Nested Redirection in BASH

Question

I have a scenario where I want to filter log files. To do this I am wanting to follow the last 10 lines or so of a log file and as new lines are read in, pipe them through an awk program and then redirect the output to a listening port on a remote host via netcat.

Currently my script has the following line in it:

echo "\`awk -f rules.awk <(tail -F $@)\`" | nc -u 127.0.0.1 5004

Where $@ is the list of arguments supplied in the initial call and rules.awk simply contains some desired patterns to match on.

Calling the script with ./script logfile.log appears to do nothing and the script hangs. I speculate that the pipe actually requires a process to terminate before its final output is actually piped to the new command.

So I tried a different approach running the following in my terminal:

nc -u 127.0.0.1 5004 <(awk -f rules.awk <(tail -F logfile.log))

This resulted in the following error:

Invalid port specification: /dev/fd/63

It appears that the redirect has actually passed the logical device of the stream rather than its 'value' but I'm just guessing again.

Could someone with a bit more BASH nous point me in the right direction with this?

Answer 1

File descriptors are a bit confusing, they are certainly more confusing than pipes. So before trying to redirect the output of a subshell (<()) as standard input to a command, try getting what you want with pipes. Also, it may help to get the output-commands working correctly before sending the input to a target command (nc).

cat <(tail -F /var/log/daemon.log ) | cat -n | awk 'NR>3 { print}'

this is equivalent to

tail -F /var/log/daemon.log | cat -n | awk 'NR>3 {print}';

but "better" since it does not rely on bash-specific syntax `<()'

Answer 2

Offhand, this seems like it should work to me

tail -F $@ | awk -f rules.awk | nc -u 127.0.0.1 5004