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 mydirectoryThis 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
