How to automatically prefill command on the Linux bash

Linux Bash is one of the most famous Linux shells. It offers a great number of features like for example spawning and controlling process, redirecting streams, supporting scripts and a flexible way to control you editing line.

Problem

How to automatically pre-populate a command on the shell after prompt.

Solution description

The shell has tree default streams: stdout, stdin and stderr. By manipulating the stdin of the process we can simulate typing a command.

Reference implementation

The original script can be found here: https://github.com/rtomaszewski/experiments/blob/master/type-command.c

	#include <strings.h>
	#include <string.h>
	#include <sys/ioctl.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>
	#include <termios.h>
	#include <stdio.h>
	#include <unistd.h>
	#include <stdlib.h>
	const char *TYPE_CMD_ENABLED = "TYPE_CMD_ENABLED";
	const char *TYPE_CMD_TYPE = "TYPE_CMD_TYPE";
	const char *PROGRAM_NAME = "type-command";
	int main(void) {
	struct termios oldtio,newtio;
	int fd=0;
	int i;
	char *is_enabled;
	char *command;
	// read the bash variables
	is_enabled = getenv(TYPE_CMD_ENABLED);
	command = getenv(TYPE_CMD_TYPE);
	// is the var defined at all
	if ( ! is_enabled ) {
	printf("%s: the variable %s is not set, set it to 'no' to surpress this message\n", PROGRAM_NAME, TYPE_CMD_ENABLED);
	return 0;
	// has the variable value different than 'yes'
	} else if ( strcmp(is_enabled, "yes") ) {
	return 0;
	}
	if ( ! command ) {
	return 0;
	}
	tcgetattr(fd,&oldtio); // save current terminal settings
	bzero(&newtio, sizeof(newtio)); // clear struct for new port settings
	memcpy(&newtio, &oldtio, sizeof(oldtio) );
	newtio.c_lflag &= ~ECHO; // do not echo the input chars
	tcflush(fd, TCIFLUSH); // set the new terminal settings
	tcsetattr(fd,TCSANOW,&newtio);
	for (i = 0; i < strlen(command); i++)
	ioctl(fd, TIOCSTI, &command[i]);
	tcsetattr(fd,TCSANOW,&oldtio);
	return 0;
	}

view raw gist-type-command.c hosted with ❤ by GitHub

Demonstration

• Compile first the program

gcc -o type-command type-command.c

• Run for the firs time

# ./type-command
type-command: the variable TYPE_CMD_ENABLED is not set, set it to 'no' to surpress this message; set the TYPE_CMD_TYPE for the command to type

Example: export TYPE_CMD_ENABLED=yes; export TYPE_CMD_TYPE=date

• Export the variable to controls if the program should try to type a command or not

# export TYPE_CMD_ENABLED=yes
# ./type-command
#

• Specify the command that you wish to be typed

# export TYPE_CMD_ENABLED=yes; export TYPE_CMD_TYPE=date
# ./type-command
# date
Sun Mar 30 19:27:55 UTC 2014>

References

http://stackoverflow.com/questions/10866005/bash-how-to-prefill-command-line-input
http://stackoverflow.com/questions/11198603/inject-keystroke-to-different-process-using-bash
http://unix.stackexchange.com/questions/48103/construct-a-command-by-putting-a-string-into-a-tty

http://fossies.org/linux/misc/old/console-tools-0.3.3.tar.gz%3at/console-tools-0.3.3/vttools/writevt.c

http://man7.org/linux/man-pages/man4/tty_ioctl.4.html
http://man7.org/linux/man-pages/man3/tcflush.3.html
http://www.tldp.org/LDP/lpg/node143.html