File globbing in Bash

File globbing, also known as wildcard expansion or globbing, is a mechanism used by command shells and programming languages to match and expand patterns in filenames or pathnames. It allows you to specify a pattern using special characters called wildcards or metacharacters, which are then interpreted by the shell or programming language to generate a list of matching filenames.

The most commonly used wildcards are:

• * (asterisk): Matches any sequence of characters (including an empty sequence).

• ? (question mark): Matches any single character.

• [ ] (square brackets): Matches any single character within the specified set or range. For example, [abc] matches either 'a', 'b', or 'c', while [0-9] matches any digit.

• [^ ] (caret and square brackets): Matches any single character not within the specified set or range. For example, [^0-9] matches any character that is not a digit.

Here are some examples to illustrate how file globbing works:

• *.txt: Matches all files with a .txt extension.

• file.*: Matches all files with a name starting with "file" and any extension.

• doc?.txt: Matches files like "doc1.txt", "docA.txt", but not "doc.txt" or "doc10.txt".

• [ab]*.txt: Matches files with a name starting with either 'a' or 'b', followed by any characters and a .txt extension.

File globbing is commonly used in command-line interfaces to specify file patterns for various operations, such as copying, deleting, or processing multiple files at once. It provides a convenient way to work with groups of files that share a common naming pattern or extension.

Example:

[ $a == z* ]

Let's break down the expression:

• [ and ] are used to enclose the conditional expression. They are part of the syntax and are required in most shell scripting languages for conditional statements.

• $a represents the value of the variable a.

• == is a comparison operator used to check if two values are equal.

• z* is a pattern or wildcard that matches any string starting with the letter 'z' followed by any number of characters.

Putting it all together, the expression [ $a == z* ] is evaluating whether the value of the variable a matches the pattern z*. If the value of a starts with 'z', the expression will evaluate to true; otherwise, it will evaluate to false.