How to get success status of last Bash shell command inside a shell script?

Question

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.

A shell script is an entirely new shell, so a new call. Do you need it as a separate shell script, or is defining a function or an alias in your bashrc enough? — Wrikken
– Wrikken, Commented Jan 30, 2014 at 21:24
@Wrikken: I'd need it as a shell script. Is there a way, given the parent shell's PID (which is accessible), to access the parent shell's $? variable (the one holding the result of the last command?). Maybe this should be a new question? — bitcoinNeverSleeps
– bitcoinNeverSleeps, Commented Jan 31, 2014 at 1:32
Not to my knowledge, no.. You could add your requirement that it should be a shell script to your question explicitly, maybe someone more knowledgeable knows something, but I'm afraid the answer will be there is no practical way to do it. — Wrikken
– Wrikken, Commented Jan 31, 2014 at 9:17

Thorsten Staerk · Accepted Answer · 2014-01-30 21:52:00Z

4

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

answered Jan 30, 2014 at 21:52

Thorsten Staerk

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Comments

Matt Parkins · Accepted Answer · 2014-10-02 10:13:34Z

2

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.

answered Oct 2, 2014 at 10:13

Matt Parkins

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Antediluvian · Accepted Answer · 2014-01-30 21:45:55Z

1

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..."

answered Jan 30, 2014 at 21:45

Antediluvian

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