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What is Shell Scripting?

• A shell script is a series of commands aggregated within a single structured file

• A command shell executes this file line-by-line, one command at a time as if individually executed on the command line

• A shell script may use control structures, variables and dynamic inputs to control how and when commands are executed

• In this sense, shell scripting is a form of computer programming

• Shell scripts allow users to automate repetitive, multi-step tasks and often complex tasks on a Linux installation

• Shell script is also used to create custom tools designed to perform highly specific tasks for which no pre-made solution is available

• Scripts are usually designed to be executed in a command-line interface, although they can be triggered in other ways, e.g. a scheduled chron job.

Shell Script Commands, Keywords and Utilities

• The bash commands, keywords and utilities that follow are those you will use to complete your workshop activities and assignments

• You may not need to use all of those listed, but you will probably end up using most of them

• Most, but not all, are specifically addressed in the unit’s lecture materials

• You will find that the definitions and usage of those that are not can be found at https://ss64.com/bash/

Essential bash Commands

mkdir

• The mkdir command is used to create directories

• The flags commonly associated with the mkdir command are shown in the table

EXAMPLE:

mkdir -m 755 -v mydirectory

• This command creates the directory mydirectory with permissions set to read, write, and execute for the owner, and read and execute for others

• Additionally, it will display a message indicating that the directory has been created

cat

• The cat command (short for concatenate) is used to display the contents of one or multiple files on the standard output

• While it doesn't have many flags of its own, it can be combined with other commands or features to achieve specific functionality.

Cat options example :

ls

• The ls command is used to list the files and directories that reside within a specified directory

• It provides a number of options that allow its output and behavior to be modified from the defaults, with the most common being:

cp

• The cp command is used to copy files and directories

• It provides several options (flags) to control its behavior, the most common being:

mv

• The mv command in bash is used to move or rename files and directories

• It provides several options (flags) to control its behavior, the most common being:

rm

• The rm command is used to remove/delete files and directories

• It provides several options (flags) to control its behavior, the most common being:

Writing a bash shell script

• Most programming beginners start out by writing a simple program to output the words Hello World on the screen

• In bash scripting, this can be accomplished with the script shown to right

Executing the script

• In most operating systems, files are not allowed to be directly executed as scripts by default

• Most scripts will need to have the correct permissions set to allow them to be executed

• A script without the execute permissions can be manually executed by invoking the bash command but this is not ideal

$ bash hello1.sh 

• Permissions can be set using the chmod command using chmod +x hello1.sh or chmod 7[n][n] hello1.sh and only needs to be done once

• After the file is marked as executable, the script can be run by prepending it with ./ and hitting Enter, i.e. ./hello1.sh

Writing and Executing a bash Script

Using the VS Code development environment:

Working with default variables and arguments

• Information in bash scripts can be stored in variables

• Once information is stored in a variable, it can be called upon in the script by invoking the variable’s name