diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 46 |
1 files changed, 45 insertions, 1 deletions
@@ -35353,6 +35353,19 @@ function creates or modifies the environment variable indicated by The .meta value string argument specifies the new value for the variable. +If +.meta value +is +.codn nil , +then +.code setenv +behaves like +.codn unsetenv , +except that it observes the +.meta overwrite-p +argument. That is to say, the meaning of a null +.meta value +is that the variable is to be removed. If the .meta overwrite-p @@ -35366,11 +35379,22 @@ effectively giving rise to a two-argument form of .code setenv which creates or overwrites environment variables. +A variable removal is deemed to be an overwrite. +Thus if both +.meta value +and +.meta overwrite-p +are +.codn nil , +then +.code setenv +does nothing. + The .code setenv function unconditionally returns .meta value -regardless of whether or not it overwrites an existing variable. +regardless of whether or not it overwrites or removes an existing variable. The .code unsetenv @@ -35380,6 +35404,26 @@ specified by if it exists. On some platforms, it instead sets the environment variable to the empty string. +Note: supporting removal semantics in +.code setenv +allows for the following simple save/modify/restore pattern: + +.cblk + (let* ((old-val (getenv "SOME-VAR"))) + (unwind-protect + (progn (setenv "SOME-VAR" new-val) + ...) + (setenv "SOME-VAR" old-val))) +.cble + +This works in the case when +.code SOME-VAR +exists, as well as in the case that it doesn't exist. +In both cases, its previous value or, respectively, non-existence, +is restored by the +.code unwind-protect +cleanup form. + .SS* System Programming .coNP Accessor @ errno .synb |