Bash alias with an argument that works with pipe and globs - bash

Question

I have a convenience function in my bashrc file that looks like following:
function get_pattern()
{
grep "P1" $1 | grep -v "BUT_NOT_THIS" | awk -F":" '{print $(1)}' | sort -u
}
alias gp='get_pattern'
This works fine if I run it on individual file like gp file_1.c. However I am unable to run it like find . -name "*.c" -type f | xargs gp. I also fail to run it like gp *.c. How do I code get_pattern so that I can have all these conveniences.
NOTE: I have simplified the function for easier understanding. Not expecting smart grep/awk/sed/sort hacks or tweaks. The question is I have an alias that takes filenames as arguments. Want it to work with pipes, and preferably with globs.

Answer 1

As pointed out by the experts, aliases are not suited for your requirement.
In your function, you are only passing the first argument to grep, as in grep "P1" $1. Change it to use all arguments, this way:
function get_pattern() {
grep "P1" "$#" | grep -v "BUT_NOT_THIS" | awk -F":" '{print $(1)}' | sort -u
}
Note:
When you invoke your function as get_pattern *.c and there are matching files, the function doesn't see *.c, it sees the list of matching files. The glob expansion is done by the shell while invoking the function, but not inside the function.
In the present format, the function doesn't read from stdin. So, piping the results of another command into your function may not work. To make it accept stdin, you need to change the first grep to:
grep "P1" - "$#"
That would mess up the invocation when you intend the function to only read the files. So, it would be better to rewrite the function this way:
function get_pattern() {
if (($# > 0)); then
# arguments passed, use them as file names to grep from
grep_args=("$#")
else
# no arguments passed, grep from stdin
grep_args=(-)
fi
grep "P1" "${grep_args[#]}" | grep -v "BUT_NOT_THIS" | awk -F":" '{print $(1)}' | sort -u
}

Answer 2

I'm just going to focus on the issue of piping data to xargs. That doesn't work because xargs doesn't know anything about the alias. However you can effectively pass the function definition in to make it work (non-portable solution which works in bash and possibly some other shells, but don't expect it to work everywhere) with export -f:
$ foo() { echo foo: $#; }
$ echo bar baz | xargs bash -c 'foo $#' _
_: foo: command not found
$ export -f foo
$ echo bar baz | xargs bash -c 'foo $#' _
foo: bar baz
I am not aware of any way to do this with an alias, but I'm also not aware of any reason to ever use an alias. Stop using aliases.