I want to write a program which executes the Linux ls command.
I really only want to type in ls and not /bin/ls, so I want to do this using execve (execvp is not an option).
I tried:
char *env[] = { "SHELL=/bin/bash",
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games",
"_=/usr/bin/env",
(char *)0 };
execve(parmList[0], parmList, env);
But it does not seem to work, ls is not recognized but /bin/ls is. Nothing is wrong with parmList because I tried this with execvp and it worked.
Any ideas?
If parmList[0] contains just ls, execve() won't find the ls command unless it is in the current directory.
With execve(), the value of PATH in the environment is immaterial — execve() does not use it. You'd use execvp() (or execvpe() if it is available) to execute the command on the current process's PATH.
Related
I want to execute a shell command with execlp, I tried with the following instruction :
execlp("sh", "sh", "-c", p_command, (char*)NULL);
p_command is a pointer to a const char representing a shell command line.
My minimal test tells me the program succeded as expected. I first choose to use "/bin/sh" instead of "sh" but I've learned that p(ath) in execlp allows us to avoid writing the full path, as if exec will complete the path for us ; so I removed "/bin/".
My concern is that I never saw a code using execlp with only "sh", as it effectively does for exemple for ls we can directly use "ls" instead of "/bin/ls".
As a beginner I am wondering what "/bin/sh" stands for, what is the difference between "sh" and "/bin/sh" in this situation and why we have to write the full path for execlp to execute a shell ?
When the path passed to execlp is sh, execlp searches for it in the directories listed in the PATH environment variable. If an attacker is able to modify the PATH variable in the environment that runs your program, they can set it to list a directory of their choosing, and they can place their own program named sh in that directory. Then your program will execute their program instead of executing the system sh program. In some cases (depending on a bit in the file’s mode bits), programs are executed with the permissions of their owners rather than the permissions of the user executing the program. Such programs must be written carefully to avoid situations like this, where an attacker would be able to exploit the program.
When the path passed to execlp is /bin/sh, execlp looks for it in the path that is /bin/sh starting from the root of the file system, called /. This will always use the sh program that the system administrator has put in the /bin directory (usually done as part of system installation).
I'm trying to run command line arguments (specifically echo) through the exec family of functions. I can get the execv function to run if I write my own executable and run it, but if I try to run touch or echo it returns -1
#include <stdio.h>
#include <unistd.h> // exec functions
#include <sys/types.h> // pid_t
#include <sys/wait.h>
#define HIGH 1
#define LOW 0
int digitalWrite(int pin, short type) {
pid_t pid = fork();
if (pid == 0) {
printf("pid == %i\n", pid);
if (type == HIGH) {
char* args[] = {"echo", "1", ">", "/sys/class/gpio/gpio67/value", NULL};
int val = execv(args[0], args);
printf("ran function execl, %i\n", val);
} else {
printf("Unable to do anything but set pin to HIGH\n");
}
} else if (pid < 0) { // pid < 0
printf("fork failed\n");
}
wait(NULL);
}
int main() {
printf("Starting digitalWrite\n");
digitalWrite(0, HIGH);
printf("Completed digitalWrite()\n");
return 0;
}
Just for context here's my build:
$ gcc wiringbeagle.c
$ ./a.out
Starting digitalWrite
pid == 0
ran function execl, -1
Completed digitalWrite()
Completed digitalWrite()
$ ls
a.out wiringbeagle.c
The command echo 1 > /sys/class/gpio/gpio67/value runs fine in the terminal on it's own, and if I create a local file (i.e. touch tmpfile.txt) and try to run echo hi > tmpfile.txt it runs as expected in my command line but doesn't run in the program.
I must be not understanding something with execv, and any assistance would be greatly appreciated!
The first argument to execv is the file to be executed. Unlike your shell, execv does not search through the directories indicated by the PATH environment variable, so you need to give it the complete path to the executable. Unless there is an executable file called echo in your current working directory, execv("echo",...) will fail with a "file not found" error. (Use perror to get better error messages).
If you want to search for the executable as the shell does, use execvp. But note that your shell probably executes echo as a built-in command, so it won't be the same echo as your shell uses. In this case, that's fine.
Once you fix that, you will encounter a different problem. Since you are just invoking a command-line utility with arguments, rather than using a shell, the argument ">" is just an argument. It is the shell which handles redirections (as well as pipes, quoting, and a bunch of other useful stuff). So all you will accomplish is to send the three arguments to stdout.
