Send stderr and stdout to a function as variable and check type

Question

I am redirecting the output of a script to a function I've written by using 2>&1 | MyFunction

This works great. However, I'm having a difficult time figuring out how to determine if the variable being passed is STDOUT or STDERR in MyFunction

Ex:

function MyFunction {
    while read line; do
        read IN
        echo "LOG: $IN" >> $LOG_FILE;
    done    
}

Is there some way the perform a check on the passed variable if it is an instance of either STDOUT or STDERR so I can perform conditional rules based upon it?

score 2 · Answer 1 · answered Jan 22 '18 at 12:54

2

The pipe will convey standard output (STDOUT) data. The 2>&1 redirects the standard error ("2") to the file descriptor "1", which is the standard output.

This means the following line : myCommand 2>&1 | MyFunction can be read as "Run myCommand, redirect the standard error to the standard output, and run MyFunction on the output (which is standard error + standard output)"

Once you've written 2>&1, there is no way to know which line was initially sent to STDOUT or STDERR.

answered Jan 22 '18 at 12:54

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Huh! Your last line probably what he is looking for. `:-)` – sjsam Jan 22 '18 at 12:55
Thanks for your reply. It gave me a better understanding of the usage of 2>&1. – Ole Haugset Jan 22 '18 at 13:55

score 1 · Answer 2 · answered Jan 22 '18 at 13:01

Not directly, no. If you don't need to be compatible to other shells than Bash, you can use a bit of trickery in Bash to add a prefix to every line; for example

bash$ { perl -le 'print "oops"; die' 2> >(sed 's/^/stderr: /') > >(sed 's/^/stdout: /'); } 2>&1 | nl
 1  stdout: oops
 2  stderr: Died at -e line 1.

As you can see, the output from the two sed processes is recombined into standard output, which is then piped as standard input to the line numbering program nl. The braces group the commands so that the final redirection 2>&1 doesn't directly affect the Perl process.

score 1 · Accepted Answer · answered Jan 22 '18 at 13:09

You can use this:

# call the function like:
# cmd | log err
# cmd | log info    
log() {
    type="${1}"
    while read -r line ; do
        if [ "${type}" = "err" ] ; then
            echo "EE ${line}"
        else
            echo "II ${line}"
        fi
    done
}

# Check this: https://stackoverflow.com/a/9113604/171318
# Use another file descriptor to separate between stdout and stderr
{ stdbuf -eL -oL cmd 2>&3 | log info ; } 3>&1 1>&2 | log err
# PS: ^ stdbuf makes sure that lines in output are in order

Send stderr and stdout to a function as variable and check type

3 Answers3