Mastering Shell Loops: From Basic for to Advanced Nested and Infinite Loops

Basic for Loop

The simplest loop in a shell script is the for loop. List the values after in and the loop iterates over them:

#!/bin/bash
for num in 1 2 3 4
do
    echo $num
done

To iterate over a sequence of letters or numbers, use brace expansion:

#!/bin/bash
for x in {a..z}
do
    echo $x
done

while Loop

The while loop evaluates its condition before each iteration. It is useful when the number of iterations is not known in advance or when the loop may run many times.

#!/bin/bash
n=1
while [ $n -le 4 ]
do
    echo $n
    ((n++))
done

Nested Loops

Loops can be nested just like in other programming languages. The example below nests a for loop inside a while loop to produce combinations of numbers and letters:

#!/bin/bash
n=1
while [ $n -lt 6 ]
do
    for l in {a..d}
    do
        echo $nl
    done
    ((n++))
done

The output will be 1a 1b 1c 1d 2a … 5d.

Dynamic Loop Content

When the values to iterate over are not known beforehand, you can generate them with commands. For example, list all users on the system:

#!/bin/bash
for user in `ls /home`
do
    echo $user
done

Or retrieve the current date and iterate over its words:

$ for word in `date`
> do
>     echo $word
> done
Thu
Apr
9
08:12:09
CST
2020

Variable Value Checks

When using a while loop that depends on a numeric variable, you often need to verify that the input is a number. Three common methods are:

Arithmetic comparison with -eq (numeric only).

Shell pattern matching using *[[:digit:]]*.

Regular‑expression match with [[ $var =~ ^[0-9]+$ ]].

Example combining input validation with a loop:

#!/bin/bash
echo -n "How many times should I say hello? "
read ans
if [ "$ans" -eq "$ans" ]; then
    n=1
    while [ $n -le $ans ]
    do
        echo hello
        ((n++))
    done
fi

Reading a Text File Line by Line

To process a file line by line, read the filename from the user and use a while read loop with input redirection:

#!/bin/bash
echo -n "File> "
read file
n=0
while read line; do
    ((n++))
    echo "$n: $line"
 done < $file

Infinite (Dead) Loop

For tasks that should run continuously, use an infinite while true loop:

#!/bin/bash
while true
do
    echo -n "Still running at "
    date
    sleep 1
done

This script prints the current time every second until interrupted with Ctrl+C.