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dateawkcommandtailprepend

How to make awk execute (i.e. re-execute) a system command (e.g. date) each time a line of input piped from tail -f is processed

I'm trying to prepend the current time to a few columns pulled from each line of a growing text file, and print to the screen.

tail -f dumpfile.txt|awk '{ "date +%T"|getline curtime;print curtime, $2, $7 }'

A sample input line from the file would be:

N 7056 65433 flags 0dbc a000 OK 0

Above command prints each line from tail, however it executes date just once, time that the command line is launched:

Desired             Actual
13:10:05 7050 OK    13:10:04 7050 OK
13:10:05 7051 OK    13:10:04 7051 OK
13:10:06 7052 OK    13:10:04 7052 OK
13:10:06 7053 OK    13:10:04 7053 OK
13:10:07 7054 OK    13:10:04 7054 OK
13:10:07 7055 OK    13:10:04 7055 OK
13:10:08 7056 OK    13:10:04 7056 OK

How can I rewrite this so that the date command is executed each time a new line comes in?

Thanks.

Solution

  • The first getline exhausts date's output (as it's just a single line), subsequent calls to getline fail and leave curtime unchanged. You need to close the file descriptor associated with date +%T every time you read from it, e.g:

    tail -f dumpfile.txt | awk 'BEGIN{cmd="date +%T"} {cmd|getline curtime; print curtime,$2,$7; close(cmd)}'