Unix Shell Environments

Shell Characteristics

 Command-line interface between the Unix Shell Environments user and the operating system  A program that automatically starts on login, waits for user to type in commands Class Meeting 6, Part I  A command interpreter that uses a programming language  Shell script is a text file containing logic for shell interpretation

Shell Interactivity Shell Programming

 Command line parsing  Variables  I/O redirection, pipes and filters  Control structures  Environment  Loops and conditionals  Text completion  Function definition and invocation  Aliases  Shell scripts  Command line editing  Next lecture  Command history  Configuration

Three standard files Redirecting stdout

 Each shell (and all other programs) automatically  Instead of sending stdout to the open three “files” when they start up terminal, you can redirect stdout to a  stdin — input character stream file using the > shell operator  Defaults to keyboard  >> shell operator is used to append  stdout — output character stream stdout to an existing file  Defaults to terminal  Examples:  stderr — receives error messages  man ls > ls_help.txt  Defaults to terminal  echo $PWD > current_directory  cat file1 >> file2

1 Redirecting stdin Pipes and Filters

 Instead of reading from a terminal, you  Pipe — a way to connect stdout of one command to stdin of another; use can tell a program to read from a file |  Filter — a program that takes input and transforms it using the < shell operator in some way  Examples:  wc — gives a count of words/lines/characters  grep — searches for lines with a given pattern (regular  mail [email protected] < message.txt expression)  sort < friends > sorted_friends  sort — sorts lines alphabetically or numerically  cut — select parts of each line to send to stdout

Examples Various Unix Shells

 ls -la | less  sh (Bourne shell, original Unix shell)  ls -la | wc  ksh (Korn shell)  man bash | grep "history"  csh (C shell, developed at Berkeley)  ps aux | grep user1 | wc -l  tcsh  who | sort > current_users  bash (Bourne again Shell)  Default user shell in Linux  http://www.faqs.org/faqs/unix-faq/shell/shell-differences/

Bourne Again Shell Environment Variables

 bash is the standard shell for this class  A set of variables the shell uses for  Superset of the Bourne shell (sh) certain operations  Borrows features from sh, csh, tcsh, and  Variables have a name and a value ksh  Current list can be displayed with the  Part of the GNU project env command  Dispaly value of varname with echo $varname

2 Environment Variable Examples Setting Environment Variables

[cs2204@acorn bin]$ echo $PS1  Set variable with varname=value  PS1=$USER@$HOSTNAME: [\u@\h \W]\$  Change default shell prompt  PS1="bash rules> "  PATH=$PATH:$HOME/bin [cs2204@acorn bin]$ echo $PATH  Append :$HOME/bin to PATH /usr/local/bin:/bin:/usr/X11R6/  PATH=$PATH:~:. bin:/usr/games:/home/courses/  Append :~:. to PATH  DATE=`date` or DATE=$(date) cs2204/bin  Capture output from date command

Text Completion Aliases

 attempts to complete the  alias shortcut=command current command or filename  Examples  push expands to  alias pu=pushd pushd  alias po=popd  pu gives the alternatives  alias l="ls –F –C" pu pup pushd  alias ll="ls –L –l –F"  In /etc, entering ll modu gives  alias up=“cd ..” modules modules.conf modules.devfs  alias hide="chmod og-rwx" [cs2204@blackberry etc]$ ll modules  alias unhide="chmod og+r"

Command History Editing on the Command Line

 Use history command to list  bash provides a number of line editing previously entered commands commands  Use fc –l to list previously  Left- and right-arrows move insertion point typed commands from m through n  Default emacs-mode commands  Esc-b Move back one word  Use up and down cursor keys to scroll  Esc-f Move forward one word through history list  Ctrl-a Move to beginning of line  Ctrl-e Move to end of line  Ctrl-k Kill text from cursor to end of line

3 Login Scripts Example .bash_profile (partial)

 You don’t want to set up your whole shell # .bash_profile environment each time you log in # include .bashrc if it exists  Startup scripts executed at each login if [ -f ~/.bashrc ]; then . ~/.bashrc  /etc/profile fi  ~/.bash_profile  ~/.bash_login (if no .bash_profile) # Set variables for a warm fuzzy environment  ~/.profile (if neither are present)  ~/.bashrc export CVSROOT=~/.cvsroot export EDITOR=/bin/vi  Script executed upon logout export PAGER=/usr/bin/less  ~/.bash_logout

Example .bashrc (partial) Login and Other Shells

# .bashrc login shell # abbreviations for some common commands /etc/profile alias bye=logout ~/.bash_profile interactive shell ~/.bashrc alias h=history ~/.bashrc alias l='ls -F -C' alias ll='ls -L -l -F' interactive shell interactive shell alias po=popd ~/.bashrc alias pu=pushd ~/.bashrc