Hi!

I have a aquestion about the unix "." (period" command)

as far as i know,that command is used to execute a shell script within the current shell,instead of creating a new subshell.

Now...
When i use that command with any of my shell scripts,it works fine.

however,when i use it with any other executable file,it says "command not found".
That happens even when the programm i'm trying to execute is within my "PATH" directories.

So..
Is this command used ONLY for shell scripts?

If yes,how does the shell distinguish between shell scripts and other executable files?

I'm asking that,because i always use "chmod +x' on my scripts to make them executable...

So...I imagine that the shell has to know if a file is a shell script or not,in order to use or not the "." command"..

right?


Thanks a lot!

You're absolutely correct:

1. Yes, the "." is used by many *nix shells to execute the shell script in the current shell rather than creating a subshell). This is the only way, for example, you can read a script that defines variables in your current shell.

2. And yes, the first argument after the "." must be the name of a shell script. It cannot be a binary.

Why?

Because the shell is expecting a text script (not an executable binary) ... and it tries to *read*, and then to *parse* that script. And of course you don't parse a binary: you execute it.

Try it:
- vi hello.c =>
#include <stdio.h>

int
main (int argc, char *argv[])
{
return printf ("Hello world!\n");
}

- cc -o hello hello.c

- ./hello =>
Hello world!
<= SO FAR, SO GOOD. WE'VE GOT A BINARY, AND WE CAN EXECUTE IT.

- od -cx hello =>0000000 177 E L F 001 001 001 \0 \0 \0 \0 \0 \0 \0 \0 \0
457f 464c 0101 0001 0000 0000 0000 0000
0000020 002 \0 003 \0 001 \0 \0 \0 \0 203 004 \b 4 \0 \0 \0
...
<= YOU'LL NOTICE THAT THE FIRST 4 BYTES OF ANY LINUX EXECUTABLE
ARE THE CHARACTERS "\0x7fELF"

- . ./hello =>
bash: ELF: command not found
<= AND YOU'LL SEE THAT THE "BASH" SHELL TRIED TO PARSE THE
"TEXT" IN THE (BINARY!) FILE "HELLO" ... AND FAILED TO UNDERSTAND IT

Your mileage may vary depending on exactly which shell you're using, and exactly how you type the command. But the bottom line is "Yes, the '.' requires a shell script."

Hope that helps .. PSM

thanks so much!
i was really confused but your answer really makes sense!


but i have one more question.

what happens when we just type "prompt> ./<some-shell-script"?

obviously our shell forks a new shell to execute the script.

however,how does the shell know how to handle the file?
it might be a a c program or simply a script.

does it examine again the first bytes of the file to determine the file type?

thanks a lot!

Think of ". some_shell_script" as more of an include cmd, rather than actually running a program.

As you rpobably know, the "dot" in "./somescript" is a totally different one from when your do ". somescript".

". somescript" includes the shell-commands in the same shell process, and "./somescript" or "./some_c_program" runs the mentioned program in a new (child-) process of the shell.

". somescript": here the dot is a shell command.
"./somescript": here the dot indicates the directory where the program or script to run resides.

"./somescript": here the shell tries to run (execute) somescript, and if the file "somescript" has execute permissions (the x's you see when you do "ls -l") it will run. The shell check the first bytes, and if they are "#!" it acutally executes the binary executable (ELF file, say "C-program") mentioned in the same line. If the bytes are not "#!" then try to make the system (kernel) execute it directly as a binary executable.