Introduction to Bash

Notes

Tilde Expansion

We can use ~ and the name of a user if we want to retrieve the home directory of a specific user.

echo ~codio

Tilde expansion lets us work in a user’s home directory even if we don’t know their user name.

We can use tilde + plus ~+ to represent the PWD, or to present the current directory that we’re working in.

We can use tilde + dash ~- to represent the OLDPWD, or the previous directory you were in if you’ve changed directories.

Brace Expansion

touch file_{1..3}{a..c}

Parameter expansion

allows us to retrieve and modify stored values. The symbol for this is ${...}.

Let’s set a parameter in the terminal:

book="Where the Sidewalk Ends"

We can retrieve the value of the parameter with a dollar sign $ and the name of the parameter.

echo $book

For example, echo ${book:6} returns the parameter’s value starting with the sixth character in the string.

echo ${book:6}

Pattern Substitution

Pattern substitution is a parameter expansion feature that lets us obtain a value and change it depending on a pattern. This is commonly used for replacing a word or character using the pattern below.

echo ${variableName/oldWord/newWord}

Below is a complete pattern substitution example.

echo ${book/Ends/Begins}

In Pattern Substitution, one slash after the parameter replaces the first instance of a match, and two slashes replaces them all.

Let’s replace all e’s with ampersands & using two forward slashes //.

echo ${book//e/&}

Now, let’s do the same thing with only one slash.

echo ${book/e/&}

Named Character Classes

Named character classes are used to print values that have been given a name. Their value is determined by the LC_CTYPE locale. A few examples include:

• [:alnum:]: Prints all files that include alphabets and numbers. Both lower and uppercase letters are taken into account.

• [:alpha:]: Prints all files that exclusively contain alphabets. Both lower and uppercase letters are taken into account.

• [:digit:]: prints all files that include digits.

• [:lower:]: prints all files with lower-case letters.

• [:punct:]: prints all files that include punctuation characters, including:

  • ! " # $ % &' () * +, –. /: ; = >? @ [] '|.

• [:space:]: prints all files that contain space characters.

• [:upper:]: prints all files with upper-case letters.