Shell Script Elements

Cybersecurity Linux Lessons 3.1.1 and 3.1.2

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Part 1

Loops

• A “while” loop executes a set of commands if a specified condition is true
• A “for” loop iterates over a sequence, like a range of numbers, and performs commands
• An “until” loop executes a set of commands if the specified condition is false

Conditionals

Control structures that allow one to make decisions and execute different sets of commands based on true/false conditions
• An if statement executes a set of commands based on the evaluation of a condition
• Switch/Case provides multiple possible execution paths based on the value of an expression

Shell Parameters

A feature in shell scripting that allows one to manipulate and expand the values of variables
• Extract substrings, perform pattern matching, etc.
• Globbing matches filenames with patterns
• Brace expressions generate arbitrary strings using curly braces

Shell Comparisons

Essential for making decisions based on the values of variables or the success/failure of commands • Using double parentheses, arithmetic operations can be performed
• Strings can be compared with operators like == (the same) or != (not the same)

Boolean Logic

• Shell scripting doesn’t have a native boolean type but still uses boolean logic
• Users can combine conditions using logical operators, && for AND, || for OR

Shell Variables and Search and Replace

• Used to store and manipulate data • Act as placeholders that can be referenced or modified within a script
• If a piece of text needs to have a part replaced, “search and replace” does that

Regular Expressions

• Regex or regexp are powerful patterns used for matching character combinations with strings
• Wildly used for tasks like string manipulation, text parsing, and pattern matching

Part 2

Shell Script Elements

• The | (Pipe) connects the output of one command as the input to another command
• This helps chain commands together
• The example below generates a fortune and uses the tool cowsay to display the fortune

Or, Oar, or Ore

• The || (OR) command executes the command following it ONLY IF the preceding command fails/returns an error
• Often used in control flow
• An alternative to an if-then-else command

Redirection

• The > (Output Redirection) redirects the standard output to a file, overwriting it if the file already exists
• The » (Also Output Redirection) appends the standard output to the end of a file, rather than overwrite it
• The < (Input Redirection) redirects the input from a file to a command
• The « (Here Document) allows multiple lines to be put into a command

And, &, and &&

• The &> combines standard output and standard error to a specified file, overwriting the file if it already exists
• The && (AND) command executes the following following it ONLY IF the preceding command succeeds - Understanding Boolean logic and truth tables makes understanding these operators a little easier

Standard Standards

• STDOUT is the standard output stream where a program writes its regular output
• STDERR is the standard error stream where a program writes its error messages

Exit Codes

• Also known as return codes or status codes
• Numeric values returned by a command or program to indicate the result of its execution - 0 typically means success
- Nonzero values typically indicate an error or some kind of failure
• The OS or shell keeps track of the exit code so other scripts or processes can proceed

Shell Built-In Commands

• Built into the shell itself
• More efficient because they don’t require a separate process
• read reads input from a user or file
• echo prints text or variables to the standard output
• source execute commands from a file in the current shell environment