11.2.5 Duplicating Output into Multiple Files

The tee program is known as a “pipe fitting.” tee copies its standard input to its standard output and also duplicates it to the files named on the command line. Its usage is as follows:

tee [-a] file …

The -a option tells tee to append to the named files, instead of truncating them and starting over.

The BEGIN rule first makes a copy of all the command-line arguments into an array named copy. ARGV[0] is not needed, so it is not copied. tee cannot use ARGV directly, because awk attempts to process each file name in ARGV as input data.

If the first argument is -a, then the flag variable append is set to true, and both ARGV[1] and copy[1] are deleted. If ARGC is less than two, then no file names were supplied and tee prints a usage message and exits. Finally, awk is forced to read the standard input by setting ARGV[1] to "-" and ARGC to two:

# tee.awk --- tee in awk
#
# Copy standard input to all named output files.
# Append content if -a option is supplied.
#
BEGIN {
    for (i = 1; i < ARGC; i++)
        copy[i] = ARGV[i]

    if (ARGV[1] == "-a") {
        append = 1
        delete ARGV[1]
        delete copy[1]
        ARGC--
    }
    if (ARGC < 2) {
        print "usage: tee [-a] file ..." > "/dev/stderr"
        exit 1
    }
    ARGV[1] = "-"
    ARGC = 2
}

The following single rule does all the work. Because there is no pattern, it is executed for each line of input. The body of the rule simply prints the line into each file on the command line, and then to the standard output:

{
    # moving the if outside the loop makes it run faster
    if (append)
        for (i in copy)
            print >> copy[i]
    else
        for (i in copy)
            print > copy[i]
    print
}

It is also possible to write the loop this way:

for (i in copy)
    if (append)
        print >> copy[i]

    else
        print > copy[i]

This is more concise, but it is also less efficient. The ‘if’ is tested for each record and for each output file. By duplicating the loop body, the ‘if’ is only tested once for each input record. If there are N input records and M output files, the first method only executes N ‘if’ statements, while the second executes N*M ‘if’ statements.

Finally, the END rule cleans up by closing all the output files:

END {
    for (i in copy)
        close(copy[i])
}