Unix Shells

1
Unix Shells

• References
• man pages
• Ray and Ray, "Visual QuickStart Guide Unix"
• Watters, Chapter 11

2
Overview

• Purpose of shells
• Common shells
• Shell commands
• bash
• POSIX environment variables
• Configuring bash shell using .bashrc

3
Purpose of Shells

• Interface between user and kernel
• User may be person or program
• Provide access to system calls

4
Common Shells

• Bourne Again SHell (/usr/bin/bash) sh-compatible
  and incorporates features of Korn and C shells
• C shell (/usr/bin/csh) uses C syntax and
  provides feature not found in Bourne shell
• Command aliasing
• History substitution
• Job control
• Korn Shell (/usr/bin/ksh) Provides more
  environmental control then Bourne shell
• Bourne shell (/usr/bin/sh) standard job control
  shell
• Provides variable, command and filename
  substitution
• Enhanced Berkley C shell (/usr/bin/tcsh) C shell
  with file name completion and command line editing

5
Other Shells of Interest

• Remote shell (/usr/bin/rsh) connects to
  specified host and executes specified command
• Job control shell (/usr/bin/jsh) same as sh
• Z shell (/usr/bin/zsh) Includes many
  enhancements to Korn shell
• Command line editing
• Built-in spelling correction
• Programmable command completion
• History

6
Identifying Current Environment

• Default shell echo SHELL
• Current search path echo PATH
• What's your login info who am i
• More info about user finger loginID
• Much more info about env set OR env
• Who's logged in who

7
Useful Commands

• Display content of a file cat filename
• Change directory cd dirpath
• Change permissions on file or directory chmod
  mode objName
• Clear display clear
• Copy file cp source destination
• Append text to a file echo string filename
• Search directory for file find path string
• Search text for string text grep i string
• View screen at a time less filename OR command
  less
• View content of a directory ls OR ls dirPath
• Get help on a command man command
• Create a directory mkdir dirName
• Change name of a file mv oldName newName
• Move a file mv fileName dest
• Display present working directory pwd
• Delete a file rm filename
• Delete a directory rmdir dirName

8
Commands to Lookup

• alias
• awk
• banner
• batch
• chown
• cut
• date
• groups
• groupadd
• kill
• ls
• netstat
• paste
• ps

• passwd
• read
• script
• sh
• spell
• sort
• su
• talk
• tail
• time
• touch
• useradd
• vi

9
bash

• Bourne shell (sh) compatible command language
  interpreter
• Executes commands for standard input or files
• Incorporates features of Korn and C shells
• Conformant implementation of IEEE POSIX Shell and
  Tools specification (IEEE 1003.2)

10
POSIX Standard

• Portable Operating Systems Interface
• Standards defines a standard operating system
  interface and environment
• Command interpreter (or "shell")
• Common utility programs to support applications
  portability at source code level

Source http//www.unix.org/single_unix_specificat
ion/
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POSIX Basic Regular Expressions

• Regular expressions used for text processing
• Includes decoding shell commands
• Zero or more characters followed by
• Ordinary characters any character in supported
  character set, except for special characters
• Special characters have special properties in
  certain context
• Matching is by bit pattern used for encoding
  character, not on graphic representation of
  character.

12
POSIX Environment Variables
13
Safe Environment Variables to Change

• CDPATH
• COLUMNS
• EDITOR
• ENV
• HISTFILE
• HISTFILESIZE
• HISTSIZE
• HOSTFILE
• IGNOREEOF
• LINES
• MAIL

• MAILCHECK
• MAIL_WARNING
• noclobber
• PATH
• PROMPT_COMMAND
• PS1 prompt
• PS2
• PS3
• PS4
• TMOUT
• VISUAL

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Types of bash Shells

• Login shell
• Default shell for user
• Or shell started with --login option
• Interactive
• Standard input and output connected to terminal
• Or shell started with i option
• Non-interactive
• Shell invoked with name sh
• Does not read startup files

15
bash Interactive Invocation

• Login shell executes following in order unless
  --noprofile used at shell's startup
• /etc/profile
• /.bash_profile
• /.bash_login
• /.profile
• Not login shell executes /.bashrc
• If exists
• If not inhibited by option norc
• Option --rcfile filename forces file named to
  execute as bashrc file and /.bashrc will not run

16
Configuring bash

• Edit /.bashrc
• Some syntax
• Comment symbol
• Continuation symbol \
  represents pressing Enter key
• Using environment variables ENV_VAR
• Setting environment variables ENV_VARvalueexpor
  t ENV_VAR1, .., ENV_VARn
• Creating aliases alias newCmd'command'

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Example .bashrc File

• This is a comment in this example
• bash run control file.
• Adding to the search path
• PATHPATH/usr/sbin
• Change prompt
• PS1"\u_at_\host\w\\"
• Make environment changes work
• export PATH, PS1
• Create aliases
• alias ll'ls al'
• alias renamemv

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Special Symbols for bash Prompt

• \u Show current loginID
• \w Show current directory with path
• \W Show current directory without path
• \t Show time
• \d Show date
• \n Force newline
• \h Show short hostname
• \H Show long hostname
• \s Show shell's name
• \v Show shell's version
• \! Show command history number

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Example PROMPT_COMMAND

• meant to be sourced from your bash environment
• this will display a colored error code if the
  previous function
• returned an error, or just the regular prompt
  otherwise.
• JPM
• showIfError ()
• AUX?
• if AUX -gt 0 then PREFIX"\\033133m\
  AUX " else PREFIX"" fi
• PS1"PREFIX\\033132m\\w\\03300m\\
  "
• PROMPT_COMMANDshowIfError

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Sending Messages

• mesg -y
• Allows messages
• If mesgn, no messages
• write DestLoginID
• DestLoginID is danaID of destination
• Type message and press Enter key
• To terminate use Ctrl-d
• Ctrl-d is End Of Transmission

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Let's Try Some of This

• What shell are you using?echo SHELL
• If it's not bash, then change to
  bash./usr/bin/bash
• Change the prompt to say "loginID rocks!
  "PS1"\u rocks!"
• Find out who's logged in.who
• Find out more about someone logged in.finger
  loginID
• See what you have done.history
• Send yourself a message.mesg ywrite
  loginIDHi, Dude!d

22
Summary

• Shell interpret commands and control jobs
• Common shells sh, bash, csh, ksh, tcsh
• ls /bin lists general commands
• ls /usr/sbin lists system admin commands
• man cmdName tells about command named
• Shells are customizable
• bash is sh-compatible and POSIX compliant
• She sells seashells by the seashore

23
Questions?