Press any key to pause shell script, press again to resume

Question

I've written a shell script for testing an API that copies files and echoes its progress after each one.

There is a two second sleep between each copy, so I would like to add the ability to press any key to pause the script to allow deeper testing. Then press any key to resume.

How can I add this in as few lines as possible?

Answer 1

You don't need to add something to your script. The shell allows such a functionality.

• Start your script in a terminal.
• While is is running and blocking the terminal use ctrl-z. The terminal is released again and your see a message that the process is stopped. (It is now in the porcess state T, stopped)
• Now do whatever you want. You can also start other processes/scripts and stop them with ctrl-z.
• Type jobs in the terminal or list all stopped jobs.
• To let your script continue, type fg (foreground). It resumes the job back into the foreground process group and the jobs continues running.

See an example:

root@host:~$ sleep 10 # sleep for 10 seconds
^Z
[1]+  Stopped                 sleep 10
root@host:~$ jobs # list all stopped jobs
[1]+  Stopped                 sleep 10
root@host:~$ fg # continue the job
sleep 10
root@host:~$ # job has finished

Answer 2

If you want to just pause the script whilst remaining inside the script then you can use read instead of sleep.

You can use

read -t to set a timeout for the read
read -n to read one character(effectively just press any key) to continue script

As you haven't provided any code, below is an example of how it could be used.
If q is pressed then read -n1 prevents the script from continuing until a key is pressed.
When a key is pressed then check is reset and the script continues in the loop as normal.

while [[ true ]]; do
    read -t2 -n1 check
    if [[ $check == "q" ]];then
        echo "pressed"
        read -n1
        check=""
    else
        echo "not pressed"
    fi
echo "Doing Something"
done

You can also add stty -echo to the start of the section and stty echo to the end to prevent typing from messing up the screen output

Answer 3

With dd you can reliably read a single byte from a file. With stty you can set a min number of bytes to qualify a terminal read and a time out in tenths of a second. Combine those two and you can do without sleep entirely, I think, and just let the terminal's read timeout do the work for you:

s=$(stty -g </dev/tty)
(while stty raw -echo isig time 20 min 0;test -z "$(
dd bs=1 count=1 2>/dev/null; stty "$s")" || (exec sh)
do echo "$SECONDS:" do your stuff here maybe                             
   echo  no sleep necessary, I think                                                          
   [ "$((i+=1))" -gt 10 ] && exit                                                             
done       
) </dev/tty

That is a little example while loop that I mocked-up for you to try out. Every two seconds dd times out on its attempted read of stdin - redirected from /dev/tty - and the while loop loops. That or dd doesn't time-out because you press a key - in which case an interactive shell is invoked.

Here is a test run - the numbers printed at the head of each line is the value of the shell variable $SECONDS:

273315: do your stuff here maybe
no sleep necessary, I think
273317: do your stuff here maybe
no sleep necessary, I think
273319: do your stuff here maybe
no sleep necessary, I think
273321: do your stuff here maybe
no sleep necessary, I think
sh-4.3$ : if you press a key you get an interactive shell
sh-4.3$ : this example loop quits after ten iterations
sh-4.3$ : or if this shell exits with a non-zero exit status
sh-4.3$ : and speaking of which, to do so you just...
sh-4.3$ exit
exit
273385: do your stuff here maybe
no sleep necessary, I think
273387: do your stuff here maybe
no sleep necessary, I think
273389: do your stuff here maybe
no sleep necessary, I think
273391: do your stuff here maybe
no sleep necessary, I think
273393: do your stuff here maybe
no sleep necessary, I think
273395: do your stuff here maybe
no sleep necessary, I think
273397: do your stuff here maybe
no sleep necessary, I think