if 语句和调用函数 if 使用 bash
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if statement and calling function in if using bash
提问by JumpOffBox
I have written a function:
我写了一个函数：
check_log(){
if [ -f "/usr/apps/appcheck.log" ]
then
return 1
else
return 0
fi
}
Then I call this function in an "if" condition:
然后我在“if”条件下调用这个函数：
if [ check_log ];
then
........statements....
fi
Will this work? I am confused here because bash returns 0 on success and 1 on failure, but my function is returning 1and the condition is checking for 1/0, it gets 1and it should give failures, but in my shell script the condition is passing.
这会起作用吗？我在这里很困惑，因为 bash 成功时返回 0，失败时返回 1，但是我的函数正在返回1并且条件正在检查1/ 0，它得到1并且应该给出失败，但是在我的 shell 脚本中，条件正在通过。
Can anyone shed light on this issue?
任何人都可以阐明这个问题吗？
回答by John Kugelman
if [ check_log ];
When you use square brackets you're invoking the testcommand. It's equivalent to if test check_logwhich is shorthand for if test -n check_log, which in turn means "if "check_log"is not an empty string". It doesn't call your check_logfunction at all.
当您使用方括号时，您正在调用test命令。它相当于if test check_logwhich 是 的简写if test -n check_log，这又意味着“如果"check_log"不是空字符串”。它根本不调用你的check_log函数。
Change it to this:
改成这样：
if check_log;
By the way, the function could be more simply written as:
顺便说一下，该函数可以更简单地写为：
check_log() {
! [ -f "/usr/apps/appcheck.log" ]
}
The return value from a function is the exit status of the last command, so no need for explicit return statements.
函数的返回值是最后一个命令的退出状态，所以不需要显式的 return 语句。
回答by markeissler
As noted by @john-kugelman, one solution (and perhaps the most correct one) is to use the following syntax:
正如@john-kugelman 所指出的，一种解决方案（也许是最正确的一种）是使用以下语法：
if check_log;
But an alternative solution is:
但另一种解决方案是：
if [[ $(check_log; echo $?) -eq 0 ]];
My personal preference is the latter as it fosters consistency among conditional statements. But the downside is thatbecause it relies on command substitution, it will fork a child process (i.e. a subshell) where as the first method will not.
我个人更喜欢后者，因为它促进了条件语句之间的一致性。但缺点是，因为它依赖于命令替换，它会分叉子进程（即子shell），而第一种方法不会。
An interesting read on the subject can be found here:
可以在此处找到有关该主题的有趣读物：
When does command substitution spawn more subshells than the same commands in isolation?
Updated (2017-10-20): to strikeout my preference for the second method as these days I'm on a mission to prevent unnecessary forking. The syntax in the first solution is not as intuitive for non-shell programming, but it certainly is more efficient.
更新（2017 年 10 月 20 日）：为了消除我对第二种方法的偏好，因为这些天我的任务是防止不必要的分叉。第一个解决方案中的语法对于非 shell 编程来说并不直观，但它肯定更有效。
