Daemon process in UNIX

Daemon process

At   TTY – if we observe ‘?’ , it indicates it a Daemon  process, which does not need any input and automatically comes  into life when the OS is switched ON.   And  the processes can be seen using commands.   Gnome – terminal Is the parent for the bash.   Example on System () call Vi p1.c #include <stdio.h> #include <stdlib.h> Main () { Print f (“Process id = % d………parent id = %d /n”,  get p id (), get p p id ()) System (“./p2 ”);// p2 is executable file of p 2.c Print f (“After system () call came back to process 1 ”); }   Vi p2.c   #include <stdio.h> #include <stdlib.h> Main () { Print f (“Process id = % d………parent id = %d /n”,  get p id (), get p p id ()); } Cc  -0 p2              p2.c Creates executable for p2.c with name p2.     Output      cc p1.c # ./a.out Process  1 id = 2963 ……….parent id = 2767 à this is PID of gnome – terminal Process 2 id = 2964……….parent id = 2763 à this is PID process 1 as process1, called process 2 After System () call came back to process 1 .   $echo $PATH / user/ Kerberos/bin : /ur/local/bin :/bin : /user/bin : /home /b59p2/bin These  are the paths available in the terminal if we have two executes like ls, cal etc……… Simply by $ ls, $ cal we can execute, but even through  a. out is an executable , $ a. out does not executes why ? $ . pwd /home/b59p2/UNIX/Sys – exec This is path of you’re a .out, Which is not available in above shown paths. Hence, it searches for a. out there in above paths  & hence not found, Results, bash : a. out : command not found. Now to execute directly like # a. out , we have two solutions.

1. Copy a. out in any of directory path

# cs    /home/b59p2/UNIX/Sys- exec/ a. out   /home /b59p2/bin # a. out

2. Another way is to update the PATH variable

# PATH = $ PATH : /root/b59p2 /UNIX/sys-exec # a. out

1. $ / bin/sh -c command
2. $ command

Ex :-  $/bin / sh  -c ls $ ls                                   Both generates same output   Here the major difference is that, in first command  bash creates sh and executes command is returns.   In 2^nd  statement executes command in present shell Bash is parent.    

 Assignments

1. create an executable named “cme exec ” , which when used with the i/p command , should be also to execute the command specified..the command is provide from the command line Vi ass1.c

Using System () call   #include   <stdio . h> #include   <stdlib . h> Main (int  argc, char ** argc ) { If (argc ! = 2) { Print f (“usage ./cmd exec  <command >” ); Return; } System ( arg v [1]); } creating  executable cc         -0          cmd exec          ass1.c   Output           ./cmd exec ls

1. a. out ass1.c cmd exec

p1.c p2 p2.c    

Using exec () call

  #include <stdio. h> #include <stdlib. h> Main (int arg c , char ** arg v) { If  (argc ! 2) { Print f (“Usage :  ./cmd exec          <command> ”); Return ; } Execlp (  args v [1], argv [1], NULL); }  

1. Q. a. p to accept any number of commands from the command line and execute them in the same sequence as they are provided.

Using system () call

  #include <stdio. h> #include <stdlib. h> Main (int arg c , char ** arg v) { Int I ; If  (argc ==  2) { Print f (“Usage :  ./a. out         <command 1>   <command 2> ………….”); Return ; }   For (I =1 ; argv [i] ; I ++ ) System (arg v [i] ); }   Output ./a .out ls date cal

1. out ass1.c ass2. C ass3. C cmd exec  cmd sys p1.c p2 thu Aug 12  00:28:23 1^st 2010

August 2010 Mon   tue wed  thu   fri  sat