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Acorn
Asked: 2010-11-02 15:26:21 +0800 CST

Getting the last match in a file using grep

  • 772

What's the best way of getting only the final match of a regular expression in a file using grep?

Also, is it possible to begin grepping from the end of the file instead of the beginning and stop when it finds the first match?

unix grep

5 Answers

  1. Best Answer

    You could try

    grep pattern file | tail -1

    or

    tac file | grep pattern | head -1

    or

    tac file | grep -m1 pattern
    • 120

  2. I am always using cat (but this makes it a little longer way): cat file | grep pattern | tail -1

    I would blame my linux admin course teacher at college who love cats :))))

    -- You don't have to cat a file first before grepping it. grep pattern file | tail -1 and is more efficient, too.

    • 2

  3. For someone working with huge text files in Unix/Linux/Mac/Cygwin. If you use Windows checkt this out about Linux tools in Windows: https://stackoverflow.com/questions/3519738/what-is-the-best-way-to-use-linux-utilities-under-windows.

    One can follow this workflow to have good performance:

    1. compress with gzip
    2. use zindex (on github: https://github.com/mattgodbolt/zindex) to index the file with appropriate key
    3. query the indexed file with zq from the package.

    Quote from its github readme:

    Creating an index

    zindex needs to be told what part of each line constitutes the index. This can be done by a regular expression, by field, or by piping each line through an external program.

    By default zindex creates an index of file.gz.zindex when asked to index file.gz.

    Example:

    create an index on lines matching a numeric regular expression. The capture group indicates the part that's to be indexed, and the options show each line has a unique, numeric index.

    $ zindex file.gz --regex 'id:([0-9]+)' --numeric --unique

    Example: create an index on the second field of a CSV file:

    $ zindex file.gz --delimiter , --field 2

    Example:

    create an index on a JSON field orderId.id in any of the items in the document root's actions array (requires jq). The jq query creates an array of all the orderId.ids, then joins them with a space to ensure each individual line piped to jq creates a single line of output, with multiple matches separated by spaces (which is the default separator).

    $ zindex file.gz --pipe "jq --raw-output --unbuffered '[.actions[].orderId.id] | join(\" \")'"

    Querying the index

    The zq program is used to query an index. It's given the name of the compressed file and a list of queries. For example:

    $ zq file.gz 1023 4443 554

    It's also possible to output by line number, so to print lines 1 and 1000 from a file:

    $ zq file.gz --line 1 1000
    • 2

  4. The above solutions only work for one single file, to print the last occurrence for many files (say with suffix .txt), use the following bash script

    #!/bin/bash
for fn in `ls *.txt`
do
    result=`grep 'pattern' $fn | tail -n 1`
echo $result
done

    where 'pattern' is what you would like to grep.

    • 1

  5. If you have several files, use inline-for:

    for a in *.txt; do grep "pattern" $a /dev/null | tail -n 1; done

    The /dev/null provides a second file so grep will list the filename where the pattern is found.

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