Bash – How to prevent word splitting without preventing empty string removal

bashnullparameter

I need to pass as a program argument a parameter expansion. The expansion results in a filename with spaces. Therefore, I double-quote it to have the filename as a single word: "$var".

As long as $var contains a filename, the program gets a single-word argument and it works fine. However, at times the expansion results in an empty string, which when passed as argument, breaks the program (which I cannot change).

Not removing the empty string is the specified behavior, according to Bash Reference Manual:

If a parameter with no value is expanded within double quotes, a null argument results and is retained.

But then, how do I manage the case where I need to quote variables, but also need to discard an empty string expansion?

EDIT:

Thanks to George Vasiliou, I see that a detail is missing in my question (just tried to keep it short 🙂 ). Running the program is a long java call, which abbreviated looks like this:

java -cp /etc/etc MyClass param1 param2 "$var" param4

Indeed, using an if statement like that described by George would solve the problem. But it would require one call with "$var" in the then clause and another without "$var" in the else clause.

To avoid the repetition, I wanted to see if there is a way to use a single call that discards the expansion of "$var" when it is empty.

Best Answer

The ${parameter:+word} parameter expansion form seems to do the job

( xyz=2; set -- ${xyz:+"$xyz"}; echo $# )
1

( xyz=; set -- ${xyz:+"$xyz"}; echo $# )
0

( unset xyz; set -- ${xyz:+"$xyz"}; echo $# )
0

So that should translate to

program ${var:+"$var"}

in your case

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Bash Shell – How to Prevent Command Substitution on the Command Line

Here (linebreaks added),

$ task add "first line doesn\'t say much
Second line says a lot but part of this line does not appear in the
resulting description 'truncate -s0 !(temp_file | temp_dir)' truncates
all files to 0 bytes as shown by: '`wc -l *`'"

the whole string is double-quoted, so command substitutions and other expansions will run there. That happens in the shell, before task sees that string, and you'll need to prevent it with backslashes or putting that part in single quotes.

E.g.

$ printf "%s\n" "...shown by: '\`wc -l *\`'"
...shown by: '`wc -l *`'

So,

task add "...shown by: '\`wc -l *\`'"

would pass the string ...shown by: '`wc -l *`' to task. It's up to it what does with that.

If you don't want to use backslashes, here's the way to put it in single quotes:

#               aaaaaaaaaaaaaaaaBBBBBBBBBBBaaa
$ printf "%s\n" "...shown by: '"'`wc -l *`'"'"
...shown by: '`wc -l *`'

(The a's mark the double-quoted parts, the B's the single-quoted parts. They are just concatenated on the shell command line. The literal single quotes are within the double-quoted strings.)

As for the single quote and the backslash, you don't need to escape a single quote within double quotes, and in fact the backslash will remain there:

$ printf "%s\n" "foo'bar"
foo'bar
$ printf "%s\n" "foo\'bar"
foo\'bar

From what you show, it seems like task removes at least the first single-quoted string from the argument (plus a word after that, since the removed part was 't say much ... 'truncate)

The shell will not do that, this works fine:

$ printf "%s\n" "a 'quoted string' to test"
a 'quoted string' to test

What's causing the shell to remove the part of the argument to task between \ (backslash) and -s,

It's highly likely it's not the shell doing that.

and how do you prevent the shell from interpreting the above single-quoted command substitution (i.e. '`wc -l *`')?

It's not single-quoted, it's double-quoted with quoted single quotes next to it.

Best Answer

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