commands works only through console on embedded ARM system

[secondmouse]

While working on an ARM based embedded system (TS-TPC-7395 ARM Touch Panel Computer w/ 7-inch TFT-LCD and PoE) I encountered something weired: there are a couple of commands: lcd_on, lcd_off in this system that are used to turn on and off the touch screen. The problem is I can only execute them in the shell through the console. If I try to use it via system("lcd_on") in a c program, I got:

sh: lcd_on: not found

I've checked $PATH, ~/.bashrc, /etc/bash.bashrc but just couldn't find either executable lcd_on or function definition of lcd_on. Also tab completion does not work for those two commands either: As we know in bash after you type in the leading characters of a command, the tab key will bring up all commands starting with the word typed, it's not the case for those two commands. Does anyone know how to locate commands that are not in the $PATH or defined as functions?

[drl]

Hi.

Perhaps they are builtins. For example:

$ bash -version
GNU bash, version 3.2.39(1)-release (x86_64-pc-linux-gnu)

$ type -a ssh
ssh is /home/drl/bin/ssh
ssh is /usr/bin/ssh
ssh is /usr/X11R6/bin/ssh

$ type gcc
gcc is /usr/bin/gcc

 $ type printf
printf is a shell builtin

See more at help type. Best wishes ... cheers, drl

[secondmouse]

drl,
Thanks for the detail explanation - I learned something new again. Now I know that's a shell built-in command, is there any way I can call it in my C program?

[drl]

Hi.

This should give you an idea of the combinations of things that work and don't work. It compares the OS utility printf (/usr/bin/printf) with the bash builtin printf:

#include <stdio.h>

/* t.c - Demonstrate system, bash, command, builtin. */

int main()
{
	void slew();

	slew();
	system("printf ' Hello, world from printf.\n'");

	slew();
	system("command printf --version");

	slew();
	system("builtin printf --version");

	slew();
	system("printf --version");

	slew();
	system("/usr/bin/printf --version | head -1");

	slew();
	system(" builtin printf ' Hi there.\n'");

	slew();
	system(" builtin printf ' Hello, world from builtin printf.\n'");

	slew();
	system("bash -c \" builtin printf ' Hi there.\n'\"");

	return( 0 );

}
void slew()
{
  printf("\n");
}

when compiled and executed, produces:

 Hello, world from printf.

sh: line 0: printf: --: invalid option
printf: usage: printf [-v var] format [arguments]

sh: line 0: printf: --: invalid option
printf: usage: printf [-v var] format [arguments]

sh: line 0: printf: --: invalid option
printf: usage: printf [-v var] format [arguments]

printf (GNU coreutils) 6.10

 Hi there.

 Hello, world from builtin printf.

 Hi there.

Best wishes ... cheers, drl

[secondmouse]

Hi drl,
Thanks for the illustration. Regarding the lcd_on command, I found out it's a function defined in one of the script files. So I create another script wrapper (named LCD_ON) with that function defined:

$ cat LCD_ON
lcd_on() {
 ...
}
lcd_on

Then I run system("LCD_ON") directly from my c program. Thanks again for the help.