CS 170 Spring 05 JShell Lab

1
CS 170 Spring 05JShell Lab
2
What is a Shell?

• A shell is a command line interpreter it reads
  lines typed by the user and interprets them as
  requests to execute other programs.

Applications
Text editor
Calculator
Shell
Operating System
Machine (Hardware)
3
What is a Shell?

• Shell is a just another application it has
• Input
• Does something (Processing)
• Output

4
Shell Input

• Commands typed at the prompt
• Needs to be parsed ? parse.h
• int parse(CMD )
• ex. error_code parse(commands)
• int cmd_free(CMD )
• Specail input -gt exit or CTRL-D
• CTRL-C check signal(SIGINT, SIG_IGN)

5
Shell Input

• int parse(CMD )
• CMD is a pointer to a pointer of CMD struct.
• / command structure /
• typedef struct command_struct
• struct command_struct prevCmd
• struct command_struct nextCmd
• char file
• char argv
• char in_redir
• char out_redir
• int appending / flag, non-zero append, zero
  truncate /
• int background / whether the line terminates
  with an /
• CMD

6
Shell input

• Very important
• Check for the return value from parse(). You have
  to handle errors and your shell must report them.
• Before accessing any pointer, make sure it is not
  NULL. Using a Null pointer will make your code
  Seg fault.

7
Shell does something (Processing)

• Calling other applications.

Applications
Text editor
Calculator
Shell
..
fork
dup
exec
Operating System
Machine (Hardware)
8
Shell does something (Processing)

• First you need to fork a new process to run the
  requested program by the user.
• ? use fork().
• How to tell which is which?
• Use getpid() or fork-return value. 0? Yes
  child, No Parent

include ltstdio.hgtint main()   pid
fork()   printf("\nHello World\n)   return
0
9
Shell does something (Processing)

• Second Shell asks the OS to run the
  user-requested program
• ?Use execvp()
• Recall difference between background processes
  and foreground ones !!
• Execvp-ed cmd input/output should be directed
  according to user request for that cmd.
• ? Use dup2()

10
Shell output

• Your Prompt ? Waiting for a new cmd.

11
General Remarks

• Use perror() to report the sys-calls error .
• Zombie processes.
• ? Use waitpid() or wait4()
• Test your code on CSIL machines
• Apply test codes.
• Provide a README file.
• Late assignments will receive a ZERO. Submit what
  you have before the deadline.

12
Zombie processes check

• Command ps a
• Zombie will show as ltdefunctgt
• Sample
• Input
• cs170_at_ella tmp2 ./jshell
• jshell sleep 3
• Output
• cs170_at_ella tmp2 ps -a
• 13858 pts/30 000000 jshell
• 13896 pts/30 000000 sleep ltdefunctgt ?
  Shouldnt show up if waitpid
• 13905 pts/1 000000 ps
  is correctly implemented.