Bash/sh - && 和 ; 之间的区别
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Bash/sh - difference between && and ;
提问by psihodelia
I normally use ;to combine more than one command in a line, but some people prefer &&. Is there any difference? For example, cd ~; cd -and cd ~ && cd -seems to make the same thing. What version is more portable, e.g. will be supported by a bash-subset like Android's shell or so?
我通常用于;在一行中组合多个命令，但有些人更喜欢&&. 有什么区别吗？例如，cd ~; cd -并且cd ~ && cd -似乎让同样的事情。哪个版本更便携，例如，像 Android 的 shell 之类的 bash 子集会支持什么？
回答by ignar
If previous command failed with ;the second one will run.
如果前一个命令失败，;第二个将运行。
But with &&the second one will not run.
但是用&&第二个就不会运行了。
This is a "lazy" logical "AND" operand between operations.
这是操作之间的“惰性”逻辑“AND”操作数。
回答by jm666
I'm using &&because a long time ago at the nearby computer:
我正在使用，&&因为很久以前在附近的计算机上：
root# pwd
/
root# cd /tnp/test; rm -rf *
cd: /tnp/test: No such file or directory
...
... and after a while ...
...
^C
but not helped... ;)
但没有帮助...;)
cd /tnp/test && rm -rf *is safe... ;)
cd /tnp/test && rm -rf *是安全的... ;）
回答by Pascal Cuoq
In cmd1 && cmd2, cmd2is only executed if cmd1succeeds (returns 0).
在cmd1 && cmd2,cmd2仅在cmd1成功时才执行（返回 0）。
In cmd1 ; cmd2, cmd2is executed in any case.
在cmd1 ; cmd2,cmd2在任何情况下都会执行。
Both constructs are part of a POSIX-compliant shell.
这两种结构都是符合 POSIX 的 shell 的一部分。
回答by Fredrik Pihl
&&means to execute next command if the previous exited with status 0.
For the opposite, use ||i.e. to be executed if previous command exits with a status not equal to 0 ;executes always.
&&表示如果前一个命令以状态 0 退出，则执行下一个命令。相反，||如果前一个命令以不等于 0 的状态退出，则使用ie;执行总是执行。
Very useful when you need to take a particular action depending on if the previous command finished OK or not.
当您需要根据上一个命令是否完成时采取特定操作时非常有用。
回答by Dave Costa
Commands separate by ;are executed sequentially regardless of their completion status.
分开的命令;将按顺序执行，而不管它们的完成状态。
With &&, the second command is executed only if the first completes successfully (returns exit status of 0).
使用&&，仅当第一个命令成功完成（返回退出状态 0）时才会执行第二个命令。
This is covered in the bash manpage under Lists. I would expect any Unix-like shell to support both of these operators, but I don't know specifically about the Android shell.
这在 .bashrc 下的 bash 联机帮助页中有介绍Lists。我希望任何类 Unix 的 shell 都支持这两个操作符，但我并不特别了解 Android shell。
回答by Marcel
&&allows for conditional execution while ;always has the second command being executed.
&&允许有条件执行，同时;始终执行第二个命令。
In e.g. command1 && command2, command2will only execute when command1has terminated with exit 0, signalling all went well, while in command1 ; command2the second command will always be executed no matter what the result was of command1.
在 eg 中command1 && command2，command2将仅在command1以 终止时执行exit 0，表明一切顺利，而在command1 ; command2第二个命令中，无论 的结果如何，都将始终执行command1。
回答by frankc
&& is logical AND in bash. Bash has short-circuit evaluation of logical AND. This idiom is a simpler way of expressing the following:
&& 在 bash 中是逻辑与。Bash 具有逻辑 AND 的短路评估。这个习语是一种更简单的表达方式：
cmd1;rc=$?
if [ $rc -eq 0 ]; then
cmd2
fi
Whereas the ; version is merely:
而 ; 版本仅仅是：
cmd1
cmd2
