Bash: Allow parentheses in command arguments (i.e. prevent "unexpected token '(' ")

Question

I am frequently using a command line program that I provide with arguments that contain parentheses. For simplicity, let's say I'm writing

echo 'bar(1,3)'

I would like to omit the quotes. However, if I do that I get syntax errror near unexpected token '('. I guess this is related to subshells. I am willing to disable those, if that's the only way. (though a subshell cannot be started if it is not at the beginning of the command anyway, so they are no reason to forbid parentheses in the arguments, as far as I can tell)

Answer 1

( and ) are special token characters in the syntax of bash that are used in a number of operators including:

• (...) subshell construct
• func() compound-command function definitions
• $(...) command substitution
• <(...), >(...) process substitution
• ((...)) arithmetic evaluation construct
• $((...)) arithmetic expansion
• a=(...), a+=(...) array assignment operators
• @(...), +(...), *(...), ?(...) glob operators (with extglob)
• [[ (a && b) || (c && d) ]] grouping conditional expression operators
• [[ ... =~ ...(...)... ]] regexp operator.

echo a=(b) is a syntax error, but not export a=(b).

echo a) is a syntax error, unless there was an opening ( in the previous lines part of one of the constructs above.

While it may be possible to write a readline hook that adds quotes where needed around (, ) to avoid a syntax error, it would be considerable effort as it would mean doing a full parsing of the shell syntax.

A better approach may be to use a shortcut that quotes the current word when you realise too late that it contains characters special to the shell.

With zsh (assuming emacs mode):

bindkey -s '\e#' '\C@\eb\Cx\Cx\e"'

To have Alt+# quote the current word.

Or an approximation with bash (also assuming emacs mode):

bind "\"\e#\": \"'\e \eb'\C-X\C-X\""

Answer 2

This might be a bad idea (see the comments), but I had the same problem and I solved it by making a function that changes [ into ( and ] into ) before using it:

echo2() {
    expression=$(echo $@ | tr '[]' '()')
    echo "$expression"
}

input:

echo2 bar[1,3]

result:

bar(1,3)

I don't know if the [ and ] are treated specially by the shell. But if you're ok with using them in place of ( and ) for your function maybe this a solution for you.