Chapter 3 The UNIX Shells

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• Chapter 3 The UNIX Shells

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Introduction

• What is shell ?
• command interpreter
• an interface between a user and the raw os
• What kinds of shell are available in UNIX
• Bourne shell
• Korn shell
• C shell

Korn shell
C shell
Bourne shell
core
core
3
Selecting a Shell

• Shell functionality
• built-in commands, scripts, variables,
  redirection
• wildcards, pipes, sequences
• subshells, background processing, command
  substitution
• Selecting a shell
• The system administrator chooses a shell for you.
• /etc/passwd
• Full pathname of Shells
• Bourne /bin/sh , /bin/bash
• Korn /bin/ksh
• C shell /bin/csh

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Selecting a Shell

• How to change your shell ?
• chsh
• Changing login shell for chang
• Old shell /bin/csh
• New shell /bin/ksh
• logout
• login chang
• passwd

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Shell sequence

read a startup file in the user's home directory
display a prompt and waits for a user command
execute the user's command
Control-D
Terminate
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Startup Files

• Bourne shell /bin/sh
• .profile
• .login
• Bourne again shell /bin/bash
• .bashrc
• .login
• C shell /bin/csh
• .cshrc
• .login

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Executable files vs. built-in commands

• several built-in commands in Shell
• echo -n arg
• cd directory
• utility programs (a file with execute permission)
• stored in a directory
• Shell locates and executes utilities usually in
  shell variable PATH
• /bin, /usr/bin, /usr/local/bin, /usr/ucb, /etc,
  /usr/bin/X11
• ls ... /bin/ls

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Output Redirection

• store the output to a file
• command gtfileName
• ls gt sample.txt
• append
• commandgtgtfileName
• cat gtgt sample.txt

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Input Redirection

• Input redirection
• execute command using the file as its standard
  input
• command lt fileName
• elm chang lt sample.txt
• Here documents
• command ltlt word
• word

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Pipe

• Pipes
• the standard output of command1 as the standard
  input of command2
• command1 command2
• ls wc -w

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Command substitution

• Command substitution
• A command surrounded by backquote() is replaced
  by its
• standard output.
• echo the date today is date

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Combination of commands

• Grouping
• (command1 ... commandn)
• executed by a child shell (subshell)
• shares the same standard input, standard output,
  and standard error
• redirected and piped as a single command
• date ls pwd gt out.txt
• (date ls pwd) gt out.txt

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Combination of commands

• Sequences
• command1 commandn
• the shell executes them in sequence, from left to
  right
• Conditional sequence
• command1 command2
• command2 is executed only if command1 completes
  successfully (return 0)
• cc my prog.c a.out
• command1 command2
• command2 is executed only if command1 complete
  unsuccessfully
• cc myprog.c echo compilation failed

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Filename substitution

• Filename substitution
• specify several files using wildcards
• Example
• ls .c
• ls /
• ls ac
• cc .c
• Wildcard Meaning
• Matches any string, including the empty
  string
• ? Matches any single character
• .. Matches any one of the characters between
  the brackets.
• A range of characters may be specified.

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Background processing

• Background
• Examples
• find . -name b.c -print
• 27174
• date pwd
• 27310
• 27311

command
command
shell
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Background processing

• Redirecting background processes
• To prevent the output from a background process
  from arriving at your terminal
• redirect output to a file
• find .-name a.c -print gt find.txt
• mail output to your self
• find . -name a.c -print mail glass
• redirecting input
• mail glass lt inputfile

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Shell Script

• What is Shell script ?
• 1.A file that contains a sequence of shell
  commands
• 2. Need execute permission.
• 3. Run it by just typing its name
• Which shell script
• 1. C shell script, if the first line is
• 2. If the first line is !pathName,
• the executable program pathName is
• used to interpret the script.
• 3. Otherwise, it is Bourne shell script.

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Shell Script Example

• script.csh
• This is sample C shell scripts
• echo -n the date today is
• date
• script.ksh
• !/bin/ksh
• This is sample Korn shell scipt.
• echo -n the date today is
• date
• ?? ??
• chmod 700 script.csh script.ksh

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Subshell

• ? An initial login shell
• when you log into UNIX
• ? When your current shell(parent) creates a new
  (child) shell ?
• 1. When a grouped command is executed, such as
  (ls pwd date).
• 2. When a script is executed.
• 3. When a background job is executed.

Parent
Child
Parent
Child
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Environment variables vs Local variables

• ? Every shell contains two data areas
• 1. environment variable space
• A child shell inherits a copy of its parent's
• 2. local variable space
• A child shell get a clean local variable space

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Environment variables

• Predefined environment variables
• echo HOMEHOME, PATHPATH, MAILMAIL

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Local variables

• ? Local variables
• - shell variables except predefined environment
  variables
• - export make local variables environment
  variables
• ? local built-in variables

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Example

• firstnameGraham
• lastnameGlass
• echo firstname lastname
• export lastname
• sh
• echo firstname lastname
• D
• echo firstname lastname

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Quoting

• ? To inhibit the shell's wildcard replacement,
  variable substitution, and command substitution
• 1. single quote (') inhibit wildcard
  replacement, variable substitution, and command
  substitution
• 2. double quote (") inhibit wildcard replacement
  only
• 3. when quotes are nested, the outer quote have
  any effect.

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Quoting

• ? Examples
• echo 3 4 12
• echo "3 4 12"
• echo '3 4 12'
• set name Graham
• echo 'my name is name and the date is 'date'
• echo "my name is name and the date is 'date' "

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Job Control

• ? Process status ps
• ps -arux
• list process status information, by default your
  processes.
• -a option all processes
• ? sleep seconds
• sleeps for the specified number of seconds and
  then terminate
• ? wait pid
• The shell suspends until the child process with
  pid terminates.
• If no arguments are supplied, the shell waits
  for all its child processes

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Job Control

• ? Signalling process kill
• kill -signalId pid kill -l
• 1. Kill(TERM signal) process or
• 2. sends the signal with code signalId to the
  list of numbered processes.
• (sleep 10 echo done)
• ps
• (sleep 10 echo done)
• 27390
• kill -9 27390
• sleep 30 sleep30 sleep 30
• kill 9 0