Order of evaluation of bash expression when using vs not using test

user_10373606 0
Viewed 218

Why does the following script gives different outputs, when the same expression is evaluated with and without test? What rules are to be taken into account: operator precedence, left-to-right or right-to-left argument evaluation ...? In my opinion it is important to understand the logic of bash at this basic level.

function a()
{
    local r=1
    printf "a%d" $r
    return $r
}

function b()
{
    local r=1
    printf "b%d" $r
    return $r
}

function c()
{
    local r=1
    printf "c%d" $r
    return $r
}

printf "$BASH_VERSION\n" #outputs 4.3.48(1)-release
a || b && c #outputs a1b1
printf "\n"
test a || b && c #outputs c1
printf "\n"
bash
2 Answers

That's because

a

runs a whereas

test a

runs test, passing a as an argument. They're completely different commands.

Similarly,

pwd

runs pwd, but

echo pwd

runs echo, passing pwd as an argument.

X || Y is not an expression, it's a compound command that first runs the X subcommand and then (if its exit status is non-zero) runs Y.

See https://www.gnu.org/software/bash/manual/bashref.html#Lists for details.

user_18486540

Your usage of test is incorrect in this context. test sets the return code to the calling shell by evaluating an expression. The way you've used it, test a does not evaluate to an expression but ends up setting a code as test 1 which does a string check and always asserts a code 0 (success error code). See

test 0; echo $?
0
test 1; echo $?
0
test false; echo $?
0
test true; echo $?
0

test evaluates the expression a||b which sets up a return code 1 to test which returns a success error code and runs the function c which prints the value you are seeing.

For cases of simplicity, I've changed the function calls to use command false which returns error code 1 to the shell as your function. 

bash -cx "test false || false && echo hi"
+ test false
+ echo hi
hi

As you can see, the moment test false|| false is run, the command sets success code to the shell and with the operand && the echo command is run.

For checking the error code from the function, perhaps you should have checked as

test a != "0"

See The classic test command

user_52910150
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