C shell 脚本中是否有短路逻辑运算符？
我认为 C shell 脚本的行为会像 C 一样，并对逻辑运算符使用短路求值。
if ((! -e $cache ) || ("`find $monitor -newer $cache`" != "")) then
...
endif
但在 if 语句中，即使第一个条件为真，也会检查第二个条件，从而给出错误。
C shell 脚本中是否有短路逻辑或？
I thought C shell script will behave like C and use short circuit evaluation for logical operators.
if ((! -e $cache ) || ("`find $monitor -newer $cache`" != "")) then
...
endif
But in the if statement, even if the first condition is true, the second is checked giving me errors.
Do we have a short circuit logical OR in C shell script?
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是的，
&&和||是 tcsh 中的短路运算符（就像它们在 C 中一样）。有两种不同的形式，一种适用于命令（如false && echo foo），另一种适用于表达式（如if (-e $file1 & ;& -e $file2))。我不完全确定这里发生了什么，除了 csh 和 tcsh 因语法定义不明确而臭名昭著。一些实验表明，将
find的输出保存在变量中可以避免错误：（请注意，额外的括号并不是真正必要的，但它们不会造成任何损害。）
但更多的改进是可能的。将 find 命令的整个输出存储在变量中并不是真正必要的。相反，您可以调用 find 并使用其返回的状态来告诉您是否找到了任何内容：
我刚刚尝试过：
并且它有效 - 除了
>& /dev/null重定向似乎在大括号内被忽略。相反，您可以单独执行“find”命令，然后检查生成的 $status。这意味着您失去了短路
||运算符的好处；您必须求助于嵌套的if语句和/或临时变量。也许真正的教训是这样的：我使用 csh 和 tcsh 的时间比我愿意承认的要长，而我能够弄清楚很多这些东西的唯一方法就是通过反复试验。您可以使用 tcsh 或 csh 来完成此操作，但最好使用不同的脚本语言。例如，在 Bourne shell 中，这相当简单：
Yes,
&&and||are short-circuit operators in tcsh (just as they are in C). And there are two distinct forms, one that works on commands (as infalse && echo foo), and one that works on expression (as inif (-e $file1 && -e $file2)).I'm not entirely sure what's going on here, except that csh and tcsh are notorious for ill-defined syntax. A bit of experimentation shows that saving the output of
findin a variable avoids the error:(Note that the extra parentheses aren't really necessary, but they don't hurt anything.)
But more improvement is possible. Storing the entire output of the find command in a variable isn't really necessary. Instead, you can invoke find and use its returned status to tell you whether it found anything:
I just tried this:
and it worked -- except that the
>& /dev/nullredirection seems to be ignored within the braces.Instead, you can execute the "find" command separately, and then examine the resulting $status. Which means that you lose the benefit of the short-circuit
||operator; you'll have to resort to nestedifstatements and/or temporary variables.Perhaps the real lesson is this: I've been using csh and tcsh for more years than I care to admit, and the only way I was able to figure out a lot of this stuff was by trial and error. You can do this using tcsh or csh, but you're probably better of using a different scripting language. For example, in Bourne shell, this is fairly straightforward:
通常，
&&和||是短路的。考虑这样的事情：在这两种情况下， foo 都不会被输出。
但是，据我所知，您不能像这样使用这种字符串比较，即使短路，csh 仍然会对整个事情进行语法检查。
Usually,
&&and||are short-circut. Consider something like this:In both cases, foo won't be put out.
But, AFAIK you cannot use this kind of string comparison like this, even if short-circuit, csh will still syntax-check the whole thing.