Bash For, Whlie Loop Examples

This article will introduce how to loop with for or while in Bash

1. while Loop

This is the syntax for a while loop. The loop will repeat as long as the condition is True, and will end when the condition is False.

while [ condition ]
do
   Statements
done

This is an example of a while loop that prints from 1 to 5.

The condition of the loop, [ "$a" -lt 5 ], means $a < 5. And $(expr $a + 1) means an operation that adds 1 to $a.

#!/bin/bash

a=0
while [ "$a" -lt 5 ]
do
    a=$(expr $a + 1)
    echo $a
done

Output:

$ bash example.sh
1
2
3
4
5

2. while Loop (Infinite loop)

In the code below, the : next to the while means True, so the loop will repeat infinitely. You can terminate the program with Ctrl + C.

#!/bin/bash

while :
do
  echo "Please type something in (^C to quit)"
  read INPUT_STRING
  echo "You typed: $INPUT_STRING"
done

$ bash example.sh
Please type something in (^C to quit)
aaa
You typed: aaa
Please type something in (^C to quit)
bbb
You typed: bbb
Please type something in (^C to quit)
^C

3. for Loop

This is the syntax for a for loop. The for loop will repeat for the number of variables inputted after the in.

Each time the loop is repeated, the variable var will be assigned to wordN.

for var in word1 word2 ... wordN
do
  Statements
done

If you look at the example below, you can easily understand how a for loop works.

#!/bin/bash

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

Output:

$ bash example.sh
1
2
3
4
5

And after in, characters other than numbers are also possible.

#!/bin/bash

for var in 1 2 a 4 HELLO
do
    echo $var
done

Output:

$ bash example.sh
1
2
a
4
HELLO

4. until Loop

The following is the syntax for an until loop. The loop will repeat until the condition is False, and will end when the condition is True.

Until is the same as while, except that the condition is reversed. That is, until [ ! condition ] will work the same as while [ condition ].

until [ condition ]
do
   Statements
done

Here is an example of using an until loop to print 1-5.

In the code, [ $a -ge 5 ] means $a >= 5, i.e. the loop will repeat until $a becomes 5.

#!/bin/bash

a=0
until [ $a -ge 5 ]
do
    a=$(expr $a + 1)
    echo $a
done

Output:

$ bash example.sh
1
2
3
4
5