Title

An interface to access environment variables.

Author

Taro Minowa (Higepon)

Status

This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-98@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

Abstract

This SRFI specifies the procedure get-environment-variable, which gets the value of the specified environment variable, and the procedure get-environment-variables, which gets an association list of all environment variables.

Rationale

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 enviornment 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.

Specification

R6RS library name
The following two procedures belong to the R6RS library named (srfi :98 os-environment-variables).
Function: get-environment-variable name
Returns 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-variables
Returns 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"))

Implementation

Gauche

(define get-environment-variable sys-getenv)
(define get-environment-variables sys-environ->alist)

Scheme48

(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)))

scsh

(define get-environment-variable getenv)
(define get-environment-variables env->alist)

SCM

(define get-environment-variable getenv)
(define get-environment-variables getenv)

Issues

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.

Appendix: Existing implementations

Scheme implementationget environment variableget 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)

Acknowledgements

Thanks to Shiro Kawai, Alexey Radul, jmuk, Kokosabu, leque and all the members of the #Lisp_Scheme IRC channel on Freenode.

Copyright

Copyright (C) Taro Minowa (Higepon) (2008). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


Editor: Mike Sperber