$?VAR在csh中是什么意思?

问题描述

我遇到了用csh编写的这段代码:

I've encountered this code written in csh:

if ( ! $?LM_LICENSE_FILE ) then
    setenv LM_LICENSE_FILE $_LmLicense
else
    switch("$LM_LICENSE_FILE")
    case *${_LmLicense}*:
         breaksw
    default:
         setenv LM_LICENSE_FILE "${_LmLicense}:$LM_LICENSE_FILE"
    endsw
endif

请注意以下一行:

if ( ! $?LM_LICENSE_FILE ) then

$?在$?LM_LICENSE_FILE中做什么?

LM_LICENSE_FILE是一个shell变量(或更确切地说是宏)，我知道$LM_LICENSE_FILE用于检索其值，但是$之后的尾随问号，即$?LM_LICENSE_FILE，是什么它的含义?

LM_LICENSE_FILE is a shell variable (or macro, more precisely), and I know that $LM_LICENSE_FILE is used to retrieve its value, but a trailing question mark following $, that is $?LM_LICENSE_FILE, what is its meaning?

推荐答案

$?VAR如果将$VAR设置为(true，则为空字符串)，则扩展为1(true)，如果未设置，则扩展为0(false). t.

$?VAR expands to 1 (true) if $VAR is set (to anything, even the empty string), 0 (false) if it isn't.

csh手册中对此进行了说明.按照链接搜索$?，或键入man csh或man tcsh.

This is documented in the csh manual; follow the link and search for $?, or type man csh or man tcsh.

哦，而且csh没有宏.

Oh, and csh doesn't have macros.

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