Why does ls accept duplicate switches?

Question

I'm curious - is there a difference between ls -l and ls -lllllllllllllllllllllllllllll?

The output appears to be the same and I'm confused on why ls allows duplicate switches. Is this a standard practice among most commands?

Answer 1

Short answer:

Because it's programmed to ignore multiple uses of a flag.

Long answer:

As you can see in the source code of ls, there is a part with the function getopt_long() and a huge switch case:

1648       int c = getopt_long (argc, argv,
1649                            "abcdfghiklmnopqrstuvw:xABCDFGHI:LNQRST:UXZ1",
1650                            long_options, &oi);
      ....
1654       switch (c)
1655         {
      ....
1707         case 'l':
1708           format = long_format;
1709           break;
      ....
1964     }

The function getopt_long() reads all paramters given to the program. In case if -l the variable format is set. So when you type multiple -lllllllll that variable is set multiple times, but that does not change anything.

Well, it changes one thing. This huge switch case statement must run through multiple times, because of multiple -l flags. ls needs longer to complete with multiple -l flags. But this time is not worth mentioning. =)

Answer 2

Because it's the right thing to do. Suppose you had a script doing something like:

ls $LS_OPTIONS -l "$dir"

where it's possible that $LS_OPTIONS already contains -l. It would be counter-intuitive and annoying for this command to produce an error and would require extra logic in the script to avoid it.

-l may not be the best example for this, but hopefully you can see how the concept applies in general. A much better example is compiler options in $CFLAGS that might duplicate explicit options in a particular invocation of the compiler.

Answer 3

ls is not a bash command, but a separate executable that you happen to launch from bash. That said, -l is just a type of Boolean flag, which if present causes ls to use a long-style format for the output. Most programs will simply ignore multiple uses (ls -ll is the same as ls -l -l) of such flags, although there are some exceptions (as an example, if -v means 'verbose', then a program may interpret multiple uses to mean "be even more verbose").

Answer 4

Shell aliases would be pretty annoying if commands like ls did not allow repeated options.

Suppose you had

alias ls='ls --color=auto'
alias rm='rm -i'

Then, if conflicting flags were not allowed, it would be an error to issue commands like ls --color=never or ls --color=auto or rm -i.

Therefore, these commands are designed to let later flags override earlier ones.