How do I inject a header line into a pipe via a shell script?

Question

I am working on a process to send data via a pipe from one server to another for processing.

Although this is not the exact command, it might look something like this:

tail -f logfile | grep "abc" | grep "def" | grep -v "ghi" | netcat -q 0 n.n.n.n 7777

I would like to wrap all those greps into a script and more importantly prepend the pipe to netcat with an identifier, so the command would look like this:

tail -f logfile | myscript.sh {id}

The script listening on the other end should receive:

{id}
[Line 1 of the logfile]
[Line 2 of the logfile]
...

Wrapping it in a script is easy:

#!/bin/sh
id=$1
grep "abc" | grep "def" | grep -v "ghi" | netcat -q 0 n.n.n.n 7777

but I cannot figure out how to inject $id at the start.

The receiving end is using

 socat -u tcp-l:7777,fork system:/dev/receivePipe

so if there is a different way I could get the id (for example somehow as a parameter to /dev/receivePipe), or via an environment variable, that would work too.

EDIT: The final answer was figured out in the comments of the accepted answer:

#!/bin/sh
{
  printf '%s\n' $1
  grep "abc" | grep "def" | grep -v "ghi" 
} | netcat -q 0 192.168.56.105 7777

Answer 1

Just do:

#! /bin/sh -
{
  printf '%s\n' "${1-default-id}"
  awk '/abc/ && /def/ && ! /ghi/'
} | socat - tcp:n.n.n.n:7777

${1-default-id} expands to the first positional parameter if specified or default-id otherwise. Replace with ${1?} to exit with an error if not passed any argument instead (or ${1?The error message} to specify an error message instead of the default).

We redirect the output of a command group that runs printf to output the ID and the filtering commands (here with your grep pipeline replaced with a single awk invocation that does the same thing a bit less clumsily) to socat/netcat.

Or to only print the ID if and when one line has been read and matches:

#! /bin/sh -
ID=${1-default-id} awk '
  /abc/ && /def/ && ! /ghi/ {
    if (!already_printed++) print ENVIRON["ID"]
    print
  }' | socat - tcp:n.n.n.n:7777

Or to prepend the ID (and a space character) to every line:

#! /bin/sh -
ID=${1-default-id} awk '
  /abc/ && /def/ && ! /ghi/ {
    print ENVIRON["ID"], $0
  }' | socat - tcp:n.n.n.n:7777

Beware awk, like grep will buffer their output when it goes to a pipe (to anything other than a tty device). With the GNU implementation of awk (aka gawk), you can add a call to fflush() after each print to force the flushing of that buffer. See also the -Winteractive of mawk. In most awk implementations, doing a system("") would also force a flush. The GNU implementation of grep has a --line-buffered option to force a flush after each line of output.

Also note that tail -f logfile is short for tail -n 10 -f logfile. Chances are you actually want either tail -n +1 -f logfile for the whole log file to be processed, and then tail carrying on following the file, or tail -n 0 -f logfile to process only the lines being added from now on.