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Unix Shell Environments

Unix Shell Environments. February 23rd, 2004 Class Meeting 6. Shell Characteristics. Command-line interface between the user and the system Automatically starts when you log in, waits for user to type in commands

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Unix Shell Environments

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  1. Unix Shell Environments February 23rd, 2004 Class Meeting 6

  2. Shell Characteristics • Command-line interface between the user and the system • Automatically starts when you log in, waits for user to type in commands • A Unix shell is both a command interpreter, which provides the user interface to the rich set of utilities, and a programming language, allowing these utilities to be combined.

  3. Main Shell Features • Interactivity • Aliases • File-name completion • Scripting language • Allows programming (shell scripting) within the shell environment • Uses variables, loops, conditionals, etc. • Next lecture

  4. Various Unix Shells • sh (Bourne shell, original Unix shell) • ksh (Korn shell) • csh (C shell, developed at Berkeley) • tcsh • bash (Bourne again SHell) • Differences mostly in level of interactivity support and scripting details http://www.faqs.org/faqs/unix-faq/shell/shell-differences/

  5. Bourne Again SHell • We will be using bash as the standard shells for this class • Superset of the Bourne shell (sh) • Borrows features from sh, csh, tcsh, and ksh • Created by the Free Software Foundation

  6. Changing Your Shell • On most Unix machines (including the lab) . . . • which bash • chsh • On some machines . . . • Ypchsh

  7. Environment Variables • A set of variables the shell uses for certain operations • Variables have a name and a value • Current list can be displayed with the env command • A particular variable’s value can be displayed with echo $<var_name>

  8. Environment Variable Examples • Some interesting environment variables: • $HOME /home/grads/callgood • $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin • $PS1 \u@\h:\w\$ • $USER callgood • $HOSTNAME mango.cslab.vt.edu • $PWD /home/grads/callgood/cs2204

  9. Setting Environment Variables • Set a variable with <name>=<value> • Examples: • PS1=myprompt> • PS1=$USER@$HOSTNAME: • PS1=“multiple word prompt> ” • PATH=$PATH:$HOME/bin • PATH=$PATH:~ • DATE=`date`

  10. Aliases • Aliases are used as shorthand for frequently-used commands • Syntax: alias <shortcut>=<command> • Examples: • alias ll=“ls –lF” • alias la=“ls –la” • alias m=more • alias up=“cd ..” • alias prompt=“echo $PS1”

  11. Repeating Commands • Use history command to list previously entered commands • Use fc –l <m> <n> to list previously typed commands from m through n

  12. Editing on the Command Line • bash provides a number of line editing commands; many are the same as emacs editing commands • M-b Move back one word • M-f Move forward one word • C-a Move to beginning of line • C-e Move to end of line • C-k Kill text from cursor to end of line

  13. Login Scripts • You don’t want to enter aliases, set environment variables, etc., each time you log in • All of these things can be done in a script that is run each time the shell is started

  14. Login Scripts (cont) • For bash, order is . . . • /etc/profile • ~/.bash_profile • ~/.bash_login (if no .bash_profile) • ~/.profile (if neither are present) • ~/.bashrc • After logout . . . • ~/.bash_logout

  15. Example .bash_profile (partial) # .bash_profile # include .bashrc if it exists if [ -f ~/.bashrc ]; then . ~/.bashrc fi # Set variables for a warm fuzzy environment export CVSROOT=~/.cvsroot export EDITOR=/usr/local/bin/emacs export PAGER=/usr/local/bin/less

  16. Example .bashrc (partial) # .bashrc # abbreviations for some common commands alias f=finger alias h=history alias j=jobs alias l='ls -lF' alias la='ls -alF' alias lo=logout alias ls='ls -F'

  17. interactive shell interactive shell interactive shell Login Shell login shell /etc/profile~/.bash_profile~/.bashrc ~/.bashrc ~/.bashrc ~/.bashrc

  18. Background Processing • Allows you to run your programs in the background callgood@mango:~/$ emacs textfile& callgood@mango:~/$

  19. stdin, stdout, and stderr • Each shell (and in fact all programs) automatically open three “files” when they start up • Standard input (stdin): Usually from the keyboard • Standard output (stdout): Usually to the terminal • Standard error (stderr): Usually to the terminal • Programs use these three files when reading (e.g. cin), writing (e.g. cout), or reporting errors/diagnostics

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