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I am rewriting the problem since it's not clearly understood as far as I see. I implement my own shell in C which needs to support all commands the original one does.

The problem is to execute all existing UNIX bash commands in C without using execvp() or system() functions which already let you do that easily.

To do that, I need to search all required directories which may consist any kind of UNIX commands. I just want to know that:

Do I really be sure that I support all possible UNIX commands in any distribution when I checked all directories in my PATH environment variable? (which becomes /bin/, /usr/bin/, /usr/local/bin in my machine)

I have also found a method which gets the full directory of a file you inserted called realpath() . But unfortunately, it returns (null) when I attempt to get the directory of the command inserted in my own shell.

What else do you suggest me to achieve this problem? As a last solution, does it make sense to search whole computer recursively from the root to find the command inserted?

If there's something unclear, please let me know to clarify. I would be very thankful if you could answer with a piece of example code and clear [on hold] tag in the question if you think it's clearly described from now.

Thanks in advance!

Yusuf Kamil AK
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    do you mean to look for files reachable with the `PATH` directory list env. variable. – Jean-François Fabre Dec 02 '17 at 11:30
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    hint: `/bin` is in the `PATH` system variable. – Jean-François Fabre Dec 02 '17 at 11:32
  • @Jean-FrançoisFabre do you suggest me to look for the contents of the PATH env. variable? If so, can I be sure that all possible UNIX commands are located only in PATH system variable? – Yusuf Kamil AK Dec 02 '17 at 11:35
  • Are you looking for `find /`? Or `find / -exec {} \;` – alk Dec 02 '17 at 11:52
  • @alk indeed, I expect user to input an existing unix command and try to find its executable file to be able to run that using execv function. By the way, I'm not allowed to use any other kind of exec functions such as execvp or so. – Yusuf Kamil AK Dec 02 '17 at 12:00
  • Why "*not allowed*"? – alk Dec 02 '17 at 12:11
  • Because it's a homework at college which we are requested not to do it in easier way, I guess :) @alk – Yusuf Kamil AK Dec 02 '17 at 12:13
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    "*do I need to recursively look for the command specified from the root on my own?*" I'd say so, yes. – alk Dec 02 '17 at 12:17
  • What you could also try 1st is to read out the `PATH` environment variable (using `getenv()`), parse the result, and check all paths in there. – alk Dec 02 '17 at 12:19
  • But: "*can I be sure that all possible UNIX commands are located only in PATH system variable?*" no, you cannot be sure. – alk Dec 02 '17 at 12:21
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    You need to define your problem a bit better. In most operating systems' environment there is a variable similar to `PATH`. This variable the user has set, or has been set for the user, to contain the directories in which the system will look for executable files. The system will *only* look in these directories, it will *not* search all directories. Searching all directories for an executable is a *significant security risk*. You need to verify *exactly* what your assignment is: are you *really* supposed to search all directories, or only directories in the user's `PATH`. – Makyen Dec 02 '17 at 16:29

2 Answers2

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It is true that a UNIX executable can be absolutely anywhere, but in the context of a homework assignment, it doesn't make sense to search the entire filesystem. What your instructor probably wants you to do is implement the functionality of execvp yourself, using execv. What execvp does is, first, it looks to see if there is a slash in the command name. If there is, it passes the command and arguments directly to execv - it doesn't search. Otherwise, it iterates over the directories in PATH and checks whether the command is an executable in each. Crucially, it does NOT scan the contents of each directory; not only would that be very slow, it wouldn't even work under some conditions (such as a directory with --x permissions) Instead, it blindly calls execv with the pathname "$dir/$cmd". If that works, execv doesn't return. If it didn't work, and errno is set to ENOENT, it goes on to try the next directory in the path.

zwol
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First, note that realpath() doesn't search anything, it just determines the absolute path of a given file.

There is no all possible UNIX command as you may think. At least any executable file can be considered as UNIX command, and executables are not necessarily the ones that have x right attached to it. Shell scripts may be executed by command like sh myscript even if executable access is not granted on it. Only binaries necessitate to have that attached right to be executed natively. So there is no true criterion that can help you. But you may have files that have x right and that are not executables!

A common usage is that executables are located in some directories /bin, /usr/bin, /usr/local/bin, and alike. Your shell has an environnement variable named PATH that contains list of directories where to search for command you specified freely on command line.

Anyway, if you choose a criterion to make an exhaustive search by yourself, say all files with x right then you can use command find like find some_starting_dir -perm +0111 to get all files that have x right somewhere.

If you want to program it then you may use either legacy readdir() function or the newer nftw() to make your own directory traversal. You will find many example of these even on SO.

Jean-Baptiste Yunès
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