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bashshellcommand-lineio-redirection

Capture stdout and stderr into different variables

Is it possible to store or capture stdout and stderr in different variables, without using a temp file? Right now I do this to get stdout in out and stderr in err when running some_command, but I'd like to avoid the temp file.

error_file=$(mktemp)
out=$(some_command 2>$error_file)
err=$(< $error_file)
rm $error_file
Solution

  • Ok, it got a bit ugly, but here is a solution:

    unset t_std t_err
eval "$( (echo std; echo err >&2) \
        2> >(readarray -t t_err; typeset -p t_err) \
         > >(readarray -t t_std; typeset -p t_std) )"

    where (echo std; echo err >&2) needs to be replaced by the actual command. Output of stdout is saved into the array $t_std line by line omitting the newlines (the -t) and stderr into $t_err.

    If you don't like arrays you can do

    unset t_std t_err
eval "$( (echo std; echo err >&2 ) \
        2> >(t_err=$(cat); typeset -p t_err) \
         > >(t_std=$(cat); typeset -p t_std) )"

    which pretty much mimics the behavior of var=$(cmd) except for the value of $? which takes us to the last modification:

    unset t_std t_err t_ret
eval "$( (echo std; echo err >&2; exit 2 ) \
        2> >(t_err=$(cat); typeset -p t_err) \
         > >(t_std=$(cat); typeset -p t_std); t_ret=$?; typeset -p t_ret )"

    Here $? is preserved into $t_ret

    Tested on Debian wheezy using GNU bash, Version 4.2.37(1)-release (i486-pc-linux-gnu).