Unix Power Tools

28.16
As an example, here’s a series of commands that puts a listing of the current
directory into a temporary file, emails the listing, then overwrites the previous
version of the file:
$ ll > $tf-1; mail -s backup joe < $tf-1; mv $tf-1 listing
I can repeat that same command later by using a history substitution (30.8)
like !ll.
• It’s useful with sleep (25.9) to run a command after a delay. The next example
shows a series of commands in a C shell alias that you might use to print
a warning and give the user a chance to abort before the last command (exit,
which ends the current shell) is executed. Be sure to read the important note
after this example:
alias bye 'echo "Type CTRL-c to abort logout"; sleep 10; exit'
Note that, in C-type shells and older Bourne-type shells, pressing your interrupt
key (24.10)—like CTRL-c—will stop execution of all jobs on the current
command line. The alias above works in shells like that. But in some shells,
like bash2, interrupting a command in a string of commands separated by
semicolons will affect only that single command. So I couldn’t rewrite the
alias above for bash2 because, if I pressed CTRL-c while the sleep command
was executing, that would simply abort sleep—and proceed to run exit,
which would log me out immediately!
• If you’re running a series of commands that take some time to complete,
you can type all the commands at once and leave them to run unattended.
For example, I have little shell scripts named nup and ndown (24.22) (which
run /sbin/ifup and /sbin/ifdown, respectively) to start and disable the network.
On a system with a dialup modem and a long file transfer to perform,
it’s nice to be able to type a series of commands that bring up the network,
do a couple of file transfers, then bring down the network. I can type this
string, go about my business somewhere else, and come back later:
$ nup;ptbk;getmail;ndown
After nup returns, the network is up (the modem has connected). So the
shell runs ptbk (38.9) to make a backup of my work on this book. Next,
getmail gets my email (it basically runs fetchmail). When getmail finishes,
ndown hangs up the modem. This can take several minutes from start to finish,
but the shell manages it all while I do something else. (If I didn’t have a
windowing system with multiple xterms, I could have put that string of commands
into a subshell (43.7) in the background (23.2).) This is one place that a
GUI interface for network control really loses to command-line utilities and
the shell.
Two related operators, && and || (35.14), work like a semicolon, but they only
execute the next command if the previous one succeeded or failed, respectively.
—JP
564 Part VI: Scripting
This is the Title of the Book, eMatter Edition
Copyright © 2009 O’Reilly & Associates, Inc. All rights reserved.