Learning the bash Shell, Second Edition

while and until

The remaining two flow control constructs bash provides are while and until. These are similar; they both allow a section of code to be run repetitively while (or until) a certain condition becomes true. They also resemble analogous constructs in Pascal (while/do and repeat/until) and C (while and do/until).

while and until are actually most useful when combined with features we will see in the next chapter, such as integer arithmetic, input/output of variables, and command-line processing. Yet we can show a useful example even with what we have covered so far.

The syntax for while is:

while condition
            do
    
            statements...
            done

For until, just substitute until for while in the above example. As with if, the condition is really a list of statements that are run; the exit status of the last one is used as the value of the condition. You can use a conditional with test here, just as you can with if.

Note that the only difference between while and until is the way the condition is handled. In while, the loop executes as long as the condition is true; in until, it runs as long as the condition is false. The until condition is checked at the top of the loop, not at the bottom as it is in analogous constructs in C and Pascal.

The result is that you can convert any until into a while by simply negating the condition. The only place where until might be more meaningful is something like this:

until command
            ; do
    
            statements...
            done

The meaning of this is essentially, “Do statements ...