Shell Programming - For Loop

In this tutorial we will learn about For loop in Shell Programming.

Loop is used to execute a block of code multiple times.

We use for loop to iterate through a list of items.

for syntax

for variable in items
do
  # code for each item
done

Where, variable holds an item from the list of multiple items.

Example #1: Write a Shell Script to print from 1 to 5 using for loop

In the following example we will use for loop to print from 1 to 5.

#!/bin/sh

for i in 1 2 3 4 5
do
  echo $i
done

Output:

$ sh example01.sh 
1
2
3
4
5

Brace expansion

We use the brace expansion {m..n} to generate string in shell script.

Example:

{1..5} will give 1 2 3 4 5

{a..f} will give a b c d e f

{Z..T} will give Z Y X W V U T

{-5..5} will give -5 -4 -3 -2 -1 0 1 2 3 4 5

{A,B,C,D} will give A B C D

{A,B,C{1..3},D} will give A B C1 C2 C3 D

Example #2: Write a Shell Script to print from 1 to 10 using brace expansion and for loop

#!/bin/sh

for i in {1..10}
do
  echo $i
done

Where, {1..10} will expand to 1 2 3 4 5 6 7 8 9 10.

Example #3: Write a Shell Script to print from A to Z using for loop

#!/bin/sh

for ch in {A..Z}
do
  echo $ch
done

Example #4: Write a Shell Script to list all the files in the current directory

For this example we will use the * which is a special character and it helps to list all the files in the current directory.

#!/bin/sh

for f in *
do
  echo $f
done

Output:

$ sh example04.sh 
example01.sh
example02.sh
example03.sh
example04.sh

seq command

We use the seq command to generate numeric sequence.

Example:

seq LAST
so, seq 5 will give
1
2
3
4
5

seq FIRST LAST
so, seq 7 10 will give
7
8
9
10

seq FIRST INCREMENT LAST
so, seq 1 2 10 will give
1
3
5
7
9

Example #5: Write a Shell Script to print all odd numbers from 1 to 10

For this we can use the seq command and set FIRST to 1, INCREMENT to 2 and LAST to 10.

#!/bin/sh

for i in $(seq 1 2 10)
do
  echo $i
done

Output:

$ sh example05.sh 
1
3
5
7
9

For loop like C programming

Following is the syntax to create for loop in shell script that looks simiarl to for loop in C programming.

for (( var=val; var<=val2; var++ ))
do
  # body of for loop
done

Example #6: Write a Shell Script to print from 1 to 5 using C style for loop

#!/bin/sh

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

Output:

$ sh example06.sh 
1
2
3
4
5

Nested for loop

We can nest one for loop inside another.

for var_outer in list_outer
do

  # outer for loop body

  for var_inner in list_inner
  do
    # inner for loop body
  done

done

Example #7: Write a Shell Script to print the following pattern

1
1 2
1 2 3
1 2 3 4

To achive this we will use nested for loops. The first loop will help to manage the rows and the second for loop will help in printing the numbers per row.

#!/bin/sh

for r in {1..4}
do
  for i in $(seq 1 $r)
  do
    printf "$i "
  done
  printf "\n"
done

In the above code we are using printf which helps in printing the result in the terminal.