10.3.2 Rereading the Current File

Another request for a new built-in function was for a function that would make it possible to reread the current file. The requesting user didn’t want to have to use getline (see Getline) inside a loop.

However, as long as you are not in the END rule, it is quite easy to arrange to immediately close the current input file and then start over with it from the top. For lack of a better name, we’ll call the function rewind():

# rewind.awk --- rewind the current file and start over

function rewind(    i)
{
    # shift remaining arguments up
    for (i = ARGC; i > ARGIND; i--)
        ARGV[i] = ARGV[i-1]

    # make sure gawk knows to keep going
    ARGC++

    # make current file next to get done
    ARGV[ARGIND+1] = FILENAME

    # do it
    nextfile
}

The rewind() function relies on the ARGIND variable (see Auto-set), which is specific to gawk. It also relies on the nextfile keyword (see Nextfile Statement). Because of this, you should not call it from an ENDFILE rule. (This isn’t necessary anyway, because gawk goes to the next file as soon as an ENDFILE rule finishes!)

You need to be careful calling rewind(). You can end up causing infinite recursion if you don’t pay attention. Here is an example use:

$ cat data
-| a
-| b
-| c
-| d
-| e

$ cat test.awk
-| FNR == 3 && ! rewound {
-| 	rewound = 1
-| 	rewind()
-| }
-| 
-| { print FILENAME, FNR, $0 }

$ gawk -f rewind.awk -f test.awk data 
-| data 1 a
-| data 2 b
-| data 1 a
-| data 2 b
-| data 3 c
-| data 4 d
-| data 5 e