How do I pass an empty string with double quotes to a bash script?

Question

I'm trying to write a script that will accept as additional arguments on the command line an empty string with double quotes. I want the script to pass those arguments along with the double quoted empty string if that is provided, but instead the command interpreter seems to interpret such an argument as an empty string and strips away the quotes. Is there any way to do this?

As a simple example I would have in the file script.sh:

#!/bin/bash
/home/myapp $1 $2

If I run at the prompt:

$ ./script.sh arg1 ""

The script just executes "/home/myapp arg1", but misses/ignores the second argument (""). I want it to preserve this empty string and instead execute: /home/myapp arg1 ""

Answer 1

You are correctly passing an empty string argument to the script.

It is the script that is messing it up:

#!/bin/bash
/home/myapp $1 $2

The script is not protecting the expansion of $1 and $2 from word-splitting. This means that if $1 and $2 contain multiple words, those turn into individual arguments, and if either of them expand to nothing, they simply disappear.

This should be:

#!/bin/bash
/home/myapp "$1" "$2"

In general, you can make your script pass all of its arguments to the invoked program, like this:

/home/myapp "$@"

The quotes are just shell syntax; they are not part of the argument data itself. When you type program "" into the shell, at the operating system level, the program receives an empty C language string: a pointer to a null byte. There are no quotes.

You can pass the argument "" (a two-character string made of two double quotes) but that is not an empty argument. A way to do that is, for instance, '""': wrap it in single quotes.

The only reason to do things like that is when you're manipulating shell syntax at a meta-level: passing around pieces of shell script source code, such as quoted tokens, empty or otherwise. The shell has a command called eval which takes source code as an argument (or multiple arguments) and evaluates it.

empty_shell_string_syntax='""'    # this variable holds the characters ""

eval empty_var=$empty_shell_string_syntax   # empty_var gets set to empty string

before eval is invoked, the command line is subject to expansion. That expansion removes the syntax $empty_shell_string_sytnax and replaces it with the contents, the characters "". So then eval gets the string empty_var="". It evaluates this, and so empty_var is set to the empty string, as the syntax indicates.