CSCI 330 The UNIX System

CSCI 330The UNIX System Bash shell

CSCI 330 - The Unix System Bash Shell Specifics • Shell login and logout files • Shell variables • Prompt • History • IO redirection

CSCI 330 - The UNIX System Invoking bash • On the command line: % sh % bash • as login shell • specified in /etc/passwd • with file as argument • file is sh-script % sh scriptfile

CSCI 330 - The Unix System Startup & Shutdown Files • /etc/profile • ~/.bash_profile • ~/.bash_login • ~/.profile • /etc/bash.bashrc • ~/.bashrc • options: • --norc don’t run initialization files • -l run as login shell • ~/.bash_logout executed for login shell executed for non-login shell

CSCI 330 - The Unix System Predefined Shell Variables

CSCI 330 - The Unix System User-defined Shell Variables • Syntax: varname=value Example: rate=7.65 echo “Rate today is: $rate” • use double quotes if the value of a variable contains white spaces Example: name=“Thomas William Flowers”

CSCI 330 - The Unix System Numeric variables • Syntax: let varname=value • can be used for simple arithmetic: let count=1 let count=$count+20 let count+=1

CSCI 330 - The Unix System Array variables • Syntax: varname=(list of words) • accessed via index: ${varname[index]} ${varname[0]} first word in array ${varname[*]} all words in array

CSCI 330 - The UNIX System Using array variables Examples: % ml=(mary ann bruce linda dara) % echo ${ml[*]} mary ann bruce linda dara % echo ${ml[2]} bruce % echo ${#ml} 4

CSCI 330 - The Unix System Exporting Variables • Environment variable is created by exporting shell variable • Syntax: export varname(s) declare –x varname(s)

CSCI 330 - The Unix System Variables commands • To delete both local and environment variables unset varname • To prohibit change readonly varname • list all shell variables (including exported) set

USING “SET” TO CHANGE OPTIONS • “set” is a builtin command of bash • “set +o” can be used to change an option • To keep I/O redirection from overwriting a file set +o noclobber CSCI 330 - The Unix System 12

CSCI 330 - The Unix System variable manipulation • use portion of a variable’s value via: ${name:offset:length} • name – the name of the variable • offset – beginning position of the value • length – the number of positions of the value Example: % SSN="123456789" % password=${SSN:5:4} % echo $password % 6789

CSCI 330 - The Unix System Special variable uses • ${#variable} number of characters in variable’s value • ${variable:-value} if variable is undefined use “value” instead • ${variable:=value} if variable is undefined use “value” instead, and set variable’s value • ${varname:?message} if variable is undefined display error “message”

CSCI 330 - The UNIX System bash Shell Prompt • can be set via “PS1” shell variable Example: % PS1="$USER > " z036473 > • Secondary prompts: PS2, PS3, PS4

CSCI 330 - The UNIX System bash Shell Prompt • special “PS1” shell variable settings: \w current work directory \h hostname \u username \! history event number \d date \t time \a ring the “bell” Example: % PS1="\u@\h-\!: " ege@turing-22:

CSCI 330 - The Unix System Command History List • View or re-issue previously executed commands • Size of history can be set via shell variables: HISTSIZE=500 HISTFILESIZE=100 • Command line editing via keys: • UP ARROW: move back one command in history list • DOWN ARROW: move forward one command • LEFT and RIGHT ARROW: move into command • BACKSPACE and DELETE: Remove information • TAB: complete current command or file name

CSCI 330 - The Unix System I/O Redirection

CSCI 330 - The Unix System positive integer for every open file process tracks its open files with this number 0 – standard input 1 – standard output 2 – standard error output bash uses file descriptor to refer to a file File Descriptor

CSCI 330 - The Unix System Redirection syntax • Output: >or1> filename 2> filename • Input: <or0< • Combining outputs: 2>&1 Example: % cat hugo > /tmp/one 2>&1

CSCI 330 - The Unix System Quoting • Quoting allows you to distinguish between the literal value of a symbol and the symbols used as code • You need a way to distinguish between the literal symbol and the symbol’s use as a metacharacter or wild card characters • To do this you must use of the following symbols: • Backslash (\) • Single quote (‘) • Double quote (“)

CSCI 330 - The Unix System A Backslash (\) • A backslash is also called the escape character • It allows you to preserve only the character immediately following it • For example: to create a file named “tools>”, enter: % touch tools\>

CSCI 330 - The Unix System The Single Quote (‘) • A single quote is used to protect the literal meaning of metacharacters. • It protects all characters within the single quotes • The only character it cannot protect is itself • A single quote cannot occur with other single quotes even if preceded by a backslash • Examples: % echo 'Joe said 'Have fun'' Joe said Have fun % echo 'Joe said "Have fun"' Joe said "Have fun"

CSCI 330 - The Unix System A Double Quote (“) • Double quotes protect all symbols and characters within the double quotes. • Double quotes will not protect these literal symbols: $ (dollar sign), ! (history event), and \ (backslash). • Examples: % echo "I've gone fishing'" I've gone fishing' % echo 'Jake won $500.00' Jake won $500.00 % echo "You've" earned '$5.00' You've earned $5.00

CSCI 330 - The Unix System Command Substitution • Used to substitute the output of a command in place of the command itself • Two forms of command substitution: $(command) `command` Examples: % echo "User $(whoami) is on $(hostname)" User ege is on lx % echo "Today is" `date` Today is Sun Jul 17 08:06:28 CDT 2007

CSCI 330 - The Unix System Using the “eval” Command • Evaluates a command line, performs all shell substitutions, and then executes the command line • Used when normal parsing of the command line is not enough • Results in a second round of variable substitution • Same capability as in C-shell

CSCI 330 The UNIX System