You could use the system function to execute a command using the shell, or you could set up the redirection yourself by freopening stdout in your child before doing the execvp.
You can get quite a lot of information about system interfaces using the man command. For example, to learn what freopen does, use man freopen. You can also read manpages on the internet, eg. http://man7.org/linux/man-pages/man3/freopen.3.html, but the documentation on your own system is right there, and also applies to the actual version of the software installed on your system (assuming you installed the documentation).
I'm not entirely certain why you're even using the exec family to run external programs in this case. The C standard library provides perfectly adequate file I/O stuff.
For example, you can simply fopen, fprintf, and fclose the file without ever starting another external process to do that work for you:
int bytesWrit = 0;
FILE *gpioHndl = fopen("/sys/class/gpio/gpio67/value");
if (gpioHndl != NULL) {
bytesWrit = fprintf(gpioHndl, "1\n");
fclose(gpioHndl);
}
if (bytesWrit != 2) {
HandleError();
}
This is probably the preferred way to do what you want, which is simply writing a fixed value to a file.
In terms of why your execv call isn't working (though it's totally irrelevant if you take my advice above), there are several things you need to be aware of.
First, while some commands are actually files on the disk that you can exec, others may be internal bash commands(a). On my system, for example:
pax:~$ type ftp
ftp is /usr/bin/ftp
pax:~$ type echo
echo is a shell builtin
One way to solve this is to run the actual bash executable (which, being an on-disk command, can be done via exec), telling it to run its internal echo command. From the command line:
pax:~$ bash -c 'echo xyzzy'
xyzzy
Second, if want to use redirection, this is normally something that's done by the shell, not the exec calls or individual executables.
Trying to do redirection via the exec family will generally only result in the >somefile being passed as a literal parameter to the executable (in the argv array), not being used to attach standard output to a file. In other words, it won't work unless the executable specifically handles redirection, which is rare.
So that means you will have to run the shell with redirection and have it run the executable after performing those redirections, even if the command is not an internal one.
Thirdly, if you want the path searched for your executable, execvp is the call you want, not execv (the latter just uses the file you explicitly provide, either relative from the current working directory or an absolute path like /bin/ls). So, in your case, you should either:
use execvp to search the path; or
fully specify the path with execv.
(a) The echo command, while it is bash-internal may also be provided as a separate executable (I believe Posix requires this), so this may not be an issue here. It may be an issue if you expect them to act exactly the same in terms of more esoteric arguments :-)
execv() does not search the PATH environment variable in order to find an executable file. Per the Linux execv() man page (bolded text added):
...
Special semantics for execlp(), execvp(), and execvpe()
The execlp(), execvp(), and execvpe() functions duplicate the actions
of the shell in searching for an executable file if the specified
filename does not contain a slash (/) character. ...
...
So, those three will search the PATH environment variable if the filename passed does not contain a / character.
You're using execv(), which is not one of those three. Therefore, execv() will not search the PATH environment variable.
Since your current working directory doesn't contain an executable file called echo, execv() fails.
You need to use execvp() per the man page.
You need to use absolute path as first parameter in execv
Then, the correct is:
char* args[] = {"/bin/echo","echo", "1", ">", "/sys/class/gpio/gpio67/value", NULL};
But to run what you want (put value '1' in file '/sys/class/gpio/gpio67/value'), you need to use command sh:
char* args[] = {"/bin/sh", "sh","-c", "/bin/echo 1 > /sys/class/gpio/gpio67/value", NULL};
The parameter to "sh -c" is a string. Then, you need to put all command together as a string
In a C program, let's say i wann use Exec functions for executing a given program, for example if i wanna just try ls -l i'll do something like
args[0]="ls";
args[1]="-l";
args[2]=NULL;
...
execvp("ls", args);
and it's all fine. Now what if i wanna also add the redirection to a file (or to stderr)?
I'm stuck, it's obvious that adding >log.txt as a 3rd entry in the array won't work, but I don't know how to proceed.
And also, what if I wanna pass some Input parameters? What if i wanna execute a GCC command like "gcc -o out in redirection>log.txt" ?
[update from comment:]
It's a C program that simulate a shell which can "run strings", string that contains a command, a list o parameters, input and a redirection.
Just set up your file descriptors as the exec-d process shall find them and then do the exec.
For that you need open, dup2 and close.
All functions in the exec-family just replace the current process with whatever one you say.
