Bash Variables

For a list of special variables are used internally by the bash(1) shell, see here.

Declaring

Variables can be explicitly declared with the declare builtin.

declare a=1 b=2 c=3

A variable can be declared without a value, and this is distinct from not being set.

declare d

Variables can also be created implicitly.

a=1
b=2
c=3

And as with declare, variables can be set empty.

d=

For declaring array variables, see here.

Within a function, local variables can be declared with the local builtin.

local a=1 b=2 c=3

Note that if declare is used within a function, those variables are also locally-scoped.

Usage

Variables are accessed by their name. A dollar sign ($) must be prefixed to the name.

a=foo
b=$a       # 'foo'

To delimit a variable name from string literals, use braces. For example:

a=foo
b=${a}bar  # 'foobar'

If a variable's value includes a character that the shell will interpret specially, quote the variable.

a=foo
b="$a bar" # 'foo bar'

Special Variables

The following variables are set automatically by bash(1).

Positional Variables

The name of a command is stored in $0.

The first argument to the command is stored in $1. And so on until the 9th argument, $9.

From the tenth argument, while arguments are stored in an indexed variable, they must be accessed differently. $10 is interpretted like ${1}0. To actually access the 10th argument, try ${10}.

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