pre-up [ $(cat /sys/class/net/eth0/carrier) -eq 1 ]

But I don't understand it.
Where does the -eq option belongs to?
To the pre-up?

I want to have dhcp to try to establish a network connection at boot time, but when there is no ethernet cable plugged in, or no wlan card, it takes up to 60 seconds before dhcp gives up.

I use this with my laptop.

root@9600k:~# cat bin/lenovo_etc_network_interfaces 
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The first card in the system added for devuan method of networking
# the pre-up is for stopping the annoying delay when the network cable is not connected.

auto eth0
    allow-hotplug eth0
    iface eth0 inet dhcp
        pre-up ip link set eth0 up && sleep 1
        pre-up [ $(cat /sys/class/net/eth0/carrier) -eq 1 ]

Forget where I got it when searching on the problem but it works perfectly every time it boots.

So let's decrease the timeout.

Edit

/etc/dhcp/dhclient.conf

Search for the line

#timeout 60;

uncomment it and change it to the value (amount of amount of seconds) you like.
For example:

timeout 15;

That's it.

Note 1: If the dhcp server did not offer an ip address before the time out runs down. you wont get an established network connection!

Note 2: udhcpc can be a replacement for isc-dhcp-client and isc-dhcp-common. I noticed, that it times out very quickly by default, when it can not find any of the interfaces defined in /etc/network/interfaces

Thanks to ralph.ronnquist (rrq) for helping with the timeout issue and mentioning udhcpc in IRC.