Bourne: if statement testing exit status

Question

What is the difference:

if IsServerStarted ; then ...

and

if [ IsServerStarted -eq 0 ] ; then ...

Seems to me that these two statements should be equivalent? Strangely the second statement is always true.

Answer 1

The following runs the shell function or executable in $PATH named IsServerStarted, and if its exit code is 0 (i.e. true), runs the then branch. If such a function or executable does not exist, the exit code will be non-0 (i.e. false) and the then branch will be skipped.

if IsServerStarted ; then ...

The following has [ (aka test) check whether IsServerStarted is an integer equal to 0, which (IsServerStarted not even containing a single digit) is always false. Thus, [ exits with a non-0 (i.e. false) code and the then branch is always skipped.

if [ IsServerStarted -eq 0 ] ; then ...