linux crash course chapter 05 3

In this chapter …• The command line • Input, output, and redirection • Process management • Wildcards and expansion • Builtins... The Command Line• It is what allows us to talk to the sh

Chapter 5:

The Shell

The Man in the Middle

In this chapter …

• The command line

• Input, output, and redirection

• Process management

• Wildcards and expansion

• Builtins

Let us recall …

• Shell is an interpreter

• Sits between the user and the kernel

• We’ll be using bash as our point of reference

The Command Line

• It is what allows us to talk to the shell

• A command line is a string of commands and arguments ending with Enter

• Shell takes the command line, interprets it,

then instructs the kernel

• command [arg1] [arg2] … [argn]

• Spaces separate command and arguments

• Arguments can be optional

• Some arguments are called Options, and are proceeded by one or more hyphens (aka

switches or flags)

Do I Need Arguments?

• Some commands work without any

arguments specified (ex., ps)

• Others need one or more

• If a command expects at least one argument, often it will display a usage message if you

execute the command without any

arguments

• Each space-delimited word in a command

line is called a token and are numbered from left to right starting at zero

• That means the command is token 0, first arg

is token 1, etc

• Token can be a filename, a string, a number

• Sometimes you can combine options

together with a a single preceding hyphen

Command Line Example

human readable format for sizes

directory to perform this

ls command upon

• Once enter is pressed, command line is

parsed by the shell

• Reads the first token and tries to find the

executable or builtin associated with it

• Passes remaining tokens to the called

program, doing expansion and manipulation

as necessary

Where’s the program?

• If not given absolute pathname, shell

searches PATH variable for location of first token

• Can’t assume PATH includes your current

• Shell begins a new process and hands

control over to called program

• Shell then goes to sleep, and waits for called program to finish running and return control

• Program also passes its exit status to shell

• Streams contain data (usually text)

• Three standard streams

– Input

– Output

– Error

• Each stream can be associated with any

number of things (remember, everything is a file!)

Standard Input

• Contains information to be passed to a

program (a utility, a shell, etc)

• Most commonly is the keyboard

• Also commonly a file

• Because everything is treated as a file,

programs never know exactly where input truly comes from

Standard Output

• Contains output from a program

• Again, program never sure where it’s

sending output to

• Most commonly the screen

• Could also go to a printer, a file, or directly into standard input (to another program)

Standard Error

• Another output stream

• Contains error messages

• Usually also goes to screen

• You can redirect both standard out and standard error to different places

Fun with cat

• cat displays the contents of files

• Try calling cat with no argument

• It takes input from standard input (instead of

a file)

• Will echo back every line you type

• Hit CTRL-D (EOF) to quit

• There are ways to permanently change

streams – beyond our scope

Redirecting Output

• Output is usually a file, but can be anything

(for example, a printer)

• The greater-than sign instructs the shell to redirect standard out to whatever is to the right

• If output exists, it will be overwritten – be

careful!

Redirecting Output con’t

• To prevent overwriting, we can set the

noclobber variable

• You can override noclobber using >|

• To append to the end of a file, use >>

• You can’t redirect output to a file you’re using

as input – the shell allocates the file for the output stream before calling the program

Redirecting Errors

• Syntax: command [args] 2> errors

• You’re redirecting the second output stream (standard error)

• Standard output can also be written 1>

• You can redirect both streams:

– find whizzbang > results 2> /dev/null

Redirecting Input

• Syntax: command [arguments] < input

• Input usually is a file

• Not all that useful if the command supports a filename for one of the arguments

• Can be handy if the command prompts for

input … you can automate the process with a file filled with the answers

• Redirects the output of one program to be

the input of another

• Reduces the need for intermediary steps

• Handy for when you have several quick

steps to perform upon some data

• Great for getting output and quickly filtering it

(ie command | grep search_string)

Getting Crazy

• tee allows you to send output to two

different locations at once

• It sends output to a file, while still passing the output onto standard output

• Cascading tee commands can go on

indefinitely

Running Jobs in the Background

• Thus far we’ve always run commands in the foreground

• Have to wait until it finishes before you can access the command line again

end of your command line before hitting

enter

Background, con’t

• Once you hit enter, it will display the job number, and the process ID number (PID)

• You may resume issuing commands

• Once the job finishes, the job number and the word Done will appear on your screen

• Remember, to check on things, use jobs

Back and Forth

• Recall that pressing CNTRL-Z suspends a process, and it gives it a job number

• To start it again in the foreground, use:

fg job_num

• To start it again in the background, use:

bg job_num

• Forgot the PID? Just use ps

• Won’t die? Don’t forget kill -KILL

• Special characters interpreted by the shell

• Also called wildcards

• Shell expands ambiguous file references into

a list of files matching that criteria

• Process is called globbing or expansion

? Character

• Used to represent any single character

• For example: foo?

– Would match food, fool, foot

– Would NOT match fooey, footsie

• Can be anywhere in the filename

Ex: b??kk??pper

• On exception – doesn’t match leading periods (for hidden files)

– Would match food, foot, footsie, foo

• Also does not match leading periods

[ ] Characters

• Used to match a list of possible characters

• Also known as a character class

• Ex: [aeiou].jpg

– Matches a.jpg, e.jpg, i.jpg, etc

• Can also do ranges of letters / numbers

– Ex: [0-9] [a-z] [A-Z]

[ ] Characters con’t

• You can negate a character set by placing

a ! or ^ after the first bracket

• Ex: [^0-9]

– Does not match the numbers 0-9

• Programs never see the metacharacters

• To let the program see them, you must quote them or escape them

• Commands that are built into the shell

• Shell does not create a new process to run a builtin

• Run very quickly

• Shell will still do metacharacter expansion for builtins

• We’ll cover them in greater detail in Ch 10