Running 'wc' using execvp() recognizes /ho

I have a program where I want to run the command wc using execvp().

The important bits are here:

char *argvNew[5];
argvNew[0] = "wc";
argvNew[1] = "/home/user/foo.txt";
argvNew[2] = NULL;
execvp(argvNew[0], arvgNew);

This works just fine and the terminal displays the output from wc. However, when I change the line to:

argvNew[1] = "~/foo.txt"

I get this error in the terminal:

wc: '~/foo.txt': No such file or directory

Running wc ~/foo.txt directly from the terminal behaves exactly as running wc /home/user/foo.txt. Why does execvp have an issue with the ~/ part?

回答1:

Tilde (~) expansion is done/understood by shells only - it's not expanded in C programs. So execvp attempts to interpret ~ as part of the filename (argument to wc).

What you can do is get the HOME directory value using getenv and prefix it with the filename (e.g., using snprintf).

回答2:

When you type a file name such as ~/foo.txt at the command line, the shell expands it for you before running the command. You therefore have to do that expansion in your C code if you want it to work — or use a shell to do the expansion for you. Neither the execvp() function nor the wc command has any problem with ~; there simply isn't a directory called ~ in the current directory (and therefore there isn't a file called foo.txt in the non-existent directory).

It's not all that hard to write the code to do the job (though doing it properly per POSIX requires attention to detail — see Tilde expansion).

However, there are also standard POSIX functions that can help and do that job, notably wordexp(). Although the standard doesn't explicitly mention tilde expansion, it does mention WRDE_NOCMD to suppress command substitution, so it is supposed to do a thorough job and tilde expansion should be included in a conforming implementation.

Here is some simple test code exercising the function:

#include "stderr.h"
#include <stdio.h>
#include <wordexp.h>

/*
int wordexp(const char *restrict words, wordexp_t *restrict pwordexp,
       int flags);
void wordfree(wordexp_t *pwordexp);
*/

static void do_wordexp(const char *name)
{
    wordexp_t wx = { 0 };
    if (wordexp(name, &wx, WRDE_NOCMD) != 0)
        err_remark("Failed to expand word [%s]\n", name);
    else
    {
        printf("Expansion of [%s]:\n", name);
        for (size_t i = 0; i < wx.we_wordc; i++)
            printf("%zu: [%s]\n", i+1, wx.we_wordv[i]);
        wordfree(&wx);
    }
}

int main(int argc, char **argv)
{
    err_setarg0(argv[0]);

    if (argc <= 1)
        do_wordexp("~/.profile");
    else
    {
        for (int i = 1; i < argc; i++)
            do_wordexp(argv[i]);
    }
    return 0;
}

And here are a couple of sample runs (program wexp19 built from wexp19.c):

$ wexp19
Expansion of [~/.profile]:
1: [/Users/jonathanleffler/.profile]
$ wexp19 '~informix/bin/oninit' '~/bin/rfmt'  '~/bin/al ~/bin/b?' '~/bin/f?' '$IXD/bin/onstat' ' ~/bin/ow '
Expansion of [~informix/bin/oninit]:
1: [/Users/informix/bin/oninit]
Expansion of [~/bin/rfmt]:
1: [/Users/jonathanleffler/bin/rfmt]
Expansion of [~/bin/al ~/bin/b?]:
1: [/Users/jonathanleffler/bin/al]
2: [/Users/jonathanleffler/bin/bk]
Expansion of [~/bin/f?]:
1: [/Users/jonathanleffler/bin/fd]
2: [/Users/jonathanleffler/bin/fl]
3: [/Users/jonathanleffler/bin/fm]
Expansion of [$IXD/bin/onstat]:
1: [/opt/informix/12.10.FC6/bin/onstat]
Expansion of [ ~/bin/ow ]:
1: [/Users/jonathanleffler/bin/ow]
$

Note in particular the result of expanding the quote-enclosed command line argument '~/bin/al ~/bin/b?'; it is split and the words are separately expanded. The next to last argument expands the environment variable $IXD from my environment, too, and the last example shows blanks are removed. The single quotes on the command line are necessary to stop the shell from doing what I want to demonstrate wordexp() doing.

Tests run on a Mac running macOS 10.13.6 High Sierra, and using GCC 8.2.0. YMMV!