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Most languages have naming conventions for variables, the most common style I see in shell scripts is MY_VARIABLE=foo. Is this the convention or is it only for global variables? What about variables local to the script?

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    The only one that I know of which everyone should follow is all uppercase names should be reserved for the shell. Don't use them to avoid accidentally clobbering something important like PATH or HOME or anything else the shell might reserve in the future.
    – jw013
    Jul 11, 2012 at 20:53
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    Actually, all uppercase names are typically used for environment variables. Some variables (like PATH) are interpreted by the shell, while others (like LANGUAGE or PRINTER) may be interpreted by other programs, but there is nothing otherwise special about them.
    – jlp
    Jul 11, 2012 at 23:01
  • 'environment variables' is indeed the proper name, I'll include it in my answer.
    – jippie
    Jul 12, 2012 at 6:45
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    While not authoritative, this Google guide has good suggestions: google.github.io/styleguide/shell.xml. It suggests sticking to all caps only for constants and exported variables, snake case for everything else. Personally I like camel case for my globals since no one else recommends it, which lowers the probability of naming collisions. Plus I like the way they read.
    – Binary Phile
    May 22, 2016 at 14:58
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    Similar question on SO: Correct Bash and shell script variable capitalization.
    – codeforester
    May 11, 2018 at 17:05

3 Answers 3

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Environment variables or shell variables introduced by the operating system, shell startup scripts, or the shell itself, etc., are usually all in CAPITALS1.

To prevent your variables from conflicting with these variables, it is a good practice to use lower_case variable names.

1A notable exception that may be worth knowing about is the path array, used by the zsh shell. This is the same as the common PATH variable but represented as an array.

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    lower_case underscore separated or camelCase?
    – Garrett Hall
    Jul 11, 2012 at 20:55
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    @GarrettHall That's entirely up to you. Once you pick one stick with it. Consistency is more important than the actual choice.
    – jw013
    Jul 11, 2012 at 20:57
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    matter of taste? I personally like the C-style camelCase because it is shorter and doesn't use the ugly underscore. Taste, style, ...
    – jippie
    Jul 11, 2012 at 20:57
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    matter of taste? I personally like underscore separated, easier to read.
    – janos
    Jul 12, 2012 at 8:22
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    For completeness, environment variables aren't the only category of conventionally all-uppercase shell variable names -- this rule also applies to builtins (like PWD, PS4, or BASH_SOURCE).
    – Charles Duffy
    Jan 7, 2015 at 23:49
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Yes, there are full code style conventions for bash, including variable names. For example, here's Google's Shell Style Guide.

As a summary for the variable names specifically:

Variable Names: Lower-case, with underscores to separate words. Ex: my_variable_name

Constants and Environment Variable Names: All caps, separated with underscores, declared at the top of the file. Ex: MY_CONSTANT

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    These convention are only Google ones for their own open source projects: though they could be very good rules, it could not apply to all projects.
    – smonff
    Feb 4, 2017 at 14:16
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Underscores to separate words seem to be the best way to go.
I have a few reasons to prefer snake_case over camelCase when I'm free to choose:

  1. Flexible: You can use upper case and lower case (e.g. MY_CONSTANT and my_variable);
  2. Consistent: The digits can be separated to make the number more readable (e.g. 1_000_000_000) and this feature is supported in many programming languages;
  3. Common: Common at the point the regex \w handles underscores like word characters and numbers ([a-zA-Z0-9_]).
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    I believe we should steer clear of attempts to rationalize stylistic decisions (or opinions in general) because the rationale is usually tenuous and can swing either way with little effort. (Also, it polarizes people, often inciting religious wars and time wasted on immaterial issues.) Specifically in re: your reasons, camelCase does not preclude #2, and #3 is trivially mitigated by considering capitals as additional word boundaries. Re: #1, there is no reason why LOUD_SNAKE can't be used with camelCase—that is a common combination, the former often being for constants/globals.
    – Ezekiel Victor
    Mar 31 at 2:01

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