Re: [Help-bash] How to use "&&" to replace "if" statement when "return" is used?

[Roman Rakus]

Hi. See comments below.
On 04/21/2013 04:19 PM, Peng Yu wrote:
> Hi,
>
> I'm trying to use "&&" to replace "if".
ok, but be very cautious. "&&" and "||"are operators of lists of 
commands. See http://www.gnu.org/software/bash/manual/bashref.html#Lists
> But the following shows that
> when "return" is used the replacement can not be done.
It can be done. See below
> Is there a way
> to use "&&" to replace "if" even "return" or "exit" is used?
It is. You used it. But you don't understand return statuses. See below.
> ~/linux/test/bash/man/operator/&&/return$ cat.sh *
> ==> main.sh <==
> #!/usr/bin/env bash
>
> set -v
> function cmd1 {
> [ '' ] && return 1
> }
>
> cmd1
> echo $?
>
> function cmd2 {
> [ x ] && return 1
> }
>
> cmd2
> echo $?
>
> function cmd1 {
> if [ '' ]
> then
>    return 1
> fi
> }
>
> cmd1
> echo $?
>
> function cmd2 {
> if [ x ]
> then
>    return 1
> fi
> }
>
> cmd2
> echo $?
>
>
> ==> main_echo.sh <==
> #!/usr/bin/env bash
>
> set -v
> function cmd1 {
> [ '' ] && echo 1
> }
>
> cmd1
>
> function cmd2 {
> [ x ] && echo 1
> }
>
> cmd2
>
> function cmd1 {
> if [ '' ]
> then
>    echo 1
> fi
> }
>
> cmd1
>
> function cmd2 {
> if [ x ]
> then
>    echo 1
> fi
> }
>
> cmd2
Here we go.
> ~/linux/test/bash/man/operator/&&/return$ ./main.sh
> function cmd1 {
> [ '' ] && return 1
Return status of builtin command `[' is 1, command after && operator is 
not executed.
> }
Function returns without explicit `return' command, so it uses last 
command's return status. It is `[' with return status 1.
> cmd1
> echo $?
> 1
This 1 return code is return code of function, which is in turn return 
code of last command executed which is `['.
> function cmd2 {
> [ x ] && return 1
> }
Similar as above. Return code of `[' is 0 so command after && is executed.
> cmd2
> echo $?
> 1
It used `return' command, but return status is also 1.
> function cmd1 {
> if [ '' ]
> then
>    return 1
> fi
> }
See 
http://www.gnu.org/software/bash/manual/bashref.html#Conditional-Constructs
Last command executed is `if' conditional statement. Return status is 
described in the document and it is 0 in this case.
> cmd1
> echo $?
> 0
return status of `if'.
> function cmd2 {
> if [ x ]
> then
>    return 1
> fi
> }
>
> cmd2
> echo $?
> 1
return status of `return' command.
> ~/linux/test/bash/man/operator/&&/return$ ./main_echo.sh
> function cmd1 {
> [ '' ] && echo 1
> }
>
> cmd1
The same for here.
> function cmd2 {
> [ x ] && echo 1
> }
>
> cmd2
> 1
>
> function cmd1 {
> if [ '' ]
> then
>    echo 1
> fi
> }
>
> cmd1
>
> function cmd2 {
> if [ x ]
> then
>    echo 1
> fi
> }
>
> cmd2
> 1

Hope it helps.

RR