Shell exec takes a string which needs to be an actual command. You are now passing it a filepath. This is not interpreted as "execute the file at this path". You could do several things.
What you need to do is call the file with a program. Call it with bash or sh as suggested in the comment:
echo shell_exec('sh /home/scripts/fix-perm.sh');
Another option could be:
$contents = file_get_contents('/home/scripts/fix-perm.sh');
echo shell_exec($contents);
I think the first option would be better however.
It is important to note that all commands for executing external programs expect actual commands and not a filepath or something else. This goes for shell_exec, exec, passthru and others.
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