Shell Programming - For Loop

Unix

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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

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.

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