

For simple numeric ranges, Bash offers a more concise syntax using brace expansion {start..end} or {start..end..step}. This is often preferred for its readability when dealing with straightforward integer sequences.

for num in {1..5};
do
  echo "Number: $num"
done

Iterating through a simple numeric range from 1 to 5.

for char in {a..e};
do
  echo "Character: $char"
done

Iterating through a range of characters.

for i in {0..10..2};
do
  echo "Even number: $i"
done

Iterating with a step value (e.g., even numbers).

Looping Through Array Elements

Bash arrays are powerful for storing collections of items. You can easily iterate through all elements of an array using the @ or * special parameters within the array expansion.

declare -a FRUITS=("Apple" "Banana" "Cherry" "Date")

for fruit in "${FRUITS[@]}";
do
  echo "I like $fruit."
done

Iterating through all elements of a Bash array.