Correct Bash and shell script variable capitalization

Question

By convention, environment variables (PAGER, EDITOR, ...) and internal shell variables (SHELL, BASH_VERSION, ...) are capitalized. All other variable names should be lower case.

Remember that variable names are case-sensitive; this convention avoids accidentally overriding environmental and internal variables.

Keeping to this convention, you can rest assured that you don't need to know every environment variable used by UNIX tools or shells in order to avoid overwriting them. If it's your variable, lowercase it. If you export it, uppercase it.

Any naming conventions followed consistently will always help. Here are a few helpful tips for shell variable naming:

• Use all caps and underscores for exported variables and constants, especially when they are shared across multiple scripts or processes. Use a common prefix whenever applicable so that related variables stand out and won't clash with Bash internal variables which are all upper case.

  Examples:

  • Exported variables with a common prefix: JOB_HOME JOB_LOG JOB_TEMP JOB_RUN_CONTROL
  • Constants: LOG_DEBUG LOG_INFO LOG_ERROR STATUS_OK STATUS_ERROR STATUS_WARNING

• Use "snake case" (all lowercase and underscores) for all variables that are scoped to a single script or a block.

  Examples: input_file first_value max_amount num_errors

  Use mixed case when local variable has some relationship with an environment variable, like: old_IFS old_HOME

• Use a leading underscore for "private" variables and functions. This is especially relevant if you ever write a shell library where functions within a library file or across files need to share variables, without ever clashing with anything that might be similarly named in the main code.

  Examples: _debug _debug_level _current_log_file

• Avoid camel case. This will minimize the bugs caused by case typos. Remember, shell variables are case sensitive.

  Examples: inputArray thisLooksBAD, numRecordsProcessed, veryInconsistent_style

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See also:

• The Open Group Base Specifications Issue 7 - Environment Variables

If shell variables are going to be exported to the environment, it’s worth considering that the POSIX (Issue 7, 2018 edition) Environment Variable Definition specifies:

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2017 consist solely of uppercase letters, digits, and the underscore ( _ ) from the characters defined in Portable Character Set and do not begin with a digit.

...

The name space of environment variable names containing lowercase letters is reserved for applications. Applications can define any environment variables with names from this name space without modifying the behavior of the standard utilities.

I do what you do. I doubt there's an authoritative source, but it seems a fairly widespread de-facto standard.