UNIX and Shell Programming (06CS36) Unit 3continued… Shrinivas R. Mangalwede Department of Computer Science and Engineering K.L.S. Gogte Institute of Technology, Belgaum. INDIA. mangalwede@yahoo.com

Customizing the Environment The Shells – A Re-look Environment Variables Common Environment Variables – their usage Aliases – Shorthand names for commands Command History In-line Command Editing – vi-like capability Miscellaneous Features - Using set –o - Tilde Substitution The Initialization Scripts - The Profile - The rc File Shrinivas R. Mangalwede, G.I.T., Belgaum

The Shells UNIX shell is both an interpreter and a scripting language. An interactive shell runs a non-interactive shell when executing a shell script. Bourne Shell – Strong programming features, weak interpreter. C Shell – Improved interpretive features, wasn’t suitable for programming. Korn Shell – Combines best of the two. Has features like aliases, command history. But lacks some features of C. Bash Shell – Superset that combined the features of Korn and C Shells. Conforms to POSIX shell specification. Shrinivas R. Mangalwede, G.I.T., Belgaum

Environment Variables Shell variables are of two types: Local and environment. PATH, HOME, SHELL etc. are examples of environment variables. Environment variables are available in the user’s total environment including the sub-shells that run the shell scripts. Local variables are more restrictive in scope. The value of a local variable is not available to child processes. (Eg. Sub shell) set: Displays all variables available in the current shell. env: Displays only environment variables. env is an external command that runs in a child process whereas set is a shell built-in. Shrinivas R. Mangalwede, G.I.T., Belgaum

Common Environment Variables Significance HOME Home directory – the directory a user is placed on login PATH List of directories searched by the shell to locate a command LOGNAME Login name of user USER MAIL Absolute pathname of user’s mailbox file MAILCHECK Mail checking interval for incoming mail TERM Type of terminal

Common Environment Variables Significance PWD Absolute pathname of current directory (Bash and Korn only) CDPATH List of directories searched by cd when used with a non-absolute pathname PS1 Primary prompt string PS2 Secondary prompt string SHELL User’s login shell

Some Examples To include the directory /home/srm/bin in the search use, PATH=$PATH:/home/srm/bin To display your home directory use, echo $HOME The location of the user’s mailbox is stored in MAIL. It is generally /var/mail or /var/spool/mail (Linux) MAILCHECK determines how often the shell checks the mailbox file for the arrival of new mail. Once the shell finds the file modified since last cjeck, it informs the user with the message, You have mail in /var/mail/srm Shrinivas R. Mangalwede, G.I.T., Belgaum

Some Examples The primary prompt string is $ and secondary prompt string is >. You can change the primary prompt string to C> as, $ PS1=“C>” C> . . . TERM indicates the terminal type. Every terminal has certain control characteristics that are defined in a separate control file in the /usr/share/lib/terminfo directory. IFS contains a string of characters that are used as field separators for command and arguments. They are normally the space, tab and newline characters. Shrinivas R. Mangalwede, G.I.T., Belgaum

Bash and Korn Shells $ PS1=‘[PWD] ‘ [/home/srm] cd progs [/home/srm/progs] _ Bash and Korn also support a history facility that treats a previous command as an event and associates it with a number. $ PS1=‘[!] ‘ $ PS1=‘[! $PWD] ‘ [42] _ [42 /home/srm/progs] _ $ PS1=“\h> “ // Host name of the machine saturn> _ Shrinivas R. Mangalwede, G.I.T., Belgaum

Aliases (bash and korn) Alias is a shorthand name that you can assign to frequently used commands $ alias dir=‘ls –l’ $ alias cdsys=“cd /usr/include/sys” You can also use aliasing to redefine an existing command. $ alias cp=“cp –i” Every time you invoke an alias, their aliased version will be executed. To use original external command, precede the command with a \ (backslash). $ \cp file1 file2 overrides the alias Shrinivas R. Mangalwede, G.I.T., Belgaum

Aliases (bash and korn) To display an alias definition use alias with the name, $ alias cp cp=“cp –i” You can list all aliases by using alias without arguments and unset an alias with the unalias statement. $ unalias cp Shrinivas R. Mangalwede, G.I.T., Belgaum

