Bash:如何将这两个if语句压缩为一个 [英] Bash : how to squeeze these 2 if statements into one

问题描述

我是Bash和脚本编写的新手，我想找出一种将这两个语句合并为1的方法.该脚本的作用是检查两个文件D1和D2是否是同一文件，如果不是，则检查它们的内容是否相同.

I am new to Bash and scripting and I want to figure out a way to combine these two statements into 1 . what this script does is checks if two files D1 and D2 are the same file and if not checks if their content is the same.

if [ ! $D1 -ef $D2 ]

 then
    echo not the same file
        if  cmp -s $D1 $D2  

           then
            echo same info
           else
                echo not same info
        fi

    else
            echo same file


fi

除此之外，我还对何时使用[]和何时跳过它们感到困惑，手册说它是有条件的使用[]，但这意味着什么?

In addition to this, I am also confused when to use [] and when to skip them, manual says when its a conditional use [], but what does that mean ?

谢谢.

推荐答案

if 语句的语法是(来自

The syntax of an if statement is (from 2.10 Shell Grammar):

if_clause        : If compound_list Then compound_list else_part Fi
                 | If compound_list Then compound_list           Fi

compound_list 最终归结为命令的地方.

Where compound_list eventually gets down to a command.

！$ D1 -ef $ D2 不是命令.

[是一个命令(也称为 test ).查看 type [或 type test 的输出，以及哪个[和哪个测试.

[ is a a command (also known as test). See the output from type [ or type test as well as which [ and which test.

所以 [！$ D1 -ef $ D2] 是 if 语句中使用的有效命令.

So [ ! $D1 -ef $D2 ] is a valid command used in the if statement.

compound_list 的返回值是 if 测试的内容.

The return value of the compound_list is what if tests.

因此，当您使用诸如 cmp 之类的命令(或任何其他命令)时，就没有理由使用 [，实际上，也使用了 [不正确.

So when you are using something like cmp (or any other command) then there is no reason to use [ and, in fact, using [ is incorrect.

由于 compound_list 可以不仅仅是将 [！$ D1 -ef $ D2] 和 cmp -s $ D1 $ D2 只需正常使用&& 即可.(也需要在 cmp 调用上使用！来将其求反以从两者中获得不相同"的测试.)

As the compound_list can be more than a single command to combine [ ! $D1 -ef $D2 ] and cmp -s $D1 $D2 simply use && as normal. (Need ! on the cmp call too to invert that to get the "not same" test from both.)

if [ ! "$D1" -ef "$D2" ] && ! cmp -s "$D1" "$D2"; then
    echo 'Not same file or same contents'
fi

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