bash shell 脚本中的单行 if 语句不起作用
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single-line if statement in a shell script not working
提问by Alexander Bird
This is my code:
这是我的代码：
#!/bin/bash
cat input | ./prog > output && if[ "" != "" ]; diff output expected;
This then happens:
然后会发生这种情况：
$ ./run.sh
./run.sh: line 2: if[ no != ]: command not found
$
I thought that I could run if statements on one line? is that what the problem is?
我以为我可以在一行上运行 if 语句？这就是问题所在吗？
回答by Alexander Bird
it turns out that there needs to be a space between the ifand the [. Also, I put in the thenand fikeywords.
事实证明，if和之间需要有一个空格[。另外，我输入了then和fi关键字。
The following worked.
以下工作。
#!/bin/bash
cat input | ./prog > output && if [ "" != "" ]; then diff output expected; fi
EDIT:
编辑：
as commented below (and in another answer), this can elegantly be shortened to:
正如下面（以及在另一个答案中）所评论的，这可以优雅地缩短为：
cat input | ./prog > output && [ "" != "" ] && diff output expected
in which case I don't even have to remember any of the rules about how to use the ifconstruct :)
在这种情况下，我什至不必记住有关如何使用if构造的任何规则:)
回答by Ярослав Рахматуллин
drop the if. [] will spawn a new "command" and you can run diff or whatever after if it exits with 0 using && (run next if previous exits ok). here:
放弃如果。[] 将产生一个新的“命令”，如果它使用 && 以 0 退出，您可以运行 diff 或其他任何内容（如果前一个退出正常，则运行下一个）。这里：
echo lol && [ $((10-1)) -eq 9 ] && echo math\!
and your one-liner:
和你的单线：
#!/bin/bash
cat input | ./prog > output && [ "" != "" ] && diff output expected
