Sobell ch 8: The Bourne Again Shell (Bash)

quoting $

"$variableName" does not work '$name' and /$name do work

-x oprtion

#! /bin/bash -x turns on debugging

commands that are symbols

(,),[,],

order of files shell looks for

.bash_profile .bash_login .profile

what will this yield false && false || true echo $?

0 true

what will this yield (false && false) || true echo $?

0 true

what will this yield? true || false && false echo $?

1 (false)

what will this yield? (true || false) && false

1 fasle

&&

Boolean AND short-circuiting control operators makes shell test exit status of command before. if command succeeds, goes to next, else skips next ex mkdir bkup && cp -r src bkup

suspending a program

CONTROL -Z temporarily stops incudes message that says stopped

keyword variables

HOME, PATH

how to declare and initialize a variable in bash

VARIABLE=value no white space characters on either side of = ex person=max echo person *person* echo $person *max*

chmod u+x filename

add execute permission (x) for the owner (u)

job

another name for process runing a pipeline can have mamny jobs running by seppearating commands with &&

(())

arithmetic expansion

&

background task shell executes task in the background and display prompt immediately to work on other tasks will display job number in [] and PID of each process running will then display done if done can layer: d & e & f &

bash shell

based on bourne shell

creating a shell to get input from a file OR supply commands to bash using standard input

bash filename do not need exectue permission for filename. only read permission OR bash < filename

#

begins a comment (whole line)

||

boolean OR short-circuiting control operators remaining commands execute only if first command failed ex mkdir backup|| echo "mkdir of bkup failed" >> /tmp/log

fg

brings a job to the foreground shell assigns job number. use that ex fg 2

is shell variable available only in the shell it was created?

by default yes. they are local

what can a shell script do

can run a utility, program (compiled), or another shell script

;;

case terminator

chsh

chamges shell pore permanently and anter password and pathname of shell you want to use (/bin/bash )

nameOfShell RETURN

changes shell login nameOfShell RETURN (bash, tcsh, another shell) give exit command to return to previous shell

[n]<&-

closes standard input or file descriptor n if specified

[n]>&-

closes standard output or file descriptor n if specified

NEWLINE

command initiator initiates execution

;

command separator

$()

command substitution

standard error

commands sending output to this. to keep from getting mixed up with information it sends to standard output by default to the screen

[[ ]]

conditional expression

.bash_logout

contains commands to log out. clean up after a session. remove temporary files

/

continue a command to next line. when command is long

PATH

determines which directories shell searches

dirs

displays the contents of directory stack if empty, displays working directory

which is the faster way to run a shell script

do not use bash to execute. instead give yourself execute permission and directly invoke script

<&m

duplicates standard input from file descriptor m

[n]>&m

duplicates standard output or file descriptor n if specified from file desccriptor m

bash -e

errexit causes bash to exit when simple command fails helps make program less fractious

exec

execute

./

execute ex ./whoson

korn shell

expanded from Bourne Shell nd similar to BASH

shell script

file that holds commands the shell can execute

startup files

files that initialize shell when it starts

login shells

first shell that displays prompt when login on system

( )

groups commands

/etc/profile

holds commands the shell executes. allows users to work with root privileges.

$?

holds exit status of preceding command false; echo$? 1

HOME

identifies home directory

exit status

if it executed successfully or not 0 is true 1 is false

jobs

lists jobs running (ex like if you put something to sleep)

chmod

makes file executable

to execute a file,

must have permission to read and execute

do any of the control operators need to be surrounded by white spcae?

no but it makes it easier to read

does sh call the original bourne shell?

no. its symbolic link to bash or dash command bash tries to mimic behavour of original bourne shell - doesnlt always work

.bashrc

nonlogin shell that interacts

bash -u

nounuset display message and exit when nit tries to eexpand an unset variable ex #! /bin/bash -eu

Sh

original Bourne Shell on UNIX system

|

pipeline returects output of one command to be input of next command

file descriptor

place a program sends its output to and gets input from execute program, shell opens: 0(standard input) 1 (standard output) 2 (standard error)

popd

pops directory off the stack working directory is new top directory popd +1

POSIX

portable operating system interface.

pushd

pushes a directory on the stack when pushd w/o arg, swaps top two directories on stake. top directory is second directory easily move back and forth between directories (2)

>> filename

redirects and appends stanard output to filename. creates if dne

<

redirects standard input

>

redirects standard output ex: cat x y > hold *error that x dne will pop up* *ys stuff will be in hold*

&> filename

redirects standard output and standard error to filename

>! filename

redirects standard output to filename

bg

sends job to the background must first suspend ctr Z

; and NEWLINE

separate commands can use ; to write a sequence of commands on one line initiate execution pressing RETURN

() groups command

shell creates a copy of itself (subshell) for each group and creates new process to execute

~

short for home directory

#!

specifies a shell. put it in first line of shell script to tell OS which shell (program) should execute file + options ex: #!/bin/bash

|&

standard error pipeline

variables

start with letter or underscore no number

where to change keywords variables?

startup files

()

subshell

[ ]

test command

- under permissions

that user does not have permission

precedence of && and ||

they are the same unless parenthesis are used. else will be equally evaluated from left to right

special paramters

those like last command executed satus of recently executed command. you cannot assign these

user created variables

those user creates can change at any time or make them readonly

control operators

tokens that separate, temrinate, and group commands

?

true false utilities do nothing. return 0 or 1 exit statuses ex: false; echo$?

ps -f

underneeath first line in shell script displau name of program executing script

having spaces in variables

use " double quotes person="mac and zac"

what should you do if you only want to run using a different shell or if bash is not you rinteractive shell

use the bash command

dash shell

user login shell. executes more quickly than bash

shell parameter

value you or shell can access like user created variables keyword variables positional parameters special parameters

declare and initialize variable in a script

variable=/home/sam/temp

run a command with variable set to something

variable=/home/sam/temp ./my_script