Run the command in a shell:
char * args[] = {
"sh",
"-c",
"ls -l >out.ls 2>err.ls <in.ls",
NULL
};
...
execvp(args[0], args);
perror("execvp() failed");
I'm writing a shell in C, and I'm having trouble understanding the filepath parameter needed for execvp(filepath,argv).
If the user typed wanted to run ls -a in their current directory ... let's say /home/user1 ... what would be the filepath and argv for running ls in said directory?
Would the filepath be the directory for where the command will be executed from /home/user1 or would it be the command's location /bin/ls?
The filepath argument to execvp() can take either of two forms — it contains a slash or it does not contain a slash.
With slash
The filepath argument specifies the pathname of the executable, either an absolute name (starts with slash) or a relative name (does not start with slash, but does contain a slash). The execvp() function makes no changes to the argument, and the program is executed (or not) assuming that the file named exists and is executable.
Without slash
The filepath arguments specifies a 'simple' name such as ls. The excevp() function then seeks to execute a program with the name ls found in one of the directories listed in the $PATH environment variable. The p in execvp() is for 'path', as in path lookup.
Applying the theory
If the user types ls -a to a shell, the normal way to execute that would be to create an array of character pointers equivalent to:
char *argv[] = { "ls", "-a", 0 };
execvp(argv[0], argv);
The execvp() will now do the path-based analysis and attempt to execute ls from one of the directories listed in $PATH.
If the user types /bin/ls -a to a shell, then the normal way to execute that would be to create an array of character pointers equivalent to:
char *argv[] = { "/bin/ls", "-a", 0 };
execvp(argv[0], argv);
The execvp() will now execute the absolute pathname specified, because that's what the user requested (as opposed to, say, /usr/bin/ls or /usr/local/bin/ls).
Note that the processing is actually the same — you split the command line into words; each word becomes an element of an array of character pointers that is terminated with a null pointer, and you pass the first word to execvp() as the 'filepath' argument, and the whole array as the second argument.
Obviously, a shell can cache the locations of the actual executables, and many shells do, so that execvp() doesn't have to do work trying to find the program (and the shells don't call execvp() but typically call execv() with the absolute pathname of the executable. But that isn't necessary; it is an optimization.
Note, too, that there is nothing to stop you doing:
char *argv[] = { "/honky/tonk/toys", "-a", 0 };
execvp("ls", argv);
Now argv[0] should be "/honky/tonk/toys" and not ls, for all it is the ls executable that is run. What you find in /proc depends on you having /proc on your system (Mac OS X does not support it, for example), but the symlink to the binary should be a link to /bin/ls. On Linux, you're apt to find that ps reports the binary name (ls), even though /proc/PID/cmdline contains the original arguments (so argv[0] is /honky/tonk/toys). Whether this is good or not depends on your viewpoint, but all the world is not Linux.
To run /bin/ls -a in the current directory you would need:
The same as argv passed into main() when a C program starts except NULL terminated, and path is the same as argv[0].
char* argv[] = { "/bin/ls", "-a", NULL };
execvp(argv[0], argv);
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Possible Duplicate:
Writing a shell - how to execute commands
I've been tasked with writing a shell in C. So far I understand that execvp will try to run the program in arg1 with arg2 as parameters. Now it seems that doing this
execvp ("ls", args); //assume args is {"ls", ">", "awd.txt"}
is not equivalent to typing this in the console
ls > awd.txt
I realize that I need to use freopen to achieve the same results but I'm curious what execvp is doing.
The exec family of functions are ultimately a system call. System calls go straight into the kernel and typically perform a very specific service that only the kernel can do.
Redirection, on the other hand, is a shell feature.
So, when one types ls > awd.txt at a shell, the shell first does a fork(2), then it closes standard output in the child, then it opens awd.txt on file descriptor one so that it's the new standard output.
Then, and only then, the shell will make an exec-family system call.
In your case you simply passed the strings > and awd.txt to the exec system call, and from there to ls. BTW, be sure you terminate your execvp arg array with a null pointer.
Note: As you can see, the redirection operators are never seen by the executed program. Before Unix, directing output to a file had to be done by every program based on an option. More trivia: most programs never know they were redirected, but ironically, ls does check to see if its output is a tty, and if so, it does the multi-column formatted output thing.
It's executing ls with 2 arguments: > and awd.txt. It is equivalent to running:
'ls' '>' 'awd.txt'
You can pass your command directly to the shell:
char * const args[] = { "sh", "-c", "ls > awd.txt", NULL};
execvp("/bin/sh", args);
But it doesn't seems like a good idea.