Thread: which utility in UNIX

You just described how to do it. Look through the PATH variable and look for the name of the program. What's the hard part?

getenv() gives you the environment variable you ask for - for example "path".
To "access" files, you could just try this:

Code:

// pseudo-code
  path = getenv("PATH")
  while more entries in path:
    get path element p
    try open for read (p + name)
    if successfully opened, show p + name; done
    else continue

I'm sure you can use boost or any other number of utilities, but if you are a beginner programmer in C, you shouldn't be using C++ tools like boost.

--
Mats

I think he meant he needs a boost, as in "a little help", rather than the boost library.

QuantumPete

Originally Posted by QuantumPete

I think he meant he needs a boost, as in "a little help", rather than the boost library.

QuantumPete

Typical - yes, I see now how it would be correctly interpreted that way.

--
Mats

[On a side-note, I don't know if it's only specific to bash, but there's the "type" builtin command who does the same thing as "which" but gives an answer if the command you are looking for is a builtin.]

Code:

while more entries in path:

how do i setup the buffer?

Code:

    get path element p
    try open for read (p + name)

What is p?

How about stat()?

Originally Posted by Ron

Code:

while more entries in path:

how do i setup the buffer?

Code:

    get path element p
    try open for read (p + name)

What is p?

How about stat()?

p is "[a copy of] a portion of the path" - it can be described in many ways, but char * would probably be a most suitable type to describe something like that.

Yes, you can (probably SHOULD) use stat.

--
Mats

Originally Posted by mkruk

there is a more convenient way to traverse a directory tree:

dirent.h

a sample code to view all files of the directory /usr would be:

Code:

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>

int main(void)
{
    DIR *directory;
    struct dirent *dp;

    if((directory = opendir("/usr")) == NULL)
        return(1);
    while((dp = readdir(directory)) != NULL)
        printf("%s\n", dp->d_name);

    closedir(directory);
    return(0);
}

based on that, you should be able to implement the functionality of "which". I won't tell anymore, since I spoonfed you enough

oh btw, I don't know if this will work on windows, but it should on all unix-like systems.

But the "which" utility will not need to traverse any directory, since it has all the information needed to find (or not find) the executable file: It needs the path environment variable, and it needs the name of the executable. Then it's just a case of splitting the path string into separate paths, and checking if the executable is in that path or not.

--
Mats