Loops in the UNIX Shell - Controlling the loops
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There are a couple of tools which will enhance your control over the loops. The "break" and the "continue" resemble heavily commands you may have already learned from C programming. The break will stop the loop and continue the execution of the script with the command line after the loop.
The "continue" will not finish the current iteration and continue to the next. In other words, it will stop the execution of any further command inside the loop, and start over with the next item, from the beginning. In an extension of the C-like break here, we can use the break to exit from nested loops as well. Jumping out from a two level loop is done with the break 2 syntax.
To make an endless loop, you can use the true and the false commands. The true will always make the $? zero, while the false it will set it to one. Let us write a little script to illustrate this.
until false
do
while true
do
read alfa
if [ "$alfa" = "one" ]
then
break;
elseif "$alfa" = "two" ]
then
break 2;
else
echo "$alfa"
continue;
fi
done
echo Finished a while loop
done
The script above will go back into the first loop if you enter one, will exit from both loops on two, and in any other case it will print back the line and continue from the start of the while.
Then there is the shift command. This will push the arguments with one to the front, and in the process, eliminate the first item.
For example, if you have inside the special variables $(number) as $1 - one, $2 - two and $3- three and the $# -3 representing the numbers of the arguments, after you call this you will have $# - 2 with $1 - two and $2 - three. It's a simple shift of these variables. We can use this to iterate through the arguments with the while loop:
while [ $# -gt 0 ]
do
echo $1
shift
done
You can call the shift command also with an argument, where the argument will represent the number with however many shifts will be made. Shift 2 will make $3 $1, and so on.
If you want to exit from a script, you should use the exit command. The exit's argument is its return value. In a successful case, it returns 0; otherwise, it returns an integer that is not zero.
For example:
#finding file alfa
for i in $(ls)
do
if [ "$i" = "alfa" ]
then
exit 0
fi
done
exit 1
If we do not use this in a script, the return value of the shell program is equal to the return value of the last executed command.
With this, we have finished the lesson on loops. Next week we will put together everything we have learned so far, extend it with a couple of new tools, and write scripts that will achieve complex tasks. Make sure you rate my article if you read it, to express your thoughts. Moreover, you can state them explicitly in the blog following this article. Live With Passion!
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