Week 7 - Shell Scripts Part 1

Question 1

Basic commands

Answer 1

ls - lists contents of directory
cp - copies files
mv - moves files rename
touch - creates file
rm - deletes file
echo - prints to standard output

Question 2

more commands

Answer 2

grep - search regular expression
less - display big files, one screen at a time (space)
pwd - print working directory
diff - see difference between 2 files
find - find file in a directory tree

Question 3

navigation commands

Answer 3

cd - change directory
mkdir - make directory
rmdir - remove directory

Question 4

echo command

Answer 4

writes arguments to standard output
- standard output = display screen
- can be changed = e.g. could be changed to a file

Question 5

find command

Answer 5

utility to search through one or more directory trees of a file system
locates files based on some user-specified criteria

Question 6

Shell Scripts

Answer 6

Series of BASH commands
Anything you can type in the command line, you can include in a script
Mainly used to automate tasls

Question 7

Shell Scripts

Answer 7

Programming Language
– Set of rules for instructing a computer how to
perform specific tasks
– C, C++, C#, Java are programming languages

Question 8

BASH Script

Answer 8

A BASH script is a list of instructions that is interpreted one instruction at a time
– we have to specify the interpreter as the first line of the script as in #!/bin/bash
- BASH is also a programming language

Question 9

Compiled Languages

Answer 9

e.g. Java, C#

Program written in language and translated all at once from
source code into executable machine code
– Compiler: program which translates source code into executable program.

Question 10

Interpreted Languages

Answer 10

e.g. BASH, PHP
– Programmer enters instructions one at a time
– Interpreter: accepts and parses instruction, then converts to
appropriate executable instruction(s), and executes it.

Question 11

Script

Answer 11

Sequence of instructions read by interpreter

Question 12

BASH Shell Script

Answer 12

– Contains a sequence of BASH commands to execute line by line
– Used in troubleshooting
– Server Maintenance
– Data Backup
– Some scripts run when the Linux system starts
* Need to know how to manage these scripts if problems occur during the boot process.

Question 13

Creating Shell Scripts

Answer 13

1.
Create a shell script:
– Create a file
– Write commands in sequence
(e.g. using vim)
– Assign execute permission for the file
2. Run a shell script:
– Enter the absolute or relative path to where it’s stored

Question 14

Variables

Answer 14

Scripts will need to:
1. Store information
2. Access stored information

Storage locations that have names
Name - value pair

$ notation used to retrieve value

Question 15

Environment Variable

Answer 15

– Placeholder for data that can change
– Gets its value automatically from the OS startup or
the shell being used
– Each user has his or her own environment variables

common environment variables
HOME, USER, PATH, HOST

Question 16

Environment Variable
env command

Answer 16

Displays a list of all environment variables and their stored values

Question 17

Environment variable
echo command

Answer 17

echo command with $ symbol before the variable name
- display value of particular environment variable

Question 18

Shell variable

Answer 18

Similar to an environment variable
– Value is assigned in a shell script
– Related to a particular script
* Not necessarily the global environment

Three methods to store information in a variable
1. Direct Assignment
2. Prompt Method
3. Positional Parameters

Question 19

Direct assignment Variable

Answer 19

Direct assignment method
– Specify the variable’s value in the command
* Output value of variable using
echo $variable_name

Question 20

The Prompt method

Answer 20

Prompt method
– User is asked to enter a value for the variable

Question 21

Positional Parameters

Answer 21

Positional parameter method
– Parameter passed as argument to command
* command [-options] [arguments]

– Uses the order of arguments in a command to assign values to variables on the command line
– Variables from
$0 to $9 are available
* Values are defined by what the user enters

$0 represents name of script,
$1 represents first argument …. $9 represents ninth argument

Question 22

Exit Status Code $?

Answer 22

Exit status code is sent to the shell
– When you quit a program or a command
– Is updated after each command is executed
* Successful commands:
Exit Status Code = 0
* Command that fails:
Exit Status Code > 0
* Code isn’t actually displayed onscreen
– Reference it with $?

Question 23

Conditions

Answer 23

Tell interpreter to skip commands based on a
condition
* if statement
– Carry out certain commands based on testing a condition

Question 24

Common condition statements used in scripts:

Answer 24

– if statement starts the condition being tested
– then statement starts the portion of code specifying what to do if the condition evaluates to true
– else statement starts the portion of code specifying what to do if the condition evaluates to false
– fi statement indicates the end of the condition being tested

Question 25

Menu Scripts

Answer 25

Menu scripts
– Allows users to choose from a list of options
– Can create a menu script with
if and then statements
* elif statement
– Combines the else and if statements
– Create multiple conditions without closing each condition

Question 26

The case statement

Answer 26

– Uses one variable to specify multiple values and matches a portion of the script to each value
Double semicolon (;;)
– Marks the end of each code portion matching a specific value
* *) character
– Runs if the value the user enters doesn’t match any of the choices specified in the case statement