Trapping Exits Caused by Interrupts (Unix Power Tools, 3rd Edition)

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35.17. Trapping Exits Caused by Interrupts

If you're running a shell script and you press your interrupt key (Section 5.8) (like CTRL-c), the shell quits right away. That can be a problem if you use temporary files in your script, because the sudden exit might leave the temporary files there. The trap command lets you tell the shell what to do before it exits. A trap can be used for a normal exit, too. See Table 35-1.

Table 35-1. Some Unix signal numbers for trap commands

Signal number	Signal name	Explanation
0	EXIT	exit command
1	HUP	When session disconnected
2	INT	Interrupt -- often CTRL-c
3	QUIT	Quit -- often CTRL-\
9	KILL	Kill, often used as a way to stop an errant program (it cannot be caught, so don't bother to trap it)
15	TERM	From kill command

Here's a script named zmore that uses a temporary file named /tmp/zmore$$ in a system temporary-file directory. The shell will replace $$ with its process ID number (Section 24.3). Because no other process will have the same ID number, that file should have a unique name. The script uncompresses (Section 15.6) the file named on its command line, then starts the more file viewer.[107] The script uses traps, so it will clean up the temporary files, even if the user presses CTRL-c. The script also sets a default exit status of 1 that's reset to 0 if more quits on its own (without an interrupt). If you are on a Linux system, you may find that gzcat is simply named zcat.

[107]The script could run gzcat $1 | more directly, but some versions of more can't back up when reading from a pipe. You may prefer to use less, at any rate.

exit Section 35.16

#!/bin/sh
# zmore - UNCOMPRESS FILE, DISPLAY WITH more
# Usage: zmore file
stat=1  # DEFAULT EXIT STATUS; RESET TO 0 BEFORE NORMAL EXIT
temp=/tmp/zmore$$
trap 'rm -f $temp; exit $stat' 0
trap 'echo "`basename $0`: Ouch! Quitting early." 1>&2' 1 2 15

case $# in
1) gzcat "$1" >$temp
   more $temp
   stat=0
   ;;
*) echo "Usage: `basename $0` filename" 1>&2 ;;
esac

There are two traps in the script:

The first trap, ending with the number 0, is executed for all shell exits -- normal or interrupted. It runs the command line between the single quotes. In this example, there are two commands separated with a semicolon (;) (Section 28.16). The first command removes the temporary file (using the -f option (Section 14.10), so rm won't give an error message if the file doesn't exist yet). The second command exits with the value stored in the stat shell variable. Look ahead at the rest of the script -- $stat will always be 1 unless the more command quit on its own, in which case stat will be reset to 0. Therefore, this shell script will always return the right exit status -- if it's interrupted before it finishes, it'll return 1; otherwise, 0.[108]

[108]It's important to use single quotes rather than double quotes around the trap. That way, the value of $stat won't be interpreted until the trap is actually executed when the script exits.
The second trap has the numbers 1 2 15 at the end. These are signal numbers that correspond to different kinds of interrupts. On newer shells, you can use signal names instead of the numbers. There's a short list in Table 35-1. For a list of all signals, type kill -l (lowercase "L") or see your online signal(3) or signal(2) manual page. Alternatively, look for a file named /usr/include/signal.h or /usr/include/linux/signal.h (which itself just includes /usr/include/asm/signal.h, which is where the constants themselves are defined).

This trap is done on an abnormal exit (like CTRL-c). It prints a message, but it could run any list of commands.

Shell scripts don't always have two traps. Look at the nom (Section 33.8) script for an example.

I usually don't trap signal 3 (QUIT) in scripts that I use myself. That gives me an easy way to abort the script without springing the trap (removing temporary files, etc.). In scripts for general use, though, I usually do trap it.

Also, notice that the echo commands in the script have 1>&2 (Section 36.16) at the end. This is the standard way to make error messages. In this particular script, that doesn't matter much because the script is used interactively. But it's a good habit to get into for all of your scripts.

If your trap runs a series of commands, it's probably neater to call a shell function (Section 29.11) than a list of commands:

trap funcname 1 2 15

--JP and SJC


35.16. Set Exit Status of a Shell		35.18. read: Reading from the Keyboard