CS 497C – Introduction to UNIX
Lecture 22: - The Shell
Chin-Chih Chang chang@cs.twsu.edu

/dev/null and /dev/tty: Two Special Files
* Quite often, you may want to test whether a program runs successfully without 
  seeing the output or the error messages on the screen.
* You have a special file that simply accepts any stream without growing in 
  size - the file /dev/null:
$ cal chap100 2>/dev/null
$ cat /dev/null

Pipes
$ who > user.lst
$ wc -l < user.lst
* The method of using two commands in sequence has certain obvious disadvantages:
 - The process is slow.
 - The intermediate file is need.
 - Temporary files can be very large.
* You can use the | (pipe) to connect two commands. 
$ who | wc -l

Pipes
* who is said to be piped to wc.
* When a sequence of commands combined together with one or more | symbols, a 
  pipeline is said to be formed. 
* You can use the col -b command to remove control characters from the 
  manual pages.
man grep | col -b > grep.man
* grep locates lines containing a specified pattern in its input.
$ cat * | grep "print"

tee: Splitting a Stream
* The UNIX tee command breaks up its input into two components; one component is 
  saved in a file, and the other is connected to the standard output.
* Being also a filter (uses standard input and standard output), tee can be 
  placed anywhere in a pipeline.
$ who | tee user.lst
$ who | tee user.lst | wc -l

Command Substitution
* The shell enables the argument of a command to be obtained from the standard 
  output of another. This feature is called command substitution.
* When scanning the command line, the ` (backquote or backtick) is another 
  metacharacter that the shell looks for. 

Command Substitution
* If the ` pair is found, the shell executed the command enclosed in the ` pair, 
  and replaces the enclosed command line with the output of the command.
$ echo Today is `date`
Today is Thu Oct 11 22:08:36 CDT 2001
* You can use this feature to generate useful message.
$ echo "There are `ls | wc -l` files"

Shell Variables
* A shell variable is of the string type, which means that the value is stored in
  ASCII rather than in binary format.
* A shell variable take on the generalized form variable=value (except in the C 
  shell).
$ x = 37; echo $x
37
$ unset x; echo $x

Shell Variables
* The C shell uses the set statement set variables.
set x = 10
* You can set a pathname or a command to a variable or substitute to set the 
  variable.
$ dir="ls -Fax"; $dir
$ mydir=`pwd`; echo $mydir

Shell Scripts
* Try executing the file containing these commands by invoking the filename:
$ date-dir.sh
date-dir.sh: Permission denied.
* Executable permission is usually necessary for any shell script to run.
$ chmod u+x date-dir.sh
$ date-dir.sh

Shell Scripts
* A shell script is an ordinary file containing a set of commands, which is 
  executed in an interpretive manner. Also known as a shell program or shell 
  procedure.
* The date-dir.sh shell script has a sequence of three commands.
directory=`pwd`
echo The date today is `date`
echo The current directory is $directory

The Shell's Treatment of the Command Line
* The shell processes the command in the following order:
 - Parsing - break up the command into words.
 - Variable evaluation - A words preceded by a $ are evaluated as variables.
 - Command substitution - Any command in back quotes is executed and replaced.
 - Redirection – redirect input and output.
 - Wild-card interpretation - scan the command line for wild cards and match 
   the file list.
 - Path evaluation - search for the command. 

The Other Shells
* The original UNIX system came with the Bourne shell. The Korn shell (ksh), 
  the bash (bash), and the C shell are widely used in UNIX.
* Korn and bash maintain near-complete compatibility with the Borne shell. 
* Because commands and integer and string handling features are built in, 
  programs run under Korn and bash execute faster than under Bourne.