Variables in

About Variables

Variables

Variables are a place to hold data. Bash has two kinds of data: strings and arrays. We'll talk about arrays later.

You assign a variable like this

name="value"

x = 10

Variables can also be called parameters. The two terms will be used interchangably in this document.

Some important parameters used by or set by bash include:

• PATH -- a colon-separated list of directories used to find external commands.
• IFS -- the "internal field separator". We'll see how it's used below.
• PWD -- your current working directory. It will be the same value as the output of the pwd command.
• RANDOM -- a pseudo-random integer between 0 and 32767.
• BASH_VERSION -- the version string of the running instance of bash.

Variable Names

A variable name consists of uppercase letters, lowercase letters, numbers and underscore. It cannot start with a number.

Parameter Expansion

You get the value of a parameter with the $varname syntax.

x=10
echo "The value of x is $x"

To prevent the variable name from being confused with surrounding text, you can enclose the variable name in braces. For example to print the string 10x10

echo "${x}x$x"
# or
echo "${x}x${x}"

Some style guides recommend using braces in most cases for readability and/or consistency.

Positional Parameters

As we discussed in the Commands and Arguments lesson, commands can take arguments. Bash scripts are no exception. The arguments to the current instance of bash are called positional parameters. They can be retrieved with these special parameters:

• $1 -- the first positional parameter
• $2 ... $9 -- subsequent parameters
• ${10} ... -- double digits require braces.
• $0 -- the name of the current script.

Arguments to a shell function are also positional parameters, handled just like this. We'll see more about functions in a later lesson.

Special Positional Parameters

There are some special parameters that help you work with the positional parameters.

• $# expands to the number of positional parameters.
• "$@" expands to the list of all of the positional parameters, each as a separate word.
• "$*" expands to a single string of all the positional parameters joined together by a character.
  • The join character is the first character of the $IFS variable, which is a space by default.

The "$@" is the safest way to pass positional parameters to another command while keeping them safely quoted. It is also what you will use to loop over the positional parameters:

for arg in "$@"; do
    do_something_with "$arg"
done

In practice, "$@" is used more often than "$*".

Command Substitution

A very frequent operation you will do in bash scripts is to capture the output of a command and store it in a variable. To do this, you will use command substitution.

var=$(command)

For example, to upper-case a string, you can do this:

text="Hello world!"
uppercase=$(echo "$text" | tr '[:lower:]' '[:upper:]')
echo "$uppercase"
# => HELLO WORLD!

More on Parameter Expansion

Bash has many builtin facilities to manipulate variables and strings so that you don't need to call out to external commands.

• Uppercasing text. In the previous section I showed how to use tr to upper case some text. To do it in bash:

  echo "${text^^}"
  

  Similarly, ${text,,} lowercases the value.

• Provide a default value if the variable is empty.

  default="nothing here"
  result=""
  echo "${result:-$default}"
  # => nothing here
  

• Search and replace.

  text="Hello world!"
  echo "${text//[aeiou]/X}"
  # => HXllX wXrld!"
  

There are many other interesting parameter expansions: extract a substring, get the string length, assign a default value, remove text from the start or end of the value, and more. Read about them in the manual.