Using Exit Status
[previous]
[next]
[table of contents] [index]
When a UNIX command runs, it can return a numeric status value to the
program that started it.
The status can tell the calling program whether the command succeeded
or failed.
Many (but not all) UNIX commands return a status of zero if everything
was okay or nonzero (1, 2, etc.) if something went wrong.
Almost all MH programs return a status.
The Bourne shell puts the exit status of each process it runs into its
question mark (?) variable; the C shell uses its status variable.
So, to see the exit status of an MH command in a particular situation,
run the MH command -- then type echo $? to the Bourne shell or
echo $status to the C shell.
Of course, you usually don't have to display the exit status in this
way, because the Bourne shell provides a couple of ways to use the exit
status of one command as a condition of further execution.
After a shell program
runs a UNIX command, it can test the exit status to see if there was a problem.
Here's a simple example: a Bourne shell script called newmsgs.
It uses the shell's if structure to run inc,
then test inc's exit status and branch.
If inc brings in some messages, newmsgs runs the
show command to show the current message (the first new message).
Otherwise, the program prints a note that there were no messages to show.
% cat newmsgs
#! /bin/sh
if inc
then
show
else
echo "Sorry; no new message to show now."
fi
A more UNIX-like shell script wouldn't tell you that there were no
messages; the no mail to incorporate
message from inc would be enough.
You could rewrite newmsgs to use the shell's && operator:
#! /bin/sh
inc && show
There's more information about && in the
Section Making Aliases and Functions.
The shells also have a || operator that runs the second command
if the first command returns a nonzero status.
|
