On Tuesday 25 July 2006 16:33, Alan McKinnon wrote:
> Um, no. Read my post again. The command 'test' and the command '['
> have *different* syntax so cannot possible be links to each other and
> still have it work. The command does behave differently depending on
> the name it is called with, but this does not change the syntax used
> on the command line that invokes it.
There are two cases:
[1] syntax checking is done by the shell, or
[2] syntax checking is done by the program.
The interesting case is [2]. Let's make an oversimplified example.
Suppose you want the commands "commandA" and "commandB".
The syntax for commandA is
commandA <arg1> <arg2>
but commandB takes a third argument, so you invoke it with
commandB <arg1> <arg2> <arg3>
This, to me, qualifies as "*different* syntax".
Here is how those checks can be implemented _using the same program_.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[]) {
char *progname;
/* the following should actually extract the basename of
the program from argv[0], but to keep things simple
let's assume that the shell does this for us */
progname = argv[0];
if (strcmp(progname, "commandA") == 0) {
/* check that we have TWO arguments */
if (argc == 3) {
printf("Ok, running as commandA with two arguments\n");
exit(0);
} else {
printf("Running as commandA, but with the wrong number of
arguments\n");
exit(1);
}
} else if (strcmp(progname, "commandB") == 0) {
/* check that we have THREE arguments */
if (argc == 4) {
printf("Ok, running as commandB with three arguments\n");
exit(0);
} else {
printf("Running as commandB, but with the wrong number of
arguments\n");
exit(1);
}
} else {
printf("Bad program name!\n");
exit(1);
}
}
In fact, as I said in a previous post, [ and test are built from the same
source file, and the "[" vs. "test" difference is used only to check for
proper syntax.
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