Bash: How to grep for empty string

veryangryman

I have an optional argument for a script (a filter) but i want the option to have no filter

So the command is

grep $filter file

I want that filter to sometimes be the empty string i.e everything if the user so wishes but grep '' does not do it

croo

veryangryman wrote: »

I have an optional argument for a script (a filter) but i want the option to have no filter

So the command is

grep $filter file

I want that filter to sometimes be the empty string i.e everything if the user so wishes but grep '' does not do it

What about

grep "$filter" file

L.Jenkins

Maybe I'm having an off day today, but would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command? As I said, I might be having an off day in thinking this way today.

croo

L.Jenkins wrote: »

...would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command

No, it returns the full file.

I've since done a simple test, wrapping it in double quotes and that does work.

#!/bin/bash
filter=$1
grep "$filter" ~/adapter.log
exit 1

Passing "" or '' will return all contents of the test file (adapter.log) and passing a value will filter to those lines.

28064212

L.Jenkins wrote: »

Maybe I'm having an off day today, but would using grep to filter an empty string simply seek out and remove all white space within a file or a returned output of a command? As I said, I might be having an off day in thinking this way today.

grep prints lines matching a pattern. All lines contain the empty string, so it will print all lines.

Also, note that the empty string is not the same as whitespace

GreeBo

what is your expected output when filter is empty?

would you not just check for empty and if so, cat the file instead of grepping it?

croo

Well I suspect the grep is not the issue... the issue, as I saw it anyway, was the OP wasn't able to utlise an empty value argument that was passed to his script. It's possible the use of grep was just an example!?