A concise tutorial on shell script while, until Loop _linux shell

One, while loop

The while loop is used to continuously execute a series of commands and to read data from an input file; a command is usually a test condition. The format is:

 code as follows:

While command
Todo
Command1
Command2
...
CommandN
Done

command completes, control returns to the top of the loop, starting from scratch until the test condition is false.

The following is a basic while loop, where the test condition is: If counter is less than 5, then the condition returns True. Counter starting from 0, counter plus 1 each time the loop is processed. Run the above script, return the number 1 to 5, and then terminate.

 code as follows:

Counter=0
While [$COUNTER-lt 5]
Todo
counter= ' Expr $COUNTER +1 '
Echo $COUNTER
Done

Run script, Output:
1
2
3
4
5

The while loop can be used to read keyboard information. In the following example, the input information is set to the variable film, and the <Ctrl-D> ends the loop.

 code as follows:

echo ' type <CTRL-D> to terminate '
Echo-n ' Enter your most liked film: '
While Read FILM
Todo
echo "yeah! Great film The $FILM "
Done

Run the script with the output similar to the following:
Type <CTRL-D> to terminate
Enter your most liked film:sound of Music
yeah! Great film The Sound of Music

Second, until cycle

The Until loop executes a series of commands until the condition is true. The Until loop and the while loop are just the opposite of how they are handled. The general while loop is better than the until loop, but at some point-and only in rare cases, the until loop is more useful.

The Until loop format is:

 code as follows:

Until conditions
Command1
Command2
...
CommandN
Done

The condition can be any test condition, the test occurs at the end of the loop, so the loop executes at least once-please note this.