Bash/sh - difference between && and ;
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 shel开发者_如何学JAVAl or so?
If previous command failed with ;
the second one will run.
But with &&
the second one will not run.
This is a "lazy" logical "AND" operand between operations.
I'm using &&
because a long time ago at the nearby computer:
root# pwd
/
root# cd /tnp/test; rm -rf *
cd: /tnp/test: No such file or directory
...
... and after a while ...
...
^C
but not helped... ;)
cd /tnp/test && rm -rf *
is safe... ;)
In cmd1 && cmd2
, cmd2
is only executed if cmd1
succeeds (returns 0).
In cmd1 ; cmd2
, cmd2
is executed in any case.
Both constructs are part of a POSIX-compliant shell.
&&
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.
Very useful when you need to take a particular action depending on if the previous command finished OK or not.
Commands separate by ;
are executed sequentially regardless of their completion status.
With &&
, the second command is executed only if the first completes successfully (returns exit status of 0).
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.
&&
allows for conditional execution while ;
always has the second command being executed.
In e.g. command1 && command2
, command2
will only execute when command1
has terminated with exit 0
, signalling all went well, while in command1 ; command2
the second command will always be executed no matter what the result was of command1
.
&& is logical AND in bash. Bash has short-circuit evaluation of logical AND. This idiom is a simpler way of expressing the following:
cmd1;rc=$?
if [ $rc -eq 0 ]; then
cmd2
fi
Whereas the ; version is merely:
cmd1
cmd2
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