Unlock Linux’s Dark Side: Master Crypt, Kill, Shred, and More

It’s time to pay attention to the scary side of Linux.

crypt

We’ve always had crypt , a command that encrypts file contents rather than merely storing junk files.

Nowadays crypt is often implemented as a script that calls the mcrypt binary to emulate the older crypt command; using mycrypt directly is a better choice.

$ mcrypt x
Enter the passphrase (maximum of 512 characters)
Please use a combination of upper and lower case letters and numbers.
Enter passphrase:
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 command terminates processes; the forcefulness depends on the signal used. Linux provides many variants such as kill, pkill, killall, killpg, rfkill, skill, tgkill, tkill, and xkill.

$ 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 with disk tools impossible. Unlike rm, which merely removes directory entries, shred actually destroys the data.

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

zombies

Zombie processes are remnants of dead processes that haven’t been fully cleaned up, indicating a flaw in the parent process.

Check for zombies with the top command; the task line will show “zombie”.

$ top

top - 18:50:38 up 6 days, 6:36, 2 users, load average: 0.00, 0.00, 0.00

Tasks: 171 total, 1 running, 167 sleeping, 0 stopped, 3 zombie <==
%Cpu(s): 0.0 us, 0.0 sy, 0.0 ni, 99.9 id, 0.1 wa, 0.0 hi, 0.0 si, 0.0 st

KiB Mem : 2003388 total, 250840 free, 545832 used, 1206716 buff/cache

KiB Swap: 9765884 total, 9765764 free, 120 used. 1156536 avail Mem

at midnight

The at midnight command schedules a one‑time task to run at a specified time, similar to a single‑run cron job.

$ 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. Many daemon names end with a “d”, indicating they run continuously to provide essential functionality.

$ ps -ef | grep sshd

root 1142 1 0 Oct19 ? 00:00:00 /usr/sbin/sshd -D

root 25342 1142 0 18:34 ? 00:00:00 sshd: shs [priv]

$ ps -ef | grep daemon | grep -v grep

message+ 790 1 0 Oct19 ? 00:00:01 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation

root 836 1 0 Oct19 ? 00:00:02 /usr/lib/accountsservice/accounts-daemon