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Most operating systems provide a mechanism for passing auxiliary parameters implicitly to child processes. Usually, this mechanism is called "the environment", and is conceptually a map from string names to string values. The string names are called environment variables.
Some applications rely on environment variables to modify their behavior according to local settings. Also, various established protocols rely on environment variables as a form of interprocess communication. For example, most implementations of the common gateway interface (CGI) use environment variables to pass Meta-Variables from the server to the script [1]. Environment variables are also required by SRFI 96: SLIB Prerequisites. Providing a means to access environment variables is therefore indispensable for writing practical programs in Scheme.
Most widely-used Scheme implementations provide a function for getting the value of a specified environment variable. The name for this function is usually getenv, but varies (see below). Some implementations also provide a way to get all the environment variables, but others do not.
This SRFI specifies a uniform interface for accessing environment variables. That should make it easier to write portable programs that need access to their environment. For example, a CGI program may portably obtain the values of the Meta-Variables "QUERY_STRING", "CONTENT_LENGTH" and "REQUEST_METHOD" as in the following examples:
(get-environment-variable "QUERY_STRING") => "foo=bar&huga=hige" (get-environment-variable "CONTENT_LENGTH") => "512" (get-environment-variable "REQUEST_METHOD") => "post"
[1] The Common Gateway Interface (CGI) Version 1.1, RFC3875, http://www.ietf.org/rfc/rfc3875.
R6RS library nameThe following two procedures belong to the R6RS library named (srfi :98 os-environment-variables).
Function: get-environment-variable nameReturns the value of the named environment variable as a string, or #f if the named environment variable is not found. The name argument is expected to be a string. get-environment-variable may use locale-setting information to encode the name and decode the value of the environment variable. If get-environment-variable can't decode the value, get-environment-variable may raise an exception.
(get-environment-variable "PATH") => "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin"
Function: get-environment-variablesReturns names and values of all the environment variables as an a-list. The same decoding considerations as for get-environment-variable apply.
(get-environment-variables) => (("PATH" . "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin") ("USERNAME" . "taro"))
(define get-environment-variable sys-getenv) (define get-environment-variables sys-environ->alist)
(define (get-environment-variable name)
(cond
((lookup-environment-variable name) => os-string->string)
(else #f)))
(define (get-environment-variables)
(map (lambda (p)
(cons (os-string->string (car p)) (os-string->string (cdr p))))
(environment-alist)))
(define get-environment-variable getenv) (define get-environment-variables env->alist)
(define get-environment-variable getenv) (define get-environment-variables getenv)
get-environment-variable is expected to return a "Scheme string". Unfortunately, many current platforms, including POSIX-like ones, do not require environment variables to be interpretable as sequences of characters. In particular, environment variables can be used to name files, and filenames on the system can amount to NULL-terminated byte vectors, which, if the Scheme program were to collect uninterpreted and pass to, say, the open call, would work just fine, but which might not represent a string in the particular encoding the program expects. While in principle it may be desirable to provide a mechanism for accessing environment variables raw, this SRFI specifies a "string" return type because that best represents the consensus of existing implementations, and captures the semantically desirable behavior in the common case that the byte sequence is interpretable as a string.
| Scheme implementation | get environment variable | get all the environment variables as an a-list |
|---|---|---|
| Bigloo | (getenv name) => (or string? false) name:string? | |
| CHICKEN | (getenv name) => (or string? false) name:string? | |
| Gambit | (getenv name . <default>) =>(or string? <default> <Unbound OS environment variable error>) name:string? | |
| Gauche | (sys-getenv name) => (or string? false) name:string? | (sys-environ) |
| Guile | (getenv name) => (or string? false) name:string? | |
| PLT | (getenv name) => (or string? false) name:string? | |
| MIT/GNU Scheme | (get-environment-variable name) => (or string? false) name:string? | |
| Scheme48 | (lookup-environment-variable name) => (or string? false) name:string? | (environment-alist) |
| SLIB | (getenv name) => (or string? false) name:string? | |
| STk | (getenv name) => (or string? false) name:string? | |
| STklos | (getenv name) => (or string? false) name:string? | (getenv) |
| SCM | (getenv name) => (or string? false) name:string? | (getenv) |
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