How to insert argument separating space into brace expansion preamble?

Question

I want to use brace expansion to generate the following argument sequence: ./some_command -c root.foo -c root.bar -c root.baz. Brace expansion at first glance looks as a perfect tool: use -c root. as a preamble and {foo,bar,baz} inside braces. However, preamble cannot contain field separators:

./some_command -c root.{foo,bar,baz} expands to -c root.foo root.bar root.baz (obviously)
./some_command -c\ root.{foo,bar,baz} expands to -c\ root.foo -c\ root.bar -c\ root.baz (obviously again). Here I get three arguments instead of six desired.

Is it possible to make a preamble with an argument separator?

Some tools treat -c whatever and -cwhatever equivalently. If ./some_command is one of them, then use ./some_command -croot.{foo,bar,baz}. — Kamil Maciorowski, Feb 19 '24 at 10:10
In fact this is a convention encouraged by The Open Group: "If the SYNOPSIS of a standard utility shows an option with a mandatory option-argument (as with [ -c option_argument] in the example), a conforming application shall use separate arguments for that option and its option-argument. However, a conforming implementation shall also permit applications to specify the option and option-argument in the same argument string without intervening characters." — Kamil Maciorowski, Feb 19 '24 at 10:30
@KamilMaciorowski It solved my current case, thanks (XY problem detected). The question is more general and may still be useful though, so I'm leaving it open and would be glad to receive the answer for the original problem. — Ivan Smirnov, Feb 19 '24 at 12:24
Remotely related: Pass each find result as an argument with prefix to exec. — Kamil Maciorowski, Feb 19 '24 at 12:30

score 0 · Answer 1 · answered Feb 19 '24 at 18:31

You can concatenate brace expansions to get the cross product

echo ./some_command {"-c root."}{foo,bar,baz}

However

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

-- https://www.gnu.org/software/bash/manual/bash.html#Brace-Expansion

So this might be OK

$ echo ./some_command {"-c root.",#}{foo,bar,baz}
./some_command -c root.foo -c root.bar -c root.baz #foo #bar #baz

The -c root.foo is considered a single word, so you might need to eval.

The best way to do this is to use arrays:

options=()
for arg in {foo,bar,baz}; do options+=( -c "root.$arg" ); done
echo ./some_command "${options[@]}"

I don't see the point of the first part of this answer. First you correctly notice that {"-c root."} in {"-c root."}{foo,bar,baz} does not expand, then you deal with this "problem" by introducing {"-c root.",#} which gives you words that start with #. They are not comments (your echo did print them), unless you eval the result. It so happens eval is a "solution" to the problem of -c root.foo being a single word, but the OP had this exact problem from -c\ root.{foo,bar,baz}. Why don't we just eval this? Why complicate things with the additional brace expansion and #? — Kamil Maciorowski, Feb 19 '24 at 19:05

How to insert argument separating space into brace expansion preamble?

1 Answers1