Passing \* as a parameter for a parameter - linux

Question

Using ksh. Trying to reuse a current script without modifying it, which basically boils down to something like this:
`expr 5 $1 $2`
How do i pass in a a multiplication command (*) as parameter $1 ?
I first attempted using "*" and even \* but that isn't working.
I've tried multiple escape backslash and quote combinations but i think im doing it wrong.

Answer 1

Without modifying the script, I don't think this can be done:
On calling, you can pass a literal * as '*', "*" or \* (any will do): this will initially protect the * from shell expansions (interpretation by the shell).
The callee (the script) will then receive literal * (as $1), but due to unquoted use of $1 inside the script, * will invariably be subject to filename expansion (globbing), and will expand to all (non-hidden) filenames in the current folder, breaking the expr command.
Trying to add an extra layer of escaping - such as "'*'" or \\\* - will NOT work, because the extra escaping will become an embedded, literal part of the argument - the target script will see literal '*' or \* and pass it as-is to expr, which will fail, because neither is a valid operator.
Here's a workaround:
Change to an empty directory.
By default, ksh will return any glob (pattern) as-is if there are no matching filenames. Thus, * (or any other glob) will be left unmodified in an empty directory, because there's nothing to match (thanks, #Peter Cordes).
For the calling script / interactive shell, you could disable globbing altogether by running set -f, but note that this will not affect the called script.
It's now safe to invoke your script with '*' (or any other glob), because it will simply be passed through; e.g., script '*' 2, will now yield 10, as expected
If both the shell you invoke from and the script's shell are ksh (or bash) with their default configuration, you can even get away with script * 2; i.e., you can make do without quoting * altogether.

Answer 2

Glob expansion happens very late, after parameter expansion, and word-splitting (in that order). Quote-removal doesn't happen on the results of earlier expansions, just what was on the command line to start with. This rules out passing in a quoted \* or similar (see mklement0's answer), by using an extra layer of quoting.
It also rules out passing in space-padded *: Word-splitting removes the spaces before pathname (glob) expansion, so it still ends up expanding * to all the filenames in the directory.
foo(){ printf '"%s"\n' "$#"; set -x; expr 5 $1 $2; set +x; }
$ foo ' * ' 4
" * "
"4"
+ expr 5 ...contents of my directory... 4
expr: syntax error
+ set +x
You should fix this buggy script before someone runs it with an arg that breaks it in a dangerous way, rather than just inconvenient.
If you don't need to support exactly the same operators as expr, you might want to use arithmetic expansion to do it without running an external command:
result=$((5 $1 $2)) # arithmetic expansion for the right-hand side
# or
((result=5 "$1" "$2")) # whole command is an arithmetic expression.
Double-quotes around parameters are optional inside an arithmetic expression, but you need to not use them in an arithmetic expansion (in bash. Apparently this works in ksh).
Normally it's not a bad habit to just always quote unless you specifically want word-splitting and glob expansion.