shell – What Does Ampersand Mean at the End of a Shell Script Line?

shell

sh sys-snap.sh &

What is sh?
What is sys-snap.sh?
Why I should put & at the end of the line?
Can anyone explain the syntax?

Without the & the script won't go back to the prompt till I press Ctrl+C.
With & I can press enter and it works.

Best Answer

sh is the default Bourne-compatible shell (usually bash or dash)

sys-snap.sh is a shell script, which contains commands that sh executes. As you do not post its content, I can only guess from its name, what it does. I can find a script related to CPanel with the same file name, that make a log file with all current processes, current memory usage, database status etc. If the script starts with a shebang line (#!/bin/sh or similar), you can make it executable with chmod +x sys-snap.sh and start it directly by using ./sys-snap.sh if it is in the current directory.

With & the process starts in the background, so you can continue to use the shell and do not have to wait until the script is finished. If you forget it, you can stop the current running process with Ctrl-Z and continue it in the background with bg (or in the foreground with fg). For more information, see job control

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Shell – Why do some commands ‘hang’ the terminal until they’ve finished

By default the terminal will run the program in the foreground, so you won't end up back at the shell until the program has finished. This is useful for programs that read from stdin and/or write to stdout -- you generally don't want many of them running at once. If you want a program to run in the background, you can start it like this:

$ lxpanel &

Or if it's already running, you can suspend it with Ctrl+Z and then run bg to move it into the background. Either way you will end up with a new shell prompt, but the program is still running and its output will appear in the terminal (so it can suddenly show up while you're in the middle of typing)

Some programs (typically daemons) will fork a separate process when they start, and then let the main process immediately exit. This lets the program keep running without blocking your shell

Bash – Kill Current Command on Ctrl+C and Continue Script

The continue keyword doesn't mean whatever you think it means. It means continue to the next iteration of a loop. It makes no sense outside of a loop.

I think you're looking for

trap ' ' INT

Since you don't want to do anything upon reception of the signal (beyond killing the foreground job), put no code in the trap. You need a non-empty string, because the empty string has a special meaning: it causes the signal to be ignored.

Best Answer

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Shell – Why do some commands ‘hang’ the terminal until they’ve finished

Bash – Kill Current Command on Ctrl+C and Continue Script

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