Chapter 8: The Shells

Boiled down:

Recall

• < filename
• > filename
• >> filename
• &> filename - redirects standard output and error to filename
• cat x y 1> hold1 2 > hold2
  redirect stdout and stderr to different files
• cat x y &> hold
  redirect stdout and stderr to single file
• cat x y 1> hold1 2 >&1
  redirects standard output to hold and declare file descriptor 2 to be a duplicate of file descriptor 1

Interactive shells

• When you login, an interactive (bash) shell shows you a command prompt
  • Command history
  • ^z, fg, bg, command &
• When you run a shell script, you get a non-interactive shell, without history of job control
• bash myScript.sh
• ./myScript.sh

Changing shells

• Recommended to stay with bash - but try the others by invoking them at the command line
  Use chsh (change shell) to change your login shell
• bash - Bourne-again shell
• csh - actually runs tcsh (a superset)
• sh - actually runs bash (a superset)
• ksh - not available
• dash - some features of ksh

Environment Variables

• env or printenv - display all of the environment variables and their values
• set - displays all variables for this shell, including local variables
• Child processes do not get copies of local variables
• Unless you export them
  workDir=/home/SRUNET/deborah.whitfield/cs207
  export workDir
  workDir=/tmp

Customizations

• Customizations are usually added to a file called .bash_login or .bash_profile in your home directory (obsidian is .bash_profile)
• PATH=$PATH:.
• PS1="\h: "
• PS1='[$PWD]'
• SHELL="/bin/csh"

aliases

• Customized commands - less typing
• See Table 8.2
• alias - displays a list of currently aliased commands
• alias f="find . -name \"*.txt\" .print.
• alias spell="/usr/bin/hunspell"

Command History

• history - shows all commands recorded
• history 5 - shows the last five commands
• !! - reexecute the last command
• !find - reexecutes the last find command
• Arrow (cursor control) up and down
  Edit using left & right arrows, then insert/delete
• Lots more in Table 8.3

~ and set

• ~ is a shorthand for your home directory
  cd ~
  cd ~deborah.whitfield

  set -o noclobber

  Prevents overwriting a file using > or >>

Writing a shell script

• edit startup file (.bash_profile, .profile, .login, .bashrc etc.) to set PATH
  PATH=$PATH:.
  PS1="\h:\w: "
  set -o noclobber
  alias h="cd $HOME"
  who | grep whitfield
• Source the startup file to run changes

Scripting

• put Unix commands into file to create a script
• Start with #!/bin/sh
• chmod u+x file to make script executable
• ./script to run file

Parameters and Variables

• myvar=abc
  sets myvar to abc, but you cannot use spaces
• to access user defined variables - $myvar
• Keyword Variables - $HOME, $PATH
• Use " " if you want spaces in string
  myvar="abc 123"
  echo $myvar
  abc 123
• But if multiple spaces...
  myvar="abc    and    123"
  echo $myvar
  abc and 123
  echo "$myvar"
  abc    and    123
• * is used as wildcard. If a directory listing included two files that start with m (m1 and m2), then ls m* would display m1 and m2
  myvar=m*
  echo "$myvar"
  m*
• Without quotes ...
  echo $myvar
  m1   m2
• unset myvar
  removes a variable
• readonly myvar
  cannot give myvar a value

  Variable attributes can be assigned with the declare statement

  declare -r myvar="I am a constant"

  -a declares as an array

  -f declares as function name

  -r makes variable read only

Command Substitution

• `whomai`
  when placed in a script will execute the whoami command
• myvar=`whoami`
  sets the variable to the output of the whoami comand

  myvar=`whoami`.`pwd`

Separator and newline

• ; newline separator
• \ continues a command

Regular Expression - Wild card characters

• * - match 0 or more
• ? - match 1 character
• LOTS more to come!

All work herein is subject to copyright. Original content to Dr. Deborah Whitfield, text content (Your UNIX/Linux) to Prentice Hall publishing.