Elegant solution to echo to either stdout or file in bash

I have a bash application that is producing some result, and I’d like to echo the result to either stdout or to a user chosen file. Because I also echo other interactive messages going to the screen, requiring the user to explicitly use the > redirection when he wants to echo the result to a file is not an option (*), as those messages would also appear in the file.

Right now I have a solution, but it’s ugly.

if [ -z $outfile ]
then
    echo "$outbuf"    # Write output buffer to the screen (stdout)
else
    echo "$outbuf" > $outfile  # Write output buffer to file
fi

I tried to have the variable $outfile to be equal to stdout, to &1 and perhaps something else but it would just write to file having that name and not actually to stdout. Is there a more elegant solution?

(*) I could cheat and use stderr for that purpose, but I think it’s also quite ugly, isn’t it?

Answers:

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Method 1

First, you should avoid echo to output arbitrary data.

On systems other than Linux-based ones, you could use:

logfile=/dev/stdout

For Linux, that works for some types of stdout, but that fails when stdout is a socket or worse, if stdout is a regular file, that would truncate that file instead of writing at the current position stdout is in the file.

Other than that, in Bourne-like shell, there’s no way to have conditional redirection, though you could use eval:

eval 'printf "%sn" "$buf" '${logfile:+'> "$logfile"'}

Instead of a variable, you could use a dedicated file descriptor:

exec 3>&1
[ -z "$logfile" ] || exec 3> "$logfile"

 printf '%sn' "$buf" >&3

A (small) downside with that is that except in ksh, that fd 3 would be leaked to every command run in the script. With zsh, you can do sysopen -wu 3 -o cloexec -- "$logfile" || exit in place of exec 3> "$logfile" but bash has no equivalent.

Another common idiom is to use a function like:

log() {
  if [ -n "$logfile" ]; then
    printf '%sn' "<a href="https://getridbug.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="250165">[email protected]</a>" >> "$logfile"
  else
    printf '%sn' "<a href="https://getridbug.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="042044">[email protected]</a>"
  fi
}

log "$buf"

Method 2

1. Set outfile to "/dev/stdout".
2. Let user choose filename, overriding outfile, or keep default value.
3. printf '%sn' "$outbuf" >"$outfile"

I used printf because of “Why is printf better than echo?“.

For caveats to this solution, see Stéphane Chazelas’ answer.

Method 3

Try this

#!/bin/bash
TOSCREEN="1" # empty OR 0 to log to file
if [[ ! -z "$TOSCREEN" && $TOSCREEN == "1" ]]; then
    echo "$outbuf"    # Write output buffer to the screen (stdout)
else
    if [[ ! -f $outfile ]]; then
      #echo -e "Making file "
      touch "$outfile"
    fi  
    echo "$outbuf" >> $outfile  # Write output buffer to file
fi

Method 4

#!/bin/bash -ue

function stdout_or_file()
{
    DUMP_FILE=${1:-}

    if [ -z "${DUMP_FILE}" ]; then
        # print stdin to stdout
        cat
    else
        # dump stdin to a file
        sed -n "w ${DUMP_FILE}"
    fi
}

echo "foo" | stdout_or_file "${outfile}"

Note that ${1:-} is bash specific and especially useful in “-u” mode. A generic one-line that even works in other shells might be

echo "foo" | if [ -z "${outfile}" ]; then cat; else sed -n "w ${outfile}"; fi

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0