Piping in bash commands are common.
Checking exit status of a command in bash scripting is essential.
How about checking exit status of the commands in a pipe?
Example:
mysqldump -u $db_username -p $db_name | gzip -c > output.sql.gz
One would just check the exit status of the above using :
echo $?
But the exit status will always be "0" as that's the exit status of the last command, which is the "gzip -c > output.sql.gz".
In order to check the exit status of a particular command in the "pipe command", examine the bash special array, "PIPESTATUS". That's where bash store exit status of the recent command from a pipe.
E.g. :
- ${PIPESTATUS[0]} refers to the first command in the "pipe command"
- ${PIPESTATUS[1]} refers to the second command in the "pipe command"
- ${PIPESTATUS[2]} refers to the third command in the "pipe command"
Here's an example of how to check a program exit code in a pipe command :
#!/usr/local/bin/bash
db_username="myusername"
db_name="mydatabase"
mysqldump -u $db_username -p $db_name | gzip -c > ./output.sql.gz;
if [ ${PIPESTATUS[0]} -ne "0" ];
then
echo "the command \"mysqldump\" failed with Error: ${PIPESTATUS[0]}";
exit 1;
else
echo "Database dump successfully!";
fi
Shalom !!!
ref : Bash manual - PIPESTATUS
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2 comments:
Thank you so much!
I was scratching my head over the exit status of
netstat -ntap | grep sshd
in a bash script, to find if ssh service is running. And i would get exit status 0 evrytime even when ssh was down.
The $PIPESTATUS variable is a great tip :)
There's a small bug with the example though, once you access PIPESTATUS it gets overwritten:
$ false
$ echo $PIPESTATUS[0]
1[0]
$ echo $PIPESTATUS[0]
0[0]
This essentially means you'll always get "failed with error 0" in the event of a failure.
Do it like this:
mysqldump -u $db_username -p $db_name | gzip -c > ./output.sql.gz;
MYSQL_RET=${PIPESTATUS[0]}
if [ ${MYSQL_RET} -ne "0" ];
etc...
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