105 Shells and Shell Scripting KT

Question 1

105.1 Customize and use the shell environment

Answer 1

The following is a partial list of the used files, terms and utilities:

.
source
/etc/bash.bashrc
/etc/profile
env
export
set
unset
~/.bash_profile
~/.bash_login
~/.profile
~/.bashrc
~/.bash_logout
function
alias

Question 2

.

Answer 2

Command: Read and execute commands from the filename argument in the current shell context.

. filename [arguments]

Question 3

source

Answer 3

Command: Read and execute commands from the filename argument in the current shell context. source is a synonym for dot/period ‘.’ in bash, but not in POSIX sh, so for maximum compatibility use the period.

source filename [arguments]

Question 4

/etc/bash.bashrc

Answer 4

System‐wide .bashrc file for interactive non-login shells.

The Debian version of bash is compiled with a special option (-DSYS_BASHRC) that makes bash read /etc/bash.bashrc before ~/.bashrc for interactive non-login shells. So, on Debian systems, /etc/bash.bashrc is to ~/.bashrc as /etc/profile is to ~/.bash_profile.

Question 5

/etc/profile

Answer 5

/etc/profile: system‐wide .profile file for the Bourne shell.

When Bash is invoked as an interactive login shell, or as a non-interactive shell with the –login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.

Question 6

env

Answer 6

Command: Set each NAME to VALUE in the environment and run COMMAND. Run a program in a modified environment.

env [OPTION]… [-] [NAME=VALUE]… [COMMAND [ARG]…]

Question 7

export

Answer 7

Command: Set export attribute for shell variables. Marks each NAME for automatic export to the environment of subsequently executed commands. If VALUE is supplied, assign VALUE before exporting.

export [-fn] [name[=value] …]
export -p

Question 8

set

Answer 8

Command: Set or unset values of shell options and positional parameters. Change the value of shell attributes and positional parameters, or display the names and values of shell variables.

set [-abefhkmnptuvxBCHP] [-o option-name] [–] [arg …]

Question 9

unset

Answer 9

Command: Unset values and attributes of shell variables and functions. For each NAME, remove the corresponding variable or function.

unset [-f] [-v] [-n] [name …]

Question 10

~/.bash_profile

Answer 10

The ~/.bash_profile file is a configuration file for configuring user environments. The users can modify the default settings and add any extra configurations in it.

Question 11

~/.bash_login

Answer 11

The ~/.bash_login file contains specific settings that are executed when a user logs in to the system.

Question 12

~/.profile

Answer 12

The ~/.profile file is yet another configuration file that is read in the absence of the ~/.bash_profile and ~/.bash_login files.

Question 13

~/.bashrc

Answer 13

User specific .bashrc file. ~/.bashrc: read and executed by bash(1) for interactive non‐login shells.

Question 14

~/.bash_logout

Answer 14

The ~/.bash_logout file contains instructions for the logout procedure.

Question 15

function

Answer 15

Command: This builtin command is used to create a new function. A function is a list of commands that will be executed when the name of the function is entered.

function [OPTIONS] NAME; BODY; end

Question 16

alias

Answer 16

Command: An alias definition provides a string value that shall replace a command name when it is encountered.

alias [alias-name[=string]…]

Question 17

105.2 Customize or write simple scripts

Answer 17

The following is a partial list of the used files, terms and utilities:

for
while
test
if
read
seq
exec
||
&&

Question 18

for

Answer 18

A for loop is classified as an iteration statement i.e. it is the repetition of a process within a bash script.

for VARIABLE in 1 2 3 4 5 .. N
do
	command1
	command2
	commandN
done

Question 19

while

Answer 19

he bash while loop is a control flow statement that allows code or commands to be executed repeatedly based on a given condition.

while [ condition ]
do
   command1
   command2
   command3
done

Question 20

test

Answer 20

Exit with the status determined by EXPRESSION. Check file types and compare values. test is the same as [.

test EXPRESSION
test
[ EXPRESSION ]
[ ]
[ OPTION

Question 21

if

Answer 21

if statements enable different courses of action to be taken in a shell script, depending on the success or failure of a condition.

if [ ]
then

fi

Question 22

read

Answer 22

Reads a single line from the standard input, or from file descriptor FD if the -u option is supplied.

read [-ers] [-a array] [-d delim] [-i text] [-n nchars]
[-N nchars] [-p prompt] [-t timeout] [-u fd] [name …]

Question 23

seq

Answer 23

Print numbers from FIRST to LAST, in steps of INCREMENT.

seq [OPTION]… LAST
seq [OPTION]… FIRST LAST
seq [OPTION]… FIRST INCREMENT LAST

Question 24

exec

Answer 24

Execute COMMAND, replacing this shell with the specified program.

exec [-cl] [-a name] [command [arguments …]]
[redirection …]

Question 25

||

Answer 25

|| will evaluate the right side only if the left side exit status is non-zero (i.e. false).

[ Eval 1 || Eval 2 ]

Question 26

&&

Answer 26

The right side of && will only be evaluated if the exit status of the left side is zero (i.e. true)

[ Eval 1 && Eval 2 ]