Use a shell variable with Awk

Use grep:

grep abc something.txt

Also note that using $2 in the shell refers to the second argument (as you seem to know), but in awk it is different. Your question seems to show a misunderstanding of this so I'll clarify it.

The shell requires the $ to refer to the value of a variable. So you refer to a variable named myvar by writing $myvar.

In awk to refer to a variable named myvar you just use its name—myvar. To refer to a literal string containing the letters m-y-v-a-r, you type "myvar".

The $ in awk is to refer to the field with a specific number. So $2 refers to the second field of the current line of the file. Or if you set myvar = "4", then $myvar refers to the fourth field of the file.

For just printing all lines of a file that match a given pattern, use grep—that's what it's designed for.

just as a disclaimer, awk isn't the right tool for this job — i'm assuming your actual use case is more complex and justified, so thanks for distilling it!

This specific issuse looks like it comes down to two things, variable "expansion" and passing args from bash to awk.

awk variables and $

once you prefix a awk variable with $, it will immediately make that a reference to a field: which is weird if you're used to the shell, php, or perl.

so an input like this

thing alive
cat yes
lamp no

piped through awk 'BEGIN{alive=2}{print $alive}' will yield

alive
yes
no

while awk 'BEGIN{alive=2}{print alive}' would yield

2
2
2

When mixing awk with bash, things get a little strange; hopefully that helps with the first part.

passing args from bash to awk

if you're set on using awk, and setting awk vars in the bash command, you're probably looking for this:

if [[ $1 = "-c" ]] && [[ $2 =~ ^[a-z A-Z]+$ ]]
then
    awk -v var="$2" 'index($0, var)' something.txt
fi

note: the index($0, var) is short for if (index($0, var)) { print $0 }

admittedly, it's not a "pretty" solution, but that gets back to awk being the wrong tool for the job. For a full awk solution check out ARGV and ARGC