Linux CLI

56 – shell scripts input text The read command is how a shell script asks the user for input. It reads a line from standard input (usually the keyboard), splits it into fields, and assigns them to variables. With a few options, it can prompt, hide input, read a fixed number of characters, or time…

Functions let you encapsulate code into reusable blocks. Instead of repeating the same commands in several places, you define them once and call them by name. This makes your scripts cleaner, shorter, and easier to maintain. Key point: Shell functions are not like functions in C or Python. They don’t have a return type, they…

Part 1 covered how to define and call functions. Part 2 is about making them robust — how functions report success or failure, how to stop a script when something breaks, and how to catch errors before they cascade. These techniques separate a fragile script from one you can trust in production. Key point: In…

59 – shell functions flow control and if Flow control is what turns a script from a straight line of commands into a program that makes decisions. The if statement is the most important of these — it lets you run different code depending on whether a condition is true or false. Combined with operators…

The test command is the engine behind every if condition you’ve written so far. [ … ] is actually a synonym for test — they do exactly the same thing. Understanding test directly makes your scripts clearer and helps you avoid quoting bugs. Key point: test evaluates a conditional expression and returns an exit status…

The older [ ] test has been around since the earliest days of Unix. [[ ]] is bash’s modern replacement — it fixes the quoting headaches, adds regex and pattern matching, and generally makes conditions safer to write. And (( )) is its arithmetic counterpart — a cleaner way to do integer math than expr…

Loops are how a script repeats work. Instead of writing the same command ten times, you write it once and let the loop run it ten times. Bash gives you two main conditional loops — while and until — plus for for iterating over lists. This chapter covers while and until; for comes next. Key…

When you have one value to check against many possible patterns, a chain of if/elif/else gets ugly fast. The case statement is the cleaner alternative — it matches a value against a list of patterns and runs the first block that matches. Key point: case compares a single value against multiple patterns. Each pattern ends…

The for loop is the most-used loop in shell scripting. Where while and until repeat until a condition changes, for iterates over a known list of items — numbers, words, files, command output. It’s the loop you reach for when you know what you’re looping over. Key point: Bash gives you five ways to write…

Parameter expansion is what turns bash variables from simple placeholders into a real text-processing toolkit. With a single ${…} expression you can extract substrings, provide defaults, replace patterns, strip prefixes and suffixes, and get the length — all without calling external commands like sed, cut, or awk. Key point: Parameter expansion happens inside the shell…