There are always things to wait for on a Linux system—upgrades to complete, processes to finish, coworkers to log in and help resolve problems, status reports to be ready.
Fortunately, you don’t have to sit twiddling your thumbs. Instead, you can get Linux to do the waiting and let you know when the work is done. You can do this with a script or you can use the wait command, a bash built-in that watches for processes running in the background to complete.
Crafting waiting within scripts
There are many ways to craft waiting within a script. Here’s a simple example of simply waiting for a period of time before moving on to the next task:
#!/bin/bash # waiting for 11 seconds num=0 while [ $num != 11 ] do echo hmmm sleep 1 ((num++)) done # add commands here
The “!= 11” portion of the commands keeps the loop going as long as this condition is true.
In this next example, the script waits for a third person to log into a Linux system before letting the person running the script know that this has happened.
#!/bin/bash # waiting for a 3rd person to log in while [ `who | wc -l` -lt 3 ] do echo waiting sleep 5 done
The “-lt 3” portion of the while command sets a “while fewer than three people are logged in” condition.
This next script waits for another script to be both available and executable before it attempts to run it.
#!/bin/bash # waiting for a script to be executable script=~/myscript2 while [ ! -x $script ] do echo waiting sleep 5 done echo $script is ready to run $script
Using the wait built-in
You can also wait for a process to finish by using the bash wait builtin. In this case, the process should be run in the background and the wait command provided with the process ID.
$ loop2 & [1] 12345 wait 12345 [1]+ Done
To view the man page information on the wait built-in, you can type “man wait” and page through all the content that proceeds this particular built-in or use a command like this which should get you close to the right spot.
$ man wait | tail -165 | head -25
wait [-fn] [-p varname] [id ...]
Wait for each specified child process and return its termina‐
tion status. Each id may be a process ID or a job specifica‐
tion; if a job spec is given, all processes in that job’s pipe‐
line are waited for. If id is not given, wait waits for all
running background jobs and the last-executed process substitu‐
tion, if its process id is the same as $!, and the return sta‐
tus is zero. If the -n option is supplied, wait waits for a
single job from the list of ids or, if no ids are supplied, any
job, to complete and returns its exit status. If none of the
supplied arguments is a child of the shell, or if no arguments
are supplied and the shell has no unwaited-for children, the
exit status is 127. If the -p option is supplied, the process
or job identifier of the job for which the exit status is re‐
turned is assigned to the variable varname named by the option
argument. The variable will be unset initially, before any as‐
signment. This is useful only when the -n option is supplied.
Supplying the -f option, when job control is enabled, forces
wait to wait for id to terminate before returning its status,
instead of returning when it changes status. If id specifies a
non-existent process or job, the return status is 127. If wait
is interrupted by a signal, the return status will be greater
than 128, as described under SIGNALS in bash(1). Otherwise,
the return status is the exit status of the last process or job
waited for.
Wrap-Up
Having Linux inform you when a process has completed, a file is ready or some condition has been met can be very useful, especially when you have a lot of other things to get done and, well, when isn’t that the case? Bookmark this post so it can remind you of a number of ways to make the waiting work best for you.