File name expansions in Linux

In Linux, file name expansion (also known as globbing) is the process of generating a list of file names that match a given pattern or criteria. File name expansion is often used in combination with commands such as ls and rm to operate on a group of files that share a common pattern in their names.

There are several wildcard characters that you can use to specify patterns in file names:

• *: Matches zero or more characters
• ?: Matches any single character
• [characters]: Matches any single character that is listed between the brackets
• [!characters]: Matches any single character that is not listed between the brackets

Here are some examples of using file name expansion:

• ls *: Lists all files in the current directory
• ls *.txt: Lists all files in the current directory with the .txt extension
• rm *~: Deletes all files in the current directory that end with a ~ character
• ls file[123].txt: Lists the files file1.txt, file2.txt, and file3.txt in the current directory

You can also use {} to specify a list of patterns to match. For example:

ls file{1,2,3}.txt: Lists the files file1.txt, file2.txt, and file3.txt in the current directory

ls *.{jpg,png,gif}: Lists all files in the current directory with the .jpg, .png, or .gif extension

mv *.{txt,doc} /new/location: Moves all files in the current directory with the .txt or .doc extension to the /new/location directory

The command history is a record of the commands that you have entered at the command prompt. The command history is stored in a file, usually ~/.bash_history, and can be accessed and used to repeat previous commands or to quickly enter frequently used commands.

To view the command history, you can use the history command:

history

This will display a list of the most recently entered commands, along with their line numbers.

To execute a command from the history, you can use the ! followed by the line number of the command:

!42

This will execute the command that is on line 42 of the history.

You can also use the ! followed by a string to execute the most recent command that begins with the string:

!ls

!grep: Executes the most recent grep command from the history

!42: Executes the command that is on line 42 of the history

This will execute the most recent ls command from the history.

You can use the !! command to execute the most recent command:

!!

Quoting in Linux

quotes are used to enclose a string or group of characters, and they are used to preserve the literal value of the characters within the quotes. There are three types of quotes that are commonly used in Linux: single quotes (‘), double quotes (“), and backticks (`). Each type of quote serves a specific purpose, and it is important to understand the differences between them to use them effectively.

Single quotes are used to enclose a string that should be treated literally, without any special interpretation. This means that any variables or special characters within the single quotes will be treated as plain text, and will not be expanded or interpreted in any way. For example:

echo 'This string will be treated literally, $foo will not be expanded'

Double quotes are used to enclose a string that should be treated literally, but with some exceptions. Within double quotes, variables and certain special characters will be expanded or interpreted. For example:

echo "This string will be treated literally, but $foo will be expanded"

Backticks are used to enclose a command that should be executed and replaced with its output. This is known as command substitution. For example:

echo The current date is `date`

This command will execute the date command and replace it with the output of the command, so the final string that is printed will be something like “The current date is Sat Dec 18 2022”.

It is important to use quotes correctly in Linux, as they can affect the way a command is interpreted and how its output is displayed. Using the wrong type of quote can lead to syntax errors or unexpected results.

special characters are characters that have a specific meaning or function within the operating system or shell. These characters include:

• $: Indicates the beginning of a variable name
• *: Matches any number of characters
• ?: Matches any single character
• [ ]: Matches any single character within the brackets
• !: Negates the character class if used as the first character within the brackets
• : Escapes the special meaning of the character that follows it

To quote a special character in Linux, you can enclose it in quotes or use the backslash () to escape its special meaning. For example:

In these examples, the * character will be treated as a literal character and will not be interpreted as a wildcard.

It is important to quote special characters correctly, as they can affect the way a command is interpreted and how its output is displayed. Using the wrong type of quote or failing to escape a special character can lead to syntax errors or unexpected results.

Escape characters

An escape character is a special character that is used to indicate that the character that follows it should be treated literally, rather than being interpreted as a special character or command. The backslash () is the most commonly used escape character in Linux.

Here are some examples of how escape characters can be used in Linux:

To escape a special character:

echo "This string contains a * wildcard character"

In this example, the * character will be treated as a literal character and will not be interpreted as a wildcard.

To escape a space character:

cp file name new file name

In this example, the space characters between the words “file”, “name”, “new”, “file”, and “name” will be treated as literal space characters and will not be interpreted as separators between arguments.

To escape a newline character:

echo "This string will span multiple lines"

In this example, the newline character after “This string” will be treated as a literal newline character and will not create a new line in the output.

Escape characters can be used in various contexts in Linux, such as in shell scripts, commands, and configuration files. They are useful for preserving the literal value of special characters and for specifying characters that would otherwise be interpreted in a special way.