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I'm wondering why optarg returns an invalid path in the following case: --foo=~/.bashrc but not if I leave a space in between --foo ~/.bashrc.

And what would be the workaround so it works on both cases.

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

int main(int argc, char *argv[]) {
    int opt = 0;
    int long_index = 0;
    char *f; 
    static struct option longopt[] = { 
        {"foo", required_argument, 0,  'd' },
        {0,0,0,0}
    };  
    while ((opt = getopt_long(argc, argv,"d:", longopt, &long_index )) != -1) {
        switch (opt) {
            case 'd' : 
                printf("\n%s\n", optarg);
                f = realpath (optarg, NULL);
                if (f) printf("%s\n", f); 
                break;
            default: 
                exit(1);
        }   
    }   
    return 0;
}

Output:

$ ./a.out --foo=~/.bashrc
  ~/.bashrc

$ ./a.out --foo ~/.bashrc
  /home/user/.bashrc
user1024718
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  • It's a problem of shell expansion. An argument starting by ~ is expanded to the home directory before the program itself is started. And because --foo=~/.bashrc is a single argument and does not start with ~, no expansion is performed here. – Marian Jan 11 '14 at 18:33
  • Thanks. So I guess the shell is unable to expand it since it considers `=~/.bashrc` to be the path. Do you know if this is a known issue across all shells? – user1024718 Jan 11 '14 at 18:39
  • It considers `--foo=~/.bashrc` as a single parameter not starting by ~. However, there must be a way to teach bash to expand `~` after `=`, because it is doing the right expansion for `dd` command. For example `dd if=~/xxx` is expanded. This is a question for bash community. – Marian Jan 11 '14 at 18:49

1 Answers1

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It happens because the "tilde expansion" is performed by the shell: it isn't a valid path by itself. The tilde ~ is expanded as the home directory only in case it is at the beginning of a string argument, that's looks like a path. For example:

$ echo ~
/home/sigi
$ echo ~/a
/home/sigi/a
$ echo ~root/a
/root/a
$ echo ~a
~a
$ echo a/~
a/~

If you want to offer this functionality in the first case too, where the shell can't help you, or more generally the word expansions that are used by the shell, you can find all the needed information to do it yourself in this reference.

Sigi
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