Re: Piping within for-loops

From: Ed Morton (morton_at_lsupcaemnt.com)
Date: 02/27/05

  • Next message: John W. Krahn: "Re: Piping within for-loops" 
    Date: Sun, 27 Feb 2005 09:35:07 -0600

    Barry Margolin wrote:

    > In article <d756f1c9.0502270452.2f902ef@posting.google.com>,
    > halsbrecherisch@yahoo.com (halsbrecherisch) wrote:
    >
    >
    >>Hi!
    >>I want to write a script, that selects words from a wordlist,
    >>which don't contain certain characters, e.g.
    >># ./filter a b c
    >>lists all words that don't contain the letters a, b and c.
    >>I've written this script:
    >>
    >>--- begin
    >> cat /usr/share/dict/words |
    >>
    >> for omit in $1
    >> do
    >> if [ ! "$omit" == "" ]; then grep -v -i "$omit" ; else cat; fi
    >> done |
    >>
    >> tee result
    >>--end
    >>
    >>My problem is, that the grep only affects the piped data during the 1st
    >>loop of the for statement. How can I solve this?
    >
    >
    > Unless you run the script like:
    >
    > ./filter "a b c"
    >
    > it looks to me like your script will only loop once anyway. Don't you
    > mean:
    >
    > for omit in "$@"
    >
    > ?
    >
    > Anyway, if you want to reread the file each time through the loop, put
    > the file reading inside the loop:
    >
    > DICT=/usr/share/dict/words
    > for omit in "$@"
    > do
    > if [ ! "$omit" == "" ]

    I realise you're just mimicing the OPs logic, but don't test negatives
    unnecessarily. Any time you find yourself using a "!", think long and
    hard about whether or not you could express the condition in a positive
    way. It makes future mauintenance much easier. In this case, this code
    could be written:

       if [ "$omit" == "" ]
       then cat "$DICT"
       else grep -v -i "$omit" "$DICT"
       fi

    > then grep -v -i "$omit" $DICT
    > else cat $DICT
    > fi
    > done | tee result

    The above logic will print most of the words from the file multiple
    times. The only words you won't see will be those that contain all of
    the letters to be skipped.

    To see just the words that contain none of the letters, and to only see
    each word once, do this:

    DICT=/usr/share/dict/words
    omit=`echo "$*" | tr -d ' '`
    if [ -n "$omit" ]
    then grep -v -i "[$omit]" "$DICT" | tee result
    else cat "$DICT" | tee result
    fi

    Depending on which shell you use, you could improve on the above, but
    that should work in any bourne-like shell.

            Ed.

  • Next message: John W. Krahn: "Re: Piping within for-loops"

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