Allowed characters in Linux environment variable n

问题:

What characters are allowed in Linux environment variable names? My cursory search of man pages and the web did only produce information about how to work with variables, but not which names are allowed.

I have a Java program that requires an defined environment variable containing a dot, like com.example.fancyproperty. With Windows I can set that variable, but I had no luck setting it in linux (tried in SuSE and Ubuntu). Is that variable name even allowed?

回答1:

From The Open Group:

These strings have the form name=value; names shall not contain the character '='. For values to be portable across systems conforming to IEEE Std 1003.1-2001, the value shall be composed of characters from the portable character set (except NUL and as indicated below).

So names may contain any character except = and NUL, but:

Environment variable names used by the utilities in the Shell and Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase letters, digits, and the '_' (underscore) from the characters defined in Portable Character Set and do not begin with a digit. Other characters may be permitted by an implementation; applications shall tolerate the presence of such names.

So while the names may be valid, your shell might not support anything besides letters, numbers, and underscores.

回答2:

The POSIX standards on shells section of IEEE Std 1003.1-2008 / IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard doesn't define the lexical convention for variable names, however a cursory look at the source reveals it uses something similar to

[a-zA-Z_]+[a-zA-Z0-9_]*

(Edit: Added missing underscore in 2nd character class.)

A quick note, as some shells don't support the + in regex, a potentially more portable regex may be:

[a-zA-Z_]{1,}[a-zA-Z0-9_]{0,}

回答3:

My quick testing showed that they basically follow the same rules as C variable names do have, namely

1. a-z, A-Z, _ and 0-9
2. May NOT begin with a number

So this excludes . inside them. Any illegal variable name is credited with unknown command.

This was tested in ZSH, which is mostly BASH-compatible.

回答4:

Depends on what you mean by 'allowed'.

Ignoring Windows for the nonce:

The environment is an array of strings, passed to the main function of a program. If you read execve(2), you will see no requirements or limits on these strings other than null-termination.

By convention, each string consists of NAME=value. There is no quoting convention, so you can't have an '=' in the name in this convention.

Normal humans set these strings by discussing them with their shell. Each shell has it's own ideas of what are valid variable NAMEs, so you have to read the man page for the shell-of-the-moment to see what it thinks.

Generally, things like com.baseball.spit=fleagh are Java system properties, and whether or not some Java program is willing to fall back to the environment, it's better to specify them with -D.

回答5:

It depends on the shell. I'm guessing you're using bash by default, in which case letters, numbers and underscores are allowed, but you can't start the variable name with a number. As of Bash v.3, periods are not allowed within variable names.