Spooky Linux Commands: Crypt, Kill, Shred, Zombies, and More

With Halloween approaching, we explore Linux commands that sound like monsters and the spooky concepts behind them.

crypt

The crypt command encrypts file contents, often implemented via a script that calls mcrypt. Using mycrypt is recommended instead.

Example:

$ mcrypt x
Enter the passphrase (maximum of 512 characters)
Please use a combination of upper and lower case letters and numbers.
Enter passphrase:
File x was encrypted.

Note: mcrypt creates a second file with the “.nc” extension and does not overwrite the original.

kill

The kill family of commands terminates processes. Linux provides many variants such as kill, pkill, killall, killpg, rfkill, skill, tgkill, tkill, and xkill.

Example:

$ killall runme
[1]   Terminated              ./runme
[2]   Terminated              ./runme
[3]-  Terminated              ./runme
[4]+  Terminated              ./runme

shred

The shred command overwrites a file to hide its previous contents, making recovery difficult. Unlike rm, which only removes the directory entry, shred actually overwrites data.

Example:

$ shred dupes.txt
$ more dupes.txt
▒oΛ▒▒9▒lm▒▒▒▒▒o▒1־▒▒f▒f▒▒▒i▒▒h^}&▒▒▒{▒▒

zombies

“Zombie” processes are dead processes that remain in the process table because their parent has not reaped them. They indicate a defect in the parent process. You can spot them with top:

$ top
... (output showing 3 zombie processes) ...

at midnight

The at command schedules a one‑time job, such as running a command at midnight.

$ at midnight
warning: commands will be executed using /bin/sh
at> echo 'the spirits of the dead have left'
at> <EOT>
job 3 at Thu Oct 31 00:00:00 2017

daemons

Linux relies heavily on background services called daemons, often named with a trailing “d”. Examples include sshd and various system daemons.

$ ps -ef | grep sshd
root      1142 1 0 Oct19 ? 00:00:00 /usr/sbin/sshd -D
$ ps -ef | grep daemon | grep -v grep
message+   790 1 0 Oct19 ? 00:00:01 /usr/bin/dbus-daemon ...
root       836 1 0 Oct19 ? 00:00:02 /usr/lib/accountsservice/accounts-daemon