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
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. 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.
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
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
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