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 copied, since it is not needed.
tee cannot use ARGV directly, since 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
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 single rule does all the work. Since 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])
}