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68 lines
2.0 KiB
Markdown
68 lines
2.0 KiB
Markdown
# Parameter
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Also the article for: [variable]{.underline}, [positional
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parameter]{.underline}, [special parameter]{.underline}
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In Bash, a parameter is simply an entity that stores values and can be
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referenced. Depending on the type, the parameters can be set directly,
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only indirectly, or only automatically by the shell.
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Bash knows 3 types of parameters:
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- variables
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- positional parameters
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- special parameters
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## variables
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A shell variable is a parameter denoted by a *variable name*:
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- containing only alphanumeric characters and underscores
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- beginning with an alphabetic character or an underscore
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A value can be assigned to a variable, using the variable's name and an
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equal-sign:
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NAME=VALUE
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Once a variable is set, it exists and can only be unset by the `unset`
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builtin command.
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The nullstring is a valid value:
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NAME=
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NAME=""
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## positional parameters
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A positional parameter is denoted by a number other than `0` (zero).
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Positional parameters reflect the shell's arguments that are not given
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to the shell itself (in practise, the script arguments, also the
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function arguments). You can't directly assign to the positional
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parameters, however, [the set builtin command](../commands/builtin/set.md)
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can be used to indirectly set them.
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The first to ninth positional parameter is referenced by `$1` to `$9`.
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All following positional parameters (tenth and above) must be referenced
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by the number given in curly braces, i.e., `${10}` or `${432}`.
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Unlike popular belief, `$0` is *not a positional parameter*.
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See also the [scripting article about handling positional
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parameters](../scripting/posparams.md).
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## special parameters
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There are a bunch of special parameters, which are set by the shell.
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Direct assignment to them is not possible. These parameter names are
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formed of one character.
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Please see [shellvars](../syntax/shellvars.md).
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## See also
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- Syntax article, internal: [pe](../syntax/pe.md)
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- Syntax article, internal: [shellvars](../syntax/shellvars.md)
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- Scripting article, internal: [posparams](../scripting/posparams.md)
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