How to display the Linux kernel command line parameters given for the current boot?

$ cat /proc/cmdline
root=/dev/xvda xencons=tty console=tty1 console=hvc0 nosep nodevfs ramdisk_size=32768 ip_conntrack.hashsize=8192 nf_conntrack.hashsize=8192 ro  devtmpfs.mount=1 
$

The kernel also printks them at the beginning of boot, see:

dmesg | grep "Command line"

Sample output:

[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.4.0-112-generic root=/dev/mapper/vg00-root ro

This can be useful information on the serial if you are hacking stuff and the kernel panics instead of booting :-)

Related: How do I find the boot parameters used by the running kernel? | Ask Ubuntu