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UNIX shell environments

UNIX shell environments. CS 2204 Class meeting 4 Created by Doug Bowman, 2001 Modified by Mir Farooq Ali, 2002. Shell characteristics. Command line interface between the user and the system Is simply a program that automatically starts when you login Waits for user to type in commands.

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UNIX shell environments

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  1. UNIX shell environments CS 2204 Class meeting 4 Created by Doug Bowman, 2001 Modified by Mir Farooq Ali, 2002

  2. Shell characteristics • Command line interface between the user and the system • Is simply a program that automatically starts when you login • Waits for user to type in commands (C) Doug Bowman, Virginia Tech, 2001

  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 (C) Doug Bowman, Virginia Tech, 2001

  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 (C) Doug Bowman, Virginia Tech, 2001

  5. The Bourne again SHell (bash) • We will be using bash as the standard shell for this class • This will be important for shell scripting assignments • Superset of the Bourne shell (sh) • Borrows features from sh, csh, tcsh & ksh • Created by Free Software Foundation (C) Doug Bowman, Virginia Tech, 2001

  6. Changing your shell • On most UNIX machines: • which bash (note path) • chsh • On the lab machines: • which ksh (note path /bin/bash) • ypchsh (C) Doug Bowman, Virginia Tech, 2001

  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> (C) Doug Bowman, Virginia Tech, 2001

  8. Some Environment variables • Some interesting variables: HOME, PATH, PS1, USER, HOSTNAME, PWD $HOME /home/gradstudents/m/miali $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin $PS1 \u@\h:\w\$ $USER miali $HOSTNAME avocado.cslab.vt.edu $PWD /home/gradstudents/m/miali (C) Doug Bowman, Virginia Tech, 2001

  9. Setting environment variables • Set a variable with <name>=<value> • Examples: • TERM=vt100 • PS1=myprompt> • PS1=$USER@$HOSTNAME: • PS1=“multiple word prompt> “ • PATH=$PATH:$HOME/bin • DATE=`date` (C) Doug Bowman, Virginia Tech, 2001

  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” (C) Doug Bowman, Virginia Tech, 2001

  11. Repeating commands • Use history to list the last 16 commands • Use fc -l <m> <n> to list previously typed commands m through n (C) Doug Bowman, Virginia Tech, 2001

  12. Editing on the command line • Some command lines can be very long and complicated - if you make a mistake you don’t want to start all over again • You can interactively edit the command line in several ways • set -o vi allows you to use vi commands to edit the command line • set -o vi-tabcomplete also lets you complete commands/filenames by entering a TAB (C) Doug Bowman, Virginia Tech, 2001

  13. Login scripts • You don’t want to enter aliases, set environment variables, set up command line editing, 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 (C) Doug Bowman, Virginia Tech, 2001

  14. Login scripts (continued) • For bash, order of files is: • /etc/profile • ~/.bash_profile • ~/.bash_login • ~/.profile • After logout • ~/.bash_logout (C) Doug Bowman, Virginia Tech, 2001

  15. Example .bash_profile (partial) # .bash_profile: executed by bash(1) for login shells umask 022 # include .bashrc if it exists if [ -f ~/.bashrc ]; then source ~/.bashrc fi # some ls aliases alias ll='ls -l' alias la='ls -A' alias l='ls -CF' (C) Doug Bowman, Virginia Tech, 2001

  16. stdin, stdout, and stderr stdout • 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 Program command stdin • Programs use these three files when reading (e.g. cin, writing (e.g. cout), or reporting errors/diagnostics (C) Doug Bowman, Virginia Tech, 2001

  17. Redirecting stdout • Instead of writing to the terminal, you can tell a program to print its output to another file using the > operator • >> operator is used to append to a file • Examples: • man ls > ls_help.txt • Echo $PWD > current_directory • cat file1 >> file2 (C) Doug Bowman, Virginia Tech, 2001

  18. Redirecting stdout • miali@avocado:~$ cat file1.txt This is a small test file. • miali@avocado:~$ cat file1.txt> file2.txt • miali@avocado:~$ cat file2.txt This is a small test file. (C) Doug Bowman, Virginia Tech, 2001

  19. Redirecting stdin • Instead of reading from the terminal, you can tell a program to read from another file using the < operator • Examples: • Mail mfali@vt.edu < message.txt • a.out < testdata1.txt (C) Doug Bowman, Virginia Tech, 2001

  20. Background processing • Allows you to run your programs in the background • miali@avocado:~/cs2204/lab3$ emacs vi_practice_edited & • miali@avocado:~/cs2204/lab3$ (C) Doug Bowman, Virginia Tech, 2001

  21. Pipes and filters • Pipe: a way to send the output of one command to the input of another • Filter: a program that takes input and transforms it in some way • wc - gives a count of words/lines/chars • grep - searches for lines with a given string • more • sort - sorts lines alphabetically or numerically (C) Doug Bowman, Virginia Tech, 2001

  22. Examples of wc • miali@avocado:~$ wc file1.txt 1 6 27 file1.txt • miali@avocado:~$ wc file1.txt file2.txt 1 6 27 file1.txt 1 6 27 file2.txt 2 12 54 total (C) Doug Bowman, Virginia Tech, 2001

  23. Examples of filtering • miali@avocado:~$ ls -la | wc 40 359 2942 • miali@avocado:~$ man ksh | grep “history\.” The name of the file used to store history. When first and last select commands from the history. Moves to the beginning of the history. Moves to the end of the history. history or search-history. move to the nth next line in the history. through the history. (C) Doug Bowman, Virginia Tech, 2001

  24. More Examples miali@avocado:~$ ls -al |grep bash -rw------- 1 miali grads 5427 May 29 08:36 .bash_history -rw-r--r-- 1 miali grads 174 May 16 02:00 .bash_logout -rw-r--r-- 1 miali grads 519 May 16 02:00 .bash_profile -rw-r--r-- 1 miali grads 1115 May 29 08:36 .bashrc -rw-r--r-- 1 miali grads 1116 May 29 08:33 .bashrc~ -rw-r--r-- 1 miali grads 876 May 16 02:00 .bashrc.dpkg-old miali@avocado:~$ ls -al |grep bash |wc 6 54 435 (C) Doug Bowman, Virginia Tech, 2001

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