mysqld_safe on Ubuntu fails to find sysctl binary

- Ubuntu trusty.
- Percona Server 5.7.17.

If --flush-caches is enabled, the service won't start with the following error:

sysctl command not found, required for --flush-caches

mysqld_safe try to find it in these places:

6632 stat("/usr/local/bin/sysctl", 0x7ffde6c3e590) = -1 ENOENT (No such file or directory)
6632 stat("/usr/bin/sysctl", 0x7ffde6c3e590) = -1 ENOENT (No such file or directory)
6632 stat("/bin/sysctl", 0x7ffde6c3e590) = -1 ENOENT (No such file or directory)
6632 stat("/usr/local/games/sysctl", 0x7ffde6c3e590) = -1 ENOENT (No such file or directory)
6632 stat("/usr/games/sysctl", 0x7ffde6c3e590) = -1 ENOENT (No such file or directory)

but it is on /sbin

The workarounds are:

1- Create a symbolic link:

# ln -s /sbin/sysctl /bin/sysctl

2- Add /sbin to mysqld_safe PATH. Edit /usr/bin/mysqld_safe and add:

PATH=$PATH:/sbin

mysqld_safe should be able to find the tool without users having to use those workarounds.