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Bash: How to grep for empty string

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  • 15-07-2020 9:38am
    #1
    veryangryman
    Registered Users Posts: 5,533 ✭✭✭


    I have an optional argument for a script (a filter) but i want the option to have no filter

    So the command is

    grep $filter file

    I want that filter to sometimes be the empty string i.e everything if the user so wishes but grep '' does not do it


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    #2
    croo
    Moderators, Technology & Internet Moderators Posts: 1,334 Mod ✭✭✭✭croo


    veryangryman wrote: »
    I have an optional argument for a script (a filter) but i want the option to have no filter

    So the command is

    grep $filter file

    I want that filter to sometimes be the empty string i.e everything if the user so wishes but grep '' does not do it
    What about
    grep "$filter" file


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    #3
    L.Jenkins
    Moderators, Computer Games Moderators, Technology & Internet Moderators Posts: 19,240 Mod ✭✭✭✭L.Jenkins


    Maybe I'm having an off day today, but would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command? As I said, I might be having an off day in thinking this way today.


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    #4
    croo
    Moderators, Technology & Internet Moderators Posts: 1,334 Mod ✭✭✭✭croo


    L.Jenkins wrote: »
    ...would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command
    No, it returns the full file.

    I've since done a simple test, wrapping it in double quotes and that does work.
    #!/bin/bash
    filter=$1
    grep "$filter" ~/adapter.log
    exit 1
    Passing "" or '' will return all contents of the test file (adapter.log) and passing a value will filter to those lines.


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    #5
    28064212
    Registered Users Posts: 10,479 ✭✭✭✭28064212


    L.Jenkins wrote: »
    Maybe I'm having an off day today, but would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command? As I said, I might be having an off day in thinking this way today.
    grep prints lines matching a pattern. All lines contain the empty string, so it will print all lines.

    Also, note that the empty string is not the same as whitespace

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    #6
    GreeBo
    Registered Users Posts: 27,053 ✭✭✭✭GreeBo


    what is your expected output when filter is empty?

    would you not just check for empty and if so, cat the file instead of grepping it?


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    #7
    croo
    Moderators, Technology & Internet Moderators Posts: 1,334 Mod ✭✭✭✭croo


    Well I suspect the grep is not the issue... the issue, as I saw it anyway, was the OP wasn't able to utlise an empty value argument that was passed to his script. It's possible the use of grep was just an example!?


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