The Shell

Figure 2.15: The shell

• Whenever you login to a Unix system you are placed in a program called the shell. The shell is perhaps the most important program on the UNIX system, from the end-user's standpoint.
• The shell is your interface with the UNIX system, the middleman between you and the kernel. The shell acts as a command interpreter; it takes each command and passes it to the operating system kernel to be acted upon. It then displays the results of this operation on your screen.
• The shell is a program that the UNIX kernel runs for you. Many basic shell commands are actually subroutines built in to the shell program. The commands that are not built in to the shell require the kernel to start another process to run them.
  • Features provided by the shell; create an environment that meets your needs, write shell scripts, define command aliases, manipulate the command history, automatically complete the command line, edit the command line, etc.
  • Special characters in UNIX; UNIX recognizes certain special characters as command directives. If you use one of the UNIX special characters in a command, make sure you understand what it does. The special characters are:
    / $<$ $>$ ! $ % ^ & * $\vert$ { } ~ and ;
    When creating files and directories on UNIX, it is safest to only use the characters A-Z, a-z, 0-9, and the period, dash, and underscore characters.
  • Getting help on UNIX; to access the on-line manuals, use the man command, followed by the name of the command you need help with. EXAMPLE: Type
    • $man ls ; to see the manual page for the ``ls'' command.
    • $man man; to get help on using the manual.
• Description of different types of shell; there are several different shells available for Unix. You can use any one of these shells if they are available on your system. And you can switch between the different shells;
  • Bourne shell (sh),
  • C shell (csh),
  • TC shell (tcsh),
  • Korn shell (ksh),
  • Bourne Again SHell (bash).
  Changing your shell; you can switch to another shell for the remainder of your login session. To do this enter the shell command name at the system prompt. For example:
  $csh This switches you from your current shell to the C shell.
• The environment variables are passed to all programs that are not built in to the shell, and may be consulted, or modified, by the program. By convention, environment variables are given in upper case letters. To view all the environment variables, use the command ;
  $printenv
  You can also view a particular environment variable using the echo command:
  $echo $TERM
  The above command echos the value of the TERM environment variable to the standard output. Important environment variables
  • TERM; environment variable defines the type of terminal that you are using. Most UNIX systems have a database of terminal types, and the capabilities of each terminal type.
  • PATH; variable contains the names of directories to be searched for programs that correspond to command names. When you issue a command to the shell, the shell searches sequentially through each directory in the PATH list until it finds an executable program with the command name you typed,
  • USER; variable contains your username. Any time you access a file or directory, the access permissions are checked against the value of USER,
  • HOME; variable contains the name of your home directory. When you issue the cd command with no directory argument, you will be placed in the directory defined in the HOME environment variable. The HOME variable is also where the shell will look for the user login scripts,
  • EDITOR; variable is used by programs that must invoke a text editor to provide the ability to edit or compose documents,
  • HOST; environment variable contains the name of the host machine that is running your shell program. When you connect to a remote host through telnet or ftp, the name of your host is relayed to the remote machine, so the administrators of the remote machine can keep track of who is connecting, and from where.
• Setting environment and shell variables; the exact mechanism for setting the environment and shell variables depends upon the type of shell you're using;
  • sh, or ksh; EDITOR=emacs;export EDITOR,
  • csh; setenv EDITOR emacs.
• Summary of shell facilities; See table 2.1.
  

  Table 2.1: Summary of shell facilities

  Facility	Bourne	C	TC	Korn	BASH	Explanation
  command history	No	Yes	Yes	Yes	Yes	(Allows previous commands to be saved, edited, and reused)
  command alias	No	Yes	Yes	Yes	Yes	(Allows the user to rename commands)
  shell scripts	Yes	Yes	Yes	Yes	Yes	(Allows the user to shell programming)
  filename completion	No	Yes*	Yes	Yes*	Yes	(Allows automatic completion of partially typed file name)
  command line editing	No	No	Yes	Yes*	Yes	(Allows the use of an editor to modify the command line text)
  job control	No	Yes	Yes	Yes	Yes	(Allows processes to be run in the background)

  
  The sign * means which is not the default setting for this shell.