The Shell

• The shell is a special purpose program designed to read commands typed by the user and execute appropriate programs in response to those commands
• Sometimes called a command interpreter
• It is the thing you type to much of the time. (Not the editor)
• Wikipedia says that a shell is either command line or gui
• Unix/LINUX is different from other os where in linux the shell is independent of the kernel.
• In fact, there are multiple shells
  • Bourne Shell (/bin/sh) (perhaps)
  • c shell (csh)
  • korn shell (ksh)
  • Bourne again shell (bash)
• Things shells do (via bash.
  • globbing - filename expansion
    • You know about this
    • The use of wildcards similar to regular expressions
      • *: matches anything,
        • sort of.
        • does not match things starting with a .
      • ?: matches a single character
      • [...]:
        • matches any one enclosed character.
        • [abc] matches the files a, b or c
        • [abc]* matches any file beginning with a,b, or c
        • [a-f] is the same as [abcdef]
        • [!abc] is [d-Z]
  • Job Control
    • processes can be run in the background, or suspended
      • The process is detached from the terminal
      • If it attempts to perform i/o to the terminal it is suspended
    • ctrl-z
    • bg
    • fg
    • prog &
    • jobs
    • kill %d
  • Command execution
    • Simple Commands : words delimited by spaces
      • ls -alrt a b c d
      • Bash will identify the command (ls)
      • And the arguments (-alrt a b c)
      • And invoke the command with the arguments
      • We will see how to do this later.
    • Pipelines
      • cmd arg arg | cmd arg arg ...
      • The shell will send the output of the first command as input to the second command.
      • We will see how to do this later
    • Lists
      • delimited by ;
      • cmd1; cmd2; ...
      • uptime; who
      • cmd1 & cmd2
      • cmd1 && cmd2
        • cmd1 is executed
        • If it has an exit status of 0 then cmd2 is executed
      • cmd1 || cmd2, same but cmd1 returns non 0
  • I/O redirection
    • cmd > file (send stdout to file)
    • cmd < file (get stdin from file)
    • cmd 2> file (standard error to file.)
    • cmd > file 2>&1 (both stdout and stderr to foo)
    • cmd >> file (append stdout to file)
    • cmd &>> file (append stdout and stderr to file)
    • There is much more here, but this will do.
  • Programming commands
    • Variables, printenv
    • loop and conditional structures
    • functions
• Shells can be used to execute programs.
• But more on this to come.