Linux Command Overview: crypt, kill, shred, zombies, at midnight, and daemons

The crypt command encrypts file contents; modern implementations invoke the mcrypt binary to simulate the older crypt utility, creating an additional file with a .nc extension without overwriting the original, and offering options for key size and algorithm.

Example usage:

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

The kill family of commands terminates processes, with variants such as kill, pkill, killall, killpg, rfkill, skill, tgkill, tkill, and xkill. An example of terminating all instances of a program:

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

The shred command securely overwrites a file to prevent data recovery, unlike rm which merely removes directory entries. Example:

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

Zombie processes are defunct processes that remain after their parent fails to reap them. They can be identified by the top command, which lists them in the “Tasks” line. Example output shows three zombie processes:

$ top<br/><br/>top - 18:50:38 up 6 days, 6:36, 2 users, load average: 0.00, 0.00, 0.00<br/><br/>Tasks: 171 total, 1 running, 167 sleeping, 0 stopped, 3 zombie <==<br/>%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<br/>KiB Mem : 2003388 total, 250840 free, 545832 used, 1206716 buff/cache<br/>KiB Swap: 9765884 total, 9765764 free, 120 used. 1156536 avail Mem

The at midnight command schedules a one‑time job to run at the specified time, similar to a single‑execution cron job. Example:

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

Daemons are background processes that provide core system services; many end with a “d” (e.g., sshd, dbus-daemon). They can be listed with ps -ef | grep daemon while excluding the grep process itself.

$ ps -ef | grep sshd<br/><br/>root 1142 1 0 Oct19 ? 00:00:00 /usr/sbin/sshd -D<br/>root 25342 1142 0 18:34 ? 00:00:00 sshd: shs [priv]<br/><br/>$ ps -ef | grep daemon | grep -v grep<br/><br/>message+ 790 1 0 Oct19 ? 00:00:01 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation<br/>root 836 1 0 Oct19 ? 00:00:02 /usr/lib/accountsservice/accounts-daemon