Grep fail on multiple matches

Question

I want to grep a search pattern but only succeed (and output the matching line) if there is only one unique match. If two lines match, grep should fail or output nothing.

Answer 1

You can't do this with grep, but you can simply count the matches. I don't know what shell, what grep or what operating system you are using, but here's an example of a bash function that can do that:

maxOne() (
    pattern="$1"
    file="$2"
    IFS=$'\n'
    set -f

    results=( $(grep -m2 -- "$pattern" "$file") )
    if [ "${#results[@]}" -eq 1 ]; then
        printf -- '%s\n' "${results[@]}"
        return 0
    else
        return 1
    fi
)

Add those lines to your ~/.bashrc or just paste them into a terminal with a running bash session, and you can then do:

maxOne foo file

To search for foo in file. Note that the -m option (maximum results) which is used here for efficiency to make grep exit after two matches, isn't supported by all versions of grep so if it gives you an error, just remove it. It isn't needed, it just speed things up.

Important: this will not work for multi-line search strings which you can use with grep -z if your grep supports that. If you need to be able to handle multi-line search patterns, you will need a different approach. Also, this will not work with patterns that match empty lines (e.g. grep '^$' file). Stéphane's solution will handle empty lines, so that would be a better option if this is an issue. His will also work on multiple files, unlike mine, which is a nice perk.

Answer 2

You could do with:

unique_egrep() (
  export ERE="$1"; shift
  exec gawk -e '
    BEGIN               {ret = 1}
    BEGINFILE           {n = 0}
    $0 ~ ENVIRON["ERE"] {if (n++) nextfile; found = $0}
    ENDFILE             {if (n == 1) {print FILENAME":"found; ret = 0}}
    END                 {exit ret}' -E /dev/null "$@"
)

And then unique_egrep pattern *.txt for instance.

Here using the -e 'code' -E /dev/null (in place of 'code') trick to be able to process arbitrary file paths.

All of -e, -E, BEGINFILE, ENDFILE and nextfile are GNU extensions (though nextfile is now found in many other implementations as well).