Command History (bash and Korn) Bash and Korn treat a previous command as an event and associate it with an event number. Using this number, you can recall previous commands, edit them if required and reexecute them. The history command displays the history list showing event number of every previously executed command By default, bash lists the complete command history while korn lists the last 16 most recently used commands. $ history 5 Bash $ history -5 Korn History is stored in $HOME/.bash_history or $HOME/.sh_history Shrinivas R. Mangalwede, G.I.T., Belgaum

Accessing previous commands By event numbers (! In bash and r in korn) $ !38 The command with event number 38 is displayed and executed (Use r 38 in korn) $ !38:p The command is displayed. You can edit and execute it $ !! Repeats previous command (Use r in korn) $ !-2 Executes command prior to the previous one ( r -2 in korn) By Context $ !v Repeats the last command beginning with v (r v in korn) Shrinivas R. Mangalwede, G.I.T., Belgaum

Substitution in previous commands If you wish to execute a previous command after some changes, you can substitute the old string with new one by substitution. If a previous command cp progs/*.doc backup is to be executed again with doc replaced with txt, $ !cp:s/doc/txt in bash $ r cp doc=txt in korn $_ is a shorthand feature to represent the directory used by the previous command. $ mkdir progs $ cs $_ Shrinivas R. Mangalwede, G.I.T., Belgaum

The History variables The command history will be maintained in default history files viz., .bash_history in Bash .sh_history in Korn Variable HISTFILE determines the filename that saves the history list. Bash uses two variables HISTSIZE for setting the size of the history list in memory and HISTFILESIZE for setting the size of disk file. Korn uses HISTSIZE for both the purposes. Shrinivas R. Mangalwede, G.I.T., Belgaum

Miscellaneous Features Using set –o set –o noclobber: Prevents overwriting of an existing file with > symbol. To override this protection, use | after the > ls –l >| file_list set –o ignoreeof: Prevents accidental logout when you press [Ctrl-d] to terminate standard input. A set option is turned off with set +o keyword. Tilde Substitution Tilde (~) acts as a shorthand representation of the home directory. A configuration file like .profile can be referred to both as $HOME/.profile and ~/.profile. Shrinivas R. Mangalwede, G.I.T., Belgaum

The initialization scripts The effect of assigning values to variables, defining aliases and using set options is applicable only for the login session; they revert to their default values when the user logs out. To make them permanent, use certain startup scripts. The startup scripts are executed when the user logs in. .profile (Bourne shell) .profile and .kshrc (Korn shell) .bash_profile (or .bash_login) and .bashrc (Bash) .login and .cshrc (C shell) You can also execute them by using a special command (called dot). $ . .profile Shrinivas R. Mangalwede, G.I.T., Belgaum

The initialization scripts When logging into an interactive login shell, login will do the authentication, set the environment and start your shell. In the case of bash, the next step is reading the general profile from /etc, if that file exists. bash then looks for ~/.bash_profile, ~/.bash_login and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. If none exists, /etc/bashrc is applied. When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists. Shrinivas R. Mangalwede, G.I.T., Belgaum

.profile and .bash_profile Sample entries $ cat .profile # User $HOME/.profile – commands executed at # login time MAIL=/var/mail/$LOGNAME PATH=$PATH:$HOME/bin:/usr/ucb:. PS1=‘$ ‘ PS2=> TERM=vt100 Stty stop ^S intr ^C erase ^? Echo “Today’s date is `date`” Shrinivas R. Mangalwede, G.I.T., Belgaum

The rc file Normally the profiles are executed only once, upon login. The rc files are designed to be executed every time a separate shell is created. There is no rc file in Bourne, but bash and korn use one. This file is defined by an environment variable BASH_ENV in Bash and ENV in Korn. export BASH_ENV=$HOME/.bashrc export ENV=$HOME/.kshrc Korn automatically executes .kshrc during login if ENV is defined. Bash merely ensures that a sub-shell executes this file. If the login shell also has to execute this file then a separate entry must be added in the profile: . ~/.bashrc Shrinivas R. Mangalwede, G.I.T., Belgaum

The rc file You should define command aliases, variable settings, and shell options in your rc file. Some sample entries of an rc file are alias cp=“cp –i” alias rm=“rm –i” set –o noclobber set –o ignoreeof set –o vi The rc file will be executed after the profile. However, if the BASH_ENV or ENV variables are not set, the shell executes only the profile. Shrinivas R. Mangalwede, G.I.T., Belgaum

To conclude, Environment-related features of the shell Common Environment Variables Environment variables specific to bask and korn Aliases Command History In-line Command editing set –o and Tilde substitution The initialization scripts The Profile file The rc file

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