How do I run grep on a zsh variable?

Question

I'd like to inline the following 2 commands:
big_query_that_returns_text > in.txt
$ printf '%sn' "foo" "bar" | grep -f /dev/stdin in.txt

that do work by finding foo and bar in in.txt
but when I try to
printf '%sn' "foo" "bar" | grep -f /dev/stdin big_query_that_returns_text

I receive
zsh: argument list too long: grep

I also tried
var=`big_query_that_returns_text`
printf '%sn' "foo" "bar" | grep -f /dev/stdin $var
printf '%sn' "foo" "bar" | grep -f /dev/stdin "$var"

but I receive the same error.

glenn jackman · Accepted Answer

This is a place for a process substitution: it's a block of code that acts like a file
Pipe the big query results to grep's stdin
big_query_that_returns_text | grep -f <(printf '%sn' "foo" "bar")

If the command to produce "foo" and "bar" is more complicated, you can help readability with arbitrary newlines inside the process substitution:
big_query_that_returns_text 
| grep -f <(
      printf '%sn' "foo" "bar"
  )

AdminBee · Answer

It seems you want to search for foo or bar in a zsh variable that basically is the output of some command. I'm not sure why you go the extra way to pipe the search strings to grep, but unless this is absolutely required, you could do
big_query_that_returns_text | grep -E "foo|bar"

If you want to use the output of big_query_that_returns_text in more than one place, and for this reason store it in a shell variable, you can still use
var=$(big_query_that_returns_text)

printf '%sn' "$var" | grep -E "foo|bar"

... (other operations on $var) ...

Here using printf '%sn'. Other alternatives would be print -r -- "$var" or echo -E - "$var" or use a here-document or here-string. All of printf, print and echo are built-in in zsh, so you won't get the argument list too long error which is a limitation of the execve() system call, so only applies to external commands that are executed (like grep which is not builtin in zsh).

How do I run grep on a zsh variable?

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