Bash：在 if 语句中使用差异的结果
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原文地址: http://stackoverflow.com/questions/3611846/
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Bash: using the result of a diff in a if statement
提问by Jary
I am writing a simple Bash script to detect when a folder has been modified.
我正在编写一个简单的 Bash 脚本来检测文件夹何时被修改。
It is something very close to:
它非常接近：
ls -lR $dir > a
ls -lR $dir > b
DIFF=$(diff a b)
if [ $DIFF -ne 0 ]
then
echo "The directory was modified"
Unfortunately, the if statement prints an error: [: -ne: unary operator expected
不幸的是，if 语句打印了一个错误：[: -ne: unary operator expected
I am not sure what is wrong with my script, would anyone please be able to help me?
我不确定我的脚本有什么问题，有人可以帮助我吗？
Thank you very much!
非常感谢！
Jary
杰瑞
采纳答案by Lou Franco
ls -lR $dir > a
ls -lR $dir > b
DIFF=$(diff a b)
if [ "$DIFF" != "" ]
then
echo "The directory was modified"
fi
回答by Paul Tomblin
if ! diff -q a b &>/dev/null; then
>&2 echo "different"
fi
回答by tangens
You are looking for the return value of diffand not the output of diffthat you are using in your example code.
您正在寻找您在示例代码中使用的返回值diff而不是输出diff。
Try this:
尝试这个：
diff a b
if [ $? -ne 0 ]; then
echo "The directory was modified";
fi
回答by Christophe Priieur
If you don't need to know what the changes are, cmpis enough.
Plus you can play with the syntactical trick provided by and ||:
如果您不需要知道更改是什么，cmp就足够了。另外，您可以使用 and 提供的语法技巧||：
cmp a b || echo 'The directory was modified'
The instruction may be interpreted as: "either a and b are equal, or i echo the message".
该指令可以解释为：“要么 a 和 b 相等，要么我回显消息”。
(The semantic of &&and ||must be handled with care, but here it's intuitive).
（语义的&&和||必须小心处理，但在这里它的直观）。
Just for the sake of readability, i actually prefer to put it on two lines:
只是为了可读性，我实际上更喜欢把它放在两行：
cmp a b \
|| echo 'The directory was modified'
回答by Aaron D. Marasco
DIFF=$(diff -u <(find dir1/ -type f -printf '%P\n' | sort) <(find dir2/ -type f -printf '%P\n' | sort))
if [ "$DIFF" ]; then
echo "Directories differ"
# Do other stuff here
fi
This uses one of my favorite bashisms, the <()process substitution.
这使用了我最喜欢的 bashism 之一，即<()过程替换。
The $DIFFvariable holds a printable difference. If you want to show it to the end user, be sure to double-quote it, e.g. echo "$DIFF".
该$DIFF变量包含可打印的差异。如果您想将其显示给最终用户，请务必将其双引号，例如echo "$DIFF".
If you want to only tell the user there was anydifference, if can be shortened to something like [ "$(diff ...)" ] && echo "Difference found"
如果你只想告诉用户有什么不同，如果可以缩短为类似[ "$(diff ...)" ] && echo "Difference found"
Note: I'm assuming the original question meant to have dir1and dir2to make a little more sense. If it was dirat time 0 and then dirat time 1, this approach obviously wouldn't work.
注：我假设意味着有原来的问题dir1，并dir2做出一点更有意义。如果它是dir在时间 0 然后dir在时间 1，这种方法显然行不通。
