Sometimes there are two commands which I often invoke in a row. However the second command only makes sense in case the first command was successful.
I wanted to do something like this:
#!/bin/bash
if [ $? -gt 0 ]
then
echo "WARNING: previous command entered at the shell prompt failed"
fi
But it doesn't work:
t@quad:~$ echo "abc" | grep def
t@quad:~$ ./warnme.sh
Last command succeeded
What I'd like is something a bit like this:
t@quad:~$ echo "abc" | grep def
t@quad:~$ echo ${PIPESTATUS[1]}
1
Where we can clearly see that the last command failed.
The result I'd like to have:
t@quad:~$ echo "abc" | grep def
t@quad:~$ ./warnme.sh
WARNING: previous command entered at the shell prompt failed
I can't find a way to do it.
command1 && command2
does exactly what you want: command 2 is executed only if command1 succeeds. For example you could do:
ls a.out && ./a.out
Then a.out would only be executed if it could be listed. I wikiblogged about this at http://www.linuxintro.org/wiki/%26%26
One option is to put this just before the list of commands you want to execute only if the previous was successful:
set -e
This will exit the script if any of the commands following it return non-zero (usually a fail). You can switch it off again with:
set +e
Or if you'd prefer to switch it off for just one line you can just logical-OR the command with true:
mycommand || true
For a lot of my scripts I have set -e at the top of the script as a safety feature to prevent the script cascading and doing something dangerous.
How about:
echo "abc" | grep def || ./warnme.sh
Where warnme.sh is now only the warning without the test. It's only called if the first command fails.
In other words, it would be the same as:
echo "abc" | grep def || echo "WARNING: That didn't work..."