CSCI 330THE UNIX SYSTEM

Bash shell

BASH SHELL SPECIFICS

Shell login and logout files Shell variables Prompt History IO redirection

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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

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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_logout4

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executed for login shell

executed for non-login shell

PREDEFINED SHELL VARIABLES

Shell Variable

Description

PWD The most recent current working directory.

OLDPWD The previous working directory.

BASH The full path name used of the bash shell.

RANDOM Generates a random integer between 0 and 32,767

HOSTNAME The current hostname of the system.

PATH A list of directories to search of commands.

HOME The home directory of the current user.

PS1 The primary prompt (also PS2, PS3, PS4).

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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”

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NUMERIC VARIABLES

Syntax:

let varname=value

can be used for simple arithmetic:

let count=1

let count=$count+20

let count+=1

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ARRAY VARIABLES

Syntax:

varname=(list of words)

accessed via index:

${varname[index]}

${varname[0]} first word in array${varname[*]} all words in array

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USING ARRAY VARIABLES

Examples:

% ml=(mary ann bruce linda dara)

% echo ${ml[*]}

mary ann bruce linda dara

% echo ${ml[2]}

bruce

% echo ${#ml}

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EXPORTING VARIABLES

Environment variable is created by exporting shell variable

Syntax: export varname(s)

declare –x varname(s)

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VARIABLES COMMANDS

To delete both local and environment variables

unset varname

To prohibit changereadonly varname

list all shell variables (including exported)set

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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 fileset +o noclobber

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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

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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”

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BASH SHELL PROMPT

can be set via “PS1” shell variable

Example: % PS1="$USER > "

z036473 >

Secondary prompts:PS2, PS3, PS4

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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: 16

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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

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I/O REDIRECTION

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Command Syntax Short Description

cmd > file Send output of cmd to file

cmd >> file Append output of cmd to file

cmd < file Take input from file

cmd << text Read stdin up to a line identical to text a.k.a “here command”

FILE DESCRIPTOR

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positive integer for every open file process tracks its open files with this

number

0 – standard input1 – standard output2 – standard error output

bash uses file descriptor to refer to a file

REDIRECTION SYNTAX

Output:> or 1> filename 2> filename

Input:< or 0<

Combining outputs:2>&1

Example: % cat hugo > /tmp/one 2>&1

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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 (“)

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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\>

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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"23

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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 $5.0024

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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 25

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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

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