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?
You don't need to add something to your script. The shell allows such a functionality.
T, stopped)jobs in the terminal or list all stopped jobs.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 
sleep 10; notify-send hello and press CTRL + Z to stop, notify-send hello get executed. if second command is getting executed how come the first process is stopped ? after that if type fg i cant see anything happening, which is obvious, since second command is already executed cmd; cmd; cmd; is like writing cmd <newline> cmd <newline> .... Alternatively to a script you can write ( cmd; cmd; cmd; ), it would behave like the script, bacuse of the subshell generated by ( sleep 10. when i press CTRL + Z after 3 seconds and resumed after few second and noticed that sleep command died in less than 7 seconds. which is opposite to what you said, since command nevers gets stopped it just runs in background. straced the sleep command and found out that the system call used is nanosleep(). It seems to be a defined behavoir of the nanosleep systemcall. restart_syscall() restarts the interrupted system call with a time argument that is suitably adjusted to account for the time that has already elapsed (including the time where the process was stopped by a signal). Read that manpage: man7.org/linux/man-pages/man2/restart_syscall.2.html fg %1) or the background (ex: bg %1). (if jobs gives out only 1 number, ie only 1 suspended process, as in the exemple chaos showed: only [1]+ stopped sleep 10, you can omit the %n part. if there are several background processes (running or stopped), you need to designate the one you want with: %n (ex: fg %2 to have %2 resume in the foreground)) 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 [[ $check == "q" ]];then echo "pressed" read -n1 check="" else echo "not pressed" fi echo "Doing Something" done You can also add stty -echo to the start of the section and stty echo to the end to prevent typing from messing up the screen output
With dd you can reliably read a single byte from a file. With stty you can set a min number of bytes to qualify a terminal read and a time out in tenths of a second. Combine those two and you can do without sleep entirely, I think, and just let the terminal's read timeout do the work for you:
s=$(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 
stty sane after changing stty setting, I may be wrong but it doesn't look like you reset them anywhere ? s=$(stty -g </dev/tty). Immediately after calling dd I then restore it with stty "$s". The terminal state doesn't care about subshells and so those settings stick regardless of the parent shell. stty sane is not necessarily what you wanna do - it is better to restore the state to the way you found it than to assume the state was sane at that point. If I wasn't restoring it those echos would be all over the place. Figuring that out is partly why I came so late - your answer wasnt here when I started testing.