I came across a if statement like this:
if [ -f <file path> -size +0 ] I know that -f checks for file existed, but what does this -size +0 do here?
I can't get help form man page also tried searching forums, little curious to find this help me.
Most likely, this is erroneous. The form -size +0 is legal for the find standard utility (and means a non-empty file), but not for test (a.k.a. [). It seems that the script author reused it without keeping proper context. If this is ksh, it should have the following check:
–s file True, if file exists and has size greater than zero. Alternatively, this script is for a (unknown to me) extended shell for which test is internal and extended for such complex constructions. (Is your ksh a specific extended version?)
It's meaningless.
That's the way ksh93 handle if there're too many arguments for an unary operator and the unary operator start with -, then the second one will be pick as the argument and the rest will be ignored:
$ ksh -c '[ -f a.out foo bar ] && echo yes' yes $ ksh -c '[ -e a.out foo a ] && echo yes' yes (Except when the second argument is -a/-o, it won't be ignored)
Checking the source of ksh93 test confirm that behavior.
Also doing strace:
$ { strace -p "$$" & sleep 1; [ -f a.out -size +0 ]; kill "$!"; } [1] 18467 Process 18455 attached restart_syscall(<... resuming interrupted call ...>) = 0 stat(".", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0 stat("a.out", {st_mode=S_IFREG|0755, st_size=8320, ...}) = 0 kill(18467, SIGTERMProcess 18455 detached <detached ...> Other Bourne-like shells will report the error with this case. The behavior is unspecified by POSIX.
If you want to check whether a file exist and have size greater than 0, then standard shells have -s test operator:
[ -s file ] && echo 'file exist and size greater than 0' ksh88 also behave the same. With my Solaris 10 VM:
$ ksh -c '[ -f a.out foo bar ] && echo yes' yes $ strings /usr/bin/ksh | grep -i version @(#)Version M-11/16/88i Pre-POSIX shells also behave like that, include Bourne shell in Solaris 10, the heirloom Bourne shell, Schily osh and Schily sh.
[ -s file ] is not the same as [ -f file ] && [ -s file ] as [ -s file ] doesn't check that file is a regular file (after symlink resolution). [ -e ./*.txt ] would check that the first file resulting from the glob expansion exists which would be a quick way to check that a glob matches (if we ignore problems with broken links). -f is -o or -a (binary or/and), it won't be ignored.