In fish true == 0, but false != 1?-CodePudding

In fish, true seems to equal 0:

❯ if true == 0; echo "YES"; else; echo "NO"; end
YES

But false seems to not equal 1:

❯ if false == 1; echo "YES"; else; echo "NO"; end
NO

In bash both of them are not equivalent to their numeric value:

$ if [ true == 0 ]; then echo "YES"; else echo "NO"; fi
NO
$ if [ false == 1 ]; then echo "YES"; else echo "NO"; fi
NO

It seems strange that fish would be consider one truth value equal to its numeric counterpart but not the other.

Maybe there is an explanation for that?

CodePudding user response：

The command true ignores arguments and exits with zero exit status meaning success.

if true you can put literally anything here; echo "YES"; else; echo "NO"; end

The command false ignores arguments and exits with non-zero exit status which means failure.

if false you can put literally anything here too; echo "YES"; else; echo "NO"; end

The equivalent Bash code is:

if true anything here; then echo "YES"; else echo "NO"; fi
if false anything here too; then echo "YES"; else echo "NO"; fi

The command [ true == 0 ] executes the command [ and compares the string true to the string 0. Because true and 0 are different strings, the comparison is (logically) false, so [ command exits with non-zero exit status. Similarly with false == 0. Note that == is an extension to [ - it's [ true = 0 ] in standard [.

You can compare that in Fish the [ command also "both of them are not equivalent to their numeric value" too:

if [ true == 0 ]; echo "YES"; else; echo "NO"; end
if [ false == 1 ]; echo "YES"; else; echo "NO"; end