watch is an external program that starts a new program every time interval.
In your case, neither the started default shell sh nor the explicitly started bash can know about your function f – it's not being passed to them.
You include the function definition in what you run. For example, you could write
#!/bin/bash function f() { date } f into a text file "myscript.sh", make it executable and run it with watch ./myscript.sh.
Alternatively, you could also
function f () { … } typeset -fx f # ^---------- modify the type of a name # ^----- work only on functions # ^---- export to environment watch -x bash -c "f" # ^-------- use `exec` rather than `system` to start bash; # makes no sense to start a shell from a shell you # only start to start a shell (but omitting -x has # no downside other than launching an unnecessary # middle layer of `sh`) to export the function declaration to an environment variable and thus make it known to bash as launched from watch, as subprocesses inherit environment variables.
I'd recommend you go with the script option - less confusing, and it doesn't mess with the environment, which might have surprising side effects. It has no performance benefits: in both cases, the shell parses the source code of your function declaration in the new subshell.
Lastly, you could do without watch:
while true ; do tput clear # to clear the screen f sleep 1 done NB: Save your terminal's scroll buffer before using tput clear because it will clear the scroll buffer and you will lose any history you were planning to capture from the scroll buffer.tput clear will clear the scroll buffer; if you had anything on that previously, you might want to save it before.
