linux crash course chapter 08 3

Some history• Very early UNIX shell was written by Steve Bourne – called the Bourne Shell sh • Another popular shell from UNIX days was the C shell csh – a C programmer’s best friend • L

Chapter 8:

The Bourne Again Shell

It’s a command interpreter, it’s a programming language, and it makes a

mean martini

Some history

• Very early UNIX shell was written by Steve

Bourne – called the Bourne Shell (sh)

• Another popular shell from UNIX days was the C shell (csh) – a C programmer’s best friend

• Linux sought to emulate Bourne and make it

better – Bourne Again Shell (bash)

• UNIX stole from bash, David Korn released the Korn shell (ksh)

• POSIX standards …

Starting bash

• When bash is called, various

startup files are run to issue

commands and define environmental variables

• Which startup file(s) begin

depends upon how bash is called

• Use these startup files to make

the shell work for you

Login shells, con’t

• Commands in those three files can override the defaults in

/etc/profile

• Once one of those files are

executed, control is passed to the user

• When the user logs out, bash runs

~/.bash_logout

– Usually clears temporary information

Interactive nonlogin shells

• Shells that you spawn yourself by typing bash

Noninteractive shells

• These are the shells used to run

scripts

• These shells do not run any of the

aforementioned startup files

• They do however inherit the calling

shell’s environmental variables marked for export

• So basically anything you set for the login shell is set for the

noninteractive shell

Working with Startup Files

• In the end, these startup files are just shell scripts

• Obey the same rules and conventions that scripts must use for the

particular shell you’re using

• Most important files are

probably bashrc and bash_profile

• Simplify – have bash_profile

call bashrc

Startup Files, con’t

• Just edit the startup files in

your favorite editor

• When done, you can apply changes

to your current shell using

either or source

• Otherwise, logout and login again

Symbol Commands in bash

• ( ) – run contents in a separate subshell

– Creates a separate process with unique ID

• $( ) – command substitution

– Runs contents and replaces with output

• (( )) – evaluate contents as

arithmetic expression or equation

• $(( )) – same as above, just

expressions (no equal signs)

Redirection Revisited

• Standard Input < (or 0<)

• Standard Output > (or 1>)

• Standard Error 2>

• Recall that we can redirect standard out and standard error to different

locations

• When you use the pipe ( | ), remember

it only passes standard output to

standard input; standard error still goes to it’s normal location

Redirection con’t

• Ex: file x exists, file y does not cat x y | tr “[a-z]” “[A-Z]”

THIS IS FILE X

cat: y: No such file or directory

• Notice that only standard out is piped to tr

• So how would we pipe standard

error as well?

Redirection con’t

• Duplicating streams

• [n]>&[m]

– Redirects stream n to stream m

• So to send std error to std out, 2>&1

• So for our previous example

cat x y 2>&1 | tr “[a-z]” “[A-Z]”

THIS IS FILE X

CAT: Y: NO SUCH FILE OR DIRECTORY

Writing simple scripts

• A shell script is, in its simplest form, just a list of shell commands

in a file

• File must have read and execute

permissions to call implicitly

• If script is in PATH, just call it from command line

• If not in PATH, include pathname

(such as /myscript)

Shell Scripts

• To specify what shell (or program)

to use to execute commands, make

first line:

#![absolute path to program/shell]

• Without this line, whatever shell you are in when calling the script

is used

• To make comments in the script,

begin the line with #

Shell Scripts con’t

• When a command is issued, fork

creates a new process (subshell)

• If the command is a binary

executable, exec makes a system

call to run the binary

• If the command is a shell script, the subshell itself runs the script

• When done, subshell closes and

returns control to calling shell

Separating and Grouping Commands

• You can have multiple commands on

a single command line, they just need to be separated

– ;

– NEWLINE

– &

• To continue a long command on

several lines, end a line with \ and press return

Separating and Grouping con’t

• To group commands to control

execution, use parenthesis

• Ex:

(a; b; c) | (d; e)

Commands a, b, and c are executed first, and once c finishes standard output is piped to the result of commands d and e

• Each set of parenthesis runs in its own subshell

• Variable names can consist of

letters, digits and underscores– Cannot start with a number

Shell Variables con’t

• In bash, to assign a value to a

variable:

VARIABLE=value

• Notice there are no spaces around

the equal sign

• If VARIABLE does not exist, it is

created and set

• If it does exist, the current value

is overwritten

– HOME – contains user’s home directory path

– PATH – contains user’s path

• Change these carefully – you can

confuse the shell and lock yourself out

of places

Shell Variables con’t

• To remove the value of a

variable, just set it equal to a null string

Shell Variable Attributes

• You can use declare or typeset to define attributes when you create

a variable

-a: create an array

-f: create a function

-i: create an integer

-r: make variable readonly

-x: make variable global (export)

Attributes con’t

• Most of these we’ll come back to

when we start doing shell scripting

• Making a variable readonly can also

be accomplished by the readonly

Keyword Variables

• HOME – your home directory

• PATH – your path

• MAIL – location of mail storage

• PS1 – your primary prompt

• PS2 – secondary prompt

• IFS – input field separator (BEWARE!!!)

• Plus many more … just type set to see current shell variables (keyword

variables usually all caps)

• Those processes fork more processes

• Etc … so we get a tree-like structure

• To see this structure use ps forest

Processes con’t

• When a process (such as the shell)

calls another process (like a command

or script), it calls fork to create a new process, then goes to sleep

• If the process is sent to the

background, fork is called but sleep

is not

• If a builtin is called, neither fork nor sleep is called

Processes con’t

• fork creates a new process,

assigns it a unique PID, and

associates it with its parent via the PPID

• When a process forks a child

process, the current shell

variables *are not* passed on,

unless marked for export

• bash contains command history

mechanism inherited from the C Shell

• Use the builtin history to view your current history

• The history is, by default, stored in your home directory in a file

called bash_history

• History entries are ordered, the last entry being the most recently executed

• fix command

• fc –l lists recent commands

• fc –l [string] lists most recent

commands, starting with one that

Readline Completion

• The Readline library brings us

the wonders of tab completion

• An alias is just another name for another command (or commands)

• Ideally, the alias is shorter

and/or easier to remember

• Syntax (bash):

alias alias=‘commands_to_alias’

• Ex: alias ll=‘ls -l colors’

• Single vs double quotes –

variable substitution

• Like a shell script stored in memory

• Set like a variable (and use unset

to delete)

• [function] funct_name()

{

}

• Or use typeset or declare with –f

• Like a function? Wanna keep it?

 bashrc

Command Line Processing

Brace Expansion

• Used to specify multiple

filenames when pathname expansion doesn’t really apply

Arithmetic Expansion

• $((expression)) is evaluated

• Any variable names inside are

substituted for their values

• You can omit the $’s in front of the variable names within the

expression

• Command is run in a separate

subshell, usual rules apply

Process Substitution

• Substitutes a process where a

filename is required

• <(command) places the output of

the command as a filename argument

• >(command) places the input of the

command as a filename argument

• Ex: uniq <(cat list | sort)