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How to evaluate if an bash environment variable is set
for example
function! Myfoo(arg)
if $SomeVar is set/exist ----> how to eval the SomeVar has been set
....
endif
endfunction
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In the command window that opens, enter echo %VARIABLE%. Replace VARIABLE with the name of the environment variable you set earlier. For example, to check if MARI_CACHE is set, enter echo %MARI_CACHE%. If the variable is set, its value is displayed in the command window.
Vim can read and write environment variables within your current shell session. Use a $ prefix to identify an environment variable, as in the following examples. Insert the contents of the PATH environment variable into the buffer: Press i to enter insert mode, press Ctrl-r then =$PATH and press Enter.
To set an environment variable, use the command " export varname=value ", which sets the variable and exports it to the global environment (available to other processes). Enclosed the value with double quotes if it contains spaces. To set a local variable, use the command " varname =value " (or " set varname =value ").
You've (intuitively?) used the correct syntax; as :help expression-syntax explains (under :help expr-env), the syntax is $VAR.
You can compare with an empty string (if $SomeVar != "") or use empty() (if !empty($SomeVar)) to check whether a (non-empty) value has been supplied. It's not so easy to differentiate between empty environment variable and non-existing environment variable, so this is best avoided. (This distinction also is rarely used in shell scripts itself, neither.)
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