Shell scripts often require repetitive tasks. The for loop is a fundamental structure for iterating over sequences, lists, or file paths. This article covers three common types of for loops: numeric ranges, character strings, and patnhame expansion.

Numeric Iteration

Shell provides several ways to loop over numbers. The following examples print (i * 3 + 1) for i from 1 to 10.

C-style syntax:

#!/bin/bash
for ((i=1; i<=10; i++)); do
    echo $((i * 3 + 1))
done

Note the double parentheses and semicolons. For an infinite loop: for ((;;)); do echo "infinite"; done.

Using seq command:

#!/bin/bash
for i in $(seq 1 10); do
    echo $((i * 3 + 1))
done

seq options:

• -f "%03g" : format with leading zeros (three digits)
• -s " " : use space as separator instead of newline
• -w : equal width output (cannot combine with -f)
• Syntax: seq [first [increment]] last

Brace expansion:

#!/bin/bash
for i in {1..10}; do
    echo $((i * 3 + 1))
done

Brace expansion {1..10} must not be quoted (single, double, or backticks).

Using awk:

awk 'BEGIN{for(i=1; i<=10; i++) print i}'

String Iteration

Loop over strings or command output.

Iterate over files in current directory:

#!/bin/bash
for i in *; do
    echo "$i is a file!"
done

Iterate over script arguments:

#!/bin/bash
for i in "$@"; do
    echo "$i is an argument!"
done

Iterate over explicit list:

#!/bin/bash
for i in item1 item2 item3; do
    echo "$i is specified"
done

Iterate over a variable containing spaces:

#!/bin/bash
list="alpha beta gamma"
for i in $list; do
    echo "$i is a word"
done

Pathname Expansion (Globbing)

Iterate over files matching a pattern.

All entries under /proc:

#!/bin/bash
for i in /proc/*; do
    echo "$i is a path"
done

All shell scripts in current directory:

#!/bin/bash
for i in *.sh; do
    echo "$i is a shell script"
done

Note: It is generally safer to quote variables ("$i") when expanding them inside the loop to handle spaces or special characters correctly.