bash “echo 'hello'; ls”与“echo 'hello' && ls”之间的区别？
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Difference between "echo 'hello'; ls" vs "echo 'hello' && ls"?
提问by never_had_a_name
I wonder what the difference between
我想知道两者有什么区别
"echo 'hello'; ls"
and
和
"echo 'hello' && ls"
is? they both do the same thing
是？他们都做同样的事情
回答by Hendra Jaya
"echo 'hello' && ls" means : execute "ls" if "echo 'hello'" runs successfully. To understand what is "successful" in bash. Try this :
"echo 'hello' && ls" 表示：如果 "echo 'hello'" 运行成功，则执行 "ls"。了解什么是 bash 中的“成功”。尝试这个 ：
bash> cd /
bash> echo $?
bash> cd /
bash> echo $?
if the previous command runs successfully, you should see 0
如果前面的命令成功运行，您应该看到 0
After that, try this :
之后，试试这个：
bash> asdfdf
bash> echo $?
bash> asdfdf
bash> echo $?
You should see a non-zero value between 1 and 255. This means previous command didn't run successfully
您应该会看到 1 到 255 之间的非零值。这意味着之前的命令没有成功运行
On the other hand, "echo 'hello'; ls" means execute "ls" whether "echo 'hello'" runs successfully or not.
另一方面，“echo 'hello'; ls”表示无论“echo 'hello'”运行成功与否，都执行“ls”。
回答by DarkDust
The &&is the logical AND operator. The idea in its use in command1 && command2is that command2 is only evaluated/run if command1 was successful. So here lswill only be run if the echocommand returned successful (which will always be the case here, but you never know ;-). You could also write this as:
该&&是逻辑运算符。其使用的想法command1 && command2是仅在 command1 成功时才评估/运行 command2。所以这里ls只会在echo命令返回成功时运行（这里总是如此，但你永远不知道;-）。你也可以这样写：
if echo 'hello'
then
ls
fi
The semicolon just delimits two commands. So you could also write echo 'hello' ; lsas:
分号只是分隔两个命令。所以你也可以写成echo 'hello' ; ls：
echo 'hello'
ls
Thus lswill also be executed even when echofails.
因此ls即使echo失败也会被执行。
BTW, successful in this context means that the program was exited with something like exit(0)and thus returned 0 as a return code (the $?shell variable tells you the return status of the last executed command).
顺便说一句，在这种情况下成功意味着程序以类似的方式退出，exit(0)因此返回 0 作为返回码（$?shell 变量告诉您上次执行的命令的返回状态）。
回答by telent
To supplement DarkDust's answer:
补充DarkDust 的回答：
So here ls will only be run if the echo command returned successful (which will always be the case here, but you never know ;-).
所以这里 ls 只会在 echo 命令返回成功时运行（这里总是这样，但你永远不知道;-）。
Well, not always. For example, the standard output stream may be unwritable
嗯，并非总是如此。例如，标准输出流可能是不可写的
:; ( echo "foo" && touch /tmp/succeeded) >/dev/full
bash: echo: write error: No space left on device
:; ls -l /tmp/succeeded
ls: cannot access /tmp/succeeded: No such file or directory
回答by Lekensteyn
Here, they do the same.
在这里，他们做同样的事情。
But take an other example:
但是再举一个例子：
cp file target && ls
cp file target; ls
In the first case, lswould only be executed if cpsucceeds.
In the second case, no matter if cpfails or succeeds, lswill always be executed.
在第一种情况下，ls只有cp成功才会执行。在第二种情况下，无论cp失败还是成功，ls都将始终执行。
