How to Write Bash WHILE-Loops

Commands, Syntax, and Examples

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You can execute a sequence of commands by writing them into a script file and then running it.

A script file is simply a text file, usually with the .SH file extension, that contains a sequence of instructions that could also be executed from the command line (shell). 

While Loop Examples

Below is an example of a while loop. When executed, this script file will print the numbers 1 through 9 on the screen. The while-statement gives you more flexibility for specifying the termination condition than the for-loop


#!/bin/bash
count=1
while [ $count -le 9 ]
do
echo "$count"
sleep 1
(( count++ ))
done

For example, you can make the previous script an infinite loop by omitting the increment statement "(( count++ ))":


#!/bin/bash
count=1
while [ $count -le 9 ]
do
echo "$count"
sleep 1
done

The "sleep 1" statement pauses the execution for 1 second on each iteration. Use the Ctrl+C keyboard shortcut to terminate the process.

You can also create an infinite loop by putting a colon as the condition:


#!/bin/bash
count=1
while :
do
echo "$count"
sleep 1
(( count++ ))
done

In order to use multiple conditions in the while-loop, you have to use the double square bracket notation:


count=1
done=0
while [[ $count -le 9 ] && [ $done == 0 ]]
do
echo "$count"
sleep 1
(( count++ ))
if [ $count == 5 ]; then $done=1
fi
done

In this script, the variable "done" is initialized to 0 and then set to 1 when the count reaches 5. The loop condition states that the while loop will continue as long as "count" is less than nine and "done" is equal to zero. Therefore the loops exit when the count equals 5.

The "&&" means logical "and" and "||" means logical "or".

An alternative notation for the conjunctions "and" and "or" in conditions is "-a" and "-o" with single square brackets. The above condition:


[[ $count -le 9 ] && [ $done == 0 ]]

...could be rewritten as:


[ $count -le 9 ] -a [ $done == 0 ]

Reading a text file is typically done with a while loop. In the following example, the bash script reads the contents line by line of a file called "inventory.txt:"


FILE=inventory.txt
exec 6

The first line assigns the input file name to the "FILE" variable. The second line saves the "standard input" in the file descriptor "6" (it could be any value between 3 and 9). This is done so that "standard input" can be restored to file descriptor "0" at the end of the script (see the statement "exec 0 In the 3rd line the input file is assigned to file descriptor "0," which is used for standard input. The "read" statement then reads a line from the file on each iteration and assigns it to the "line1" variable.

In order to prematurely exit a while-loop, you can use the break statement like this:


count=1
done=0
while [ $count -le 9 ]
do
echo "$count"
sleep 1
(( count++ ))
if [ $count == 5 ]
then
break
fi
done
echo Finished

The break statement skips program execution to the end while loop and executes any statements following it. In this case, the statement "echo Finished."

The continue statement, on the other hand, skips only the rest of the while loop statement of the current iteration and jumps directly to the next iteration:


count=1
done=0
while [ $count -le 9 ]
do
sleep 1
(( count++ ))
if [ $count == 5 ]
then
continue
fi
echo "$count"
done
echo Finished

In this case, the "continue" statement is executed when the variable "count" reaches 5. This means the subsequent statement (echo "$count") is not executed on this iteration (when the value of "count" is 5).