I'd like to find a way to find out if a program is installed using C gnu11 standard, for example, will the command 'Rscript' work if it is executed using the system function in C.
I'm not a programmer by training, so I'm not sure if I'm using the correct search terms.
I would like the C equivalent of
con@con-Inspiron-3521:~/Scripts$ which Rscript
/usr/bin/Rscript
how can I do that in C?
With a similar effort you can just parse the PATH variable and check if the given file exists and is executable:
#include <stdbool.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
bool can_run_command(const char *cmd) {
if(strchr(cmd, '/')) {
// if cmd includes a slash, no path search must be performed,
// go straight to checking if it's executable
return access(cmd, X_OK)==0;
}
const char *path = getenv("PATH");
if(!path) return false; // something is horribly wrong...
// we are sure we won't need a buffer any longer
char *buf = malloc(strlen(path)+strlen(cmd)+3);
if(!buf) return false; // actually useless, see comment
// loop as long as we have stuff to examine in path
for(; *path; ++path) {
// start from the beginning of the buffer
char *p = buf;
// copy in buf the current path element
for(; *path && *path!=':'; ++path,++p) {
*p = *path;
}
// empty path entries are treated like "."
if(p==buf) *p++='.';
// slash and command name
if(p[-1]!='/') *p++='/';
strcpy(p, cmd);
// check if we can execute it
if(access(buf, X_OK)==0) {
free(buf);
return true;
}
// quit at last cycle
if(!*path) break;
}
// not found
free(buf);
return false;
}
strchr is for the weak who loop over strings twice - once in strchr, once in strcpy. ;-) Just joking, it just felt more natural to write like this.
malloc()'s return value is also very bad.malloc cannot fail on Linux systems (unless you ask for more than physical RAM + paging file), and in general I have yet to see any reasonably sized application deal "correctly" (whatever that means) with an out of memory. If malloc returns NULL for small allocations like this there's nothing you can reasonably do to save yourself, just crash. But still, if you prefer, I'll just add a useless check for that.The which program probably does a search for the file in any of the directories specified in the PATH environment variable.
There are many ways to do this, this is one such ways
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stddef.h>
#include <sys/stat.h>
int
main(int argc, char *argv[])
{
const char *head;
const char *command;
size_t length;
if (argc < 2) {
fprintf(stderr, "insufficient number of arguments\n");
return -1;
}
command = argv[1];
length = strlen(command);
// Get the PATH environment variable
head = getenv("PATH");
if (head == NULL) {
fprintf(stderr, "the PATH variable was not set, what?\n");
return -1;
}
// Find the first separator
while (*head != '\0') {
struct stat st;
ptrdiff_t dirlen;
const char *tail;
char *path;
// Check for the next ':' if it's not found
// then get a pointer to the null terminator
tail = strchr(head, ':');
if (tail == NULL)
tail = strchr(head, '\0');
// Get the length of the string between the head
// and the ':'
dirlen = tail - head;
// Allocate space for the new string
path = malloc(length + dirlen + 2);
if (path == NULL)
return -1;
// Copy the directory path into the newly
// allocated space
memcpy(path, head, dirlen);
// Append the directory separator
path[dirlen] = '/';
// Copy the name of the command
memcpy(path + dirlen + 1, command, length);
// `null' terminate please
path[dirlen + length + 1] = '\0';
// Check if the file exists and whether it's
// executable
if ((stat(path, &st) != -1) && (S_ISREG(st.st_mode) != 0)) {
if ((st.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH)) == 0) {
fprintf(stdout, "found `%s' but it's not executable\n", path);
} else {
fprintf(stdout, "found at: %s\n", path);
}
}
// Don't forget to free!
free(path);
// Point to the next directory
head = tail + 1;
}
return 0;
}
in general the same algorithm
PATH environment variable, which consists of a sequence of directory paths separated by a : character.Check if,
S_IXUSR | S_IXGRP | S_IXOTH
popento runwhich Rscriptand capture the output.