When I get used to the C/C++ programming environment, and then when I turn to shell programming, I always feel a little unfamiliar.
The general if-then-else function format in the shell is nothing more than the following:
if condition1 then Order1 elif condition2 thenOrder2 else Order3 fi
In Linux
If the command exit status is 0, the exit is successful and there are no errors;
If the exit status is 1, the exit fails and there is an error somewhere.
In the if-then-else- function, condition 1 is a conditional test. If condition 1 returns to 0, it means that the test is true, and execute command 1; otherwise, return 1, then elif is executed.
That is, the criterion for determining whether the condition is successful or not is the return value.
This is a bit different from c/c++, and you will get it wrong if you are not careful.
For more information, see:
"Linux and Unix_shell Programming Guide" 18.3
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