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In this article, we will discuss nested for loops in bash. First, we will quickly discuss the different for loop formulas available in bash. Next, we will discuss nested for loops with examples.

Bash for loop variants

First, let's look at the syntax of the for loop in bash:

for var in something
do
    command
done

In this syntax, the variable name is the user's choice. There are multiple choices for something, which we will discuss later.

We can write any number of commands in the for body. There are several for loop variations in bash; we will see some of them shortly below.

for in the Range of Numbers

We can use a for loop to specify a list of numbers and iterate over those numbers one by one as shown in the following example:

for i in 1 3 8
do
    echo $i
done

The loop will iterate over the given list of values ​​(i.e. 1 3 8). In this particular example, the output will be:

1
3
8

for iterates over a list of values

We can define a list of values ​​and iterate over that list as follows:

x=(1 3 8)
for i in ${x[@]}
do
    echo $i
done

The result of this code will be the same as the output of the previous example. In the for loop statement, {x[@]}it means: for each element of the list x`.

for range of sequence

We can give a range of values ​​with a starting number followed by a double dot ..and then an ending number. Similarly, we can put another pair of dots after a step.

For example, let's look at these two sample loop formulas:

for i in {1..3}
do
    echo $i
done
echo "----------------------"
for i in {1..3..2}
do
    echo $i
done

The output of this code will be:

1
2
3
----------------------
1
3

for words in string

We can define a list of words in a string separated by spaces and iterate over the words in the string like this:

names="Asim Babar Munir Ali"
for name in $names
do
    echo $name
done

The output of this code will give the names in separate lines as shown below.

Asim
Babar
Munir
Ali

C-Style for Expression

We can forgive an expression in the loop similar to the for expression in C language. See the example:

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

It will produce the following output:

1
2
3
4
5

Use of continue statement in for loop

We can put some conditions in the body of a for loop, followed by a continuestatement to skip the next body of the loop. See the example:

for ((i=1;i<=5;i++))
do
    if [ $i == 3 ]
    then
        continue
    fi
    echo $i
done

Because of the if condition, the loop will skip the echo command for the value 3; therefore, the output will be: 1 2 4 5.

Looping through command line arguments

We can provide command line arguments to our bash script and run a for loop on the arguments separated by spaces. See the example:

for i in $@
do
    echo "Script arg is $i"
done

In the first line, $@the symbols represent a list of arguments separated by spaces. See the output of this example:

bash myscript.sh 13 28
Script arg is 13
Script arg is 28

Loop command output

We can run a for loop on the output of some command. See the example:

for f in $(ls)
do
  echo "**** $f *****"
done

This script takes the output of the ls command and iterates over the values. See the example output:

**** first.c *****
**** second.c *****
**** third.c *****

Loop variable sequence

We can also run some inputs at runtime based on some variables. To do this, we can use seq 1 1 $n, where n can be a variable. See the example:

n=4
for i in $(seq 1 1 $n)
do
    echo -n $i
    echo " "
done

Here, I have assigned the value 4 to a variable, but it could be some count like word count etc. See the sample output:

1
2
3
4

Nested for loops in Bash

We have various nested for loops in bash. As per our requirement, we can create multiple variations of nested for loops i.e. by combining the available for loop formulas.

Here we will see multiple examples with output:

• Print the first ten multiples of the first five positive integers.

  for number in {1..5}
  do
    for n in {1..10}
    do
        echo -n $((number*$n))
        echo -n " "
    done
    echo ""
  done
  

  The output of this code is:

  1 2 3 4 5 6 7 8 9 10
  2 4 6 8 10 12 14 16 18 20
  3 6 9 12 15 18 21 24 27 30
  4 8 12 16 20 24 28 32 36 40
  5 10 15 20 25 30 35 40 45 50
  

• Print the first 10 multiples of a list of integers.

  for number in 2 5 9
  do
    for n in {1..10}
    do
        echo -n $((number*$n))
        echo -n " "
    done
    echo ""
  done
  

  The output of this code is:

  2 4 6 8 10 12 14 16 18 20
  5 10 15 20 25 30 35 40 45 50
  9 18 27 36 45 54 63 72 81 90
  

• Prints numbers in a triangular pattern.

  for number in {1..5}
  do
    for n in $(seq 1 1 $number)
    do
        echo -n $n
    done
    echo ""
  done
  

  The output of this code is:

  1
  12
  123
  1234
  12345
  

  We may get several lines from command line arguments to print variable size patterns. See code:

  for number in $(seq 1 1 $@)
  do
    for n in $(seq 1 1 $number)
    do
        echo -n $n
    done
    echo ""
  done
  

  The output of the command bash pattern2.sh 6is:

  1
  12
  123
  1234
  12345
  123456
  

  where patter2.sh is the name of the script and 6 are the command line arguments.
• Prints a triangle pattern with an asterisk ( *) or any other character.

  for number in $(seq 1 1 $@)
  do
    for n in $(seq 1 1 $number)
    do
        echo -n "*"
    done
    echo ""
  done
  

  The output of the code with parameter 4 is:

  *
  **
  ***
  ****
  

• Print a message multiple times (for snippet).

  messages="Sea_is_rough Don't_go_too_far_in_the_sea Remain_in_the_group Don't_take_kids_inside"
  for i in 1 2 3 4 5
  do
    for message in $messages
    do
        echo $message
    done
    echo "----------------------------------"
  done
  

  The output is:

  Sea_is_rough
  Don't_go_too_far_in_the_sea
  Remain_in_the_group
  Don't_take_kids_inside
  ----------------------------------
  Sea_is_rough
  Don't_go_too_far_in_the_sea
  Remain_in_the_group
  Don't_take_kids_inside
  ----------------------------------
  Sea_is_rough
  Don't_go_too_far_in_the_sea
  Remain_in_the_group
  Don't_take_kids_inside
  ----------------------------------
  Sea_is_rough
  Don't_go_too_far_in_the_sea
  Remain_in_the_group
  Don't_take_kids_inside
  ----------------------------------
  Sea_is_rough
  Don't_go_too_far_in_the_sea
  Remain_in_the_group
  Don't_take_kids_inside
  ----------------------------------
  

• Printing on a line starts with a count of 1. You must print the count up to the command line argument list.

  for number in $@
  do
    for i in $(seq 1 1 $number)
    do
        echo -n "$i "
    done
    echo ""
  done
  

  Bash counting.sh 4 5 8 The output of this code is:

  1 2 3 4
  1 2 3 4 5
  1 2 3 4 5 6 7 8
  

• Print multiples of 10 through 20, but do not print multiples of the prime numbers in the sequence.

  for number in $(seq 10 1 20)
  do
    if [[ $number == 11 || $number == 13 || $number == 17 || $number == 19 ]]
    then
        continue
    fi
    for n in {1..10}
    do
        echo -n $((number*$n))
        echo -n " "
    done
    echo ""
  done
  

  The output of this code is:

  10 20 30 40 50 60 70 80 90 100
  12 24 36 48 60 72 84 96 108 120
  14 28 42 56 70 84 98 112 126 140
  15 30 45 60 75 90 105 120 135 150
  16 32 48 64 80 96 112 128 144 160
  18 36 54 72 90 108 126 144 162 180
  20 40 60 80 100 120 140 160 180 200
  

  Finally, we have multiple nested for loops in bash. So, we can combine the available for loop variants as needed to accomplish our task.