Utilities
Environment Variables {ENV}
Some useful links to explain the concepts of Environment Variables
Crontab
Some useful links to cover the working of Crontab
http://www.thegeekstuff.com/2009/06/15-practical-crontab-examples/
http://www.cyberciti.biz/faq/linux-show-what-cron-jobs-are-setup/
Online Utilities to generate crons
GREP
Some useful links to cover the working of Grep
https://www.linode.com/docs/tools-reference/search-and-filter-text-with-grep
http://www.cyberciti.biz/faq/searching-multiple-words-string-using-grep/
http://www.thegeekstuff.com/2011/10/grep-or-and-not-operators/
http://xmodulo.com/how-to-grep-multiple-terms-or-strings.html
https://stackoverflow.com/questions/25853722/how-to-suppress-binary-file-matching-results-in-grep
Grepping ps output without showing the grep process
https://www.safaribooksonline.com/library/view/bash-cookbook/0596526784/ch17s18.html
https://unix.stackexchange.com/questions/74185/how-can-i-prevent-grep-from-showing-up-in-ps-results
FIND
Some useful links to cover the working of Find
http://www.tecmint.com/find-and-sort-files-modification-date-and-time-in-linux/
https://www.linode.com/docs/tools-reference/tools/find-files-in-linux-using-the-command-line
https://alvinalexander.com/blog/post/linux-unix/find-how-multiple-search-patterns-filename-command
https://stackoverflow.com/questions/5475905/linux-delete-file-with-size-0/18052644#18052644
User Management
https://www.linode.com/docs/tools-reference/linux-users-and-groups
https://askubuntu.com/questions/515103/how-can-i-display-all-users-and-groups-with-a-command
File Permissions
Some useful links to cover the working of File Permissions
https://www.linux.com/learn/getting-know-linux-file-permissions
http://www.penguintutor.com/linux/file-permissions-reference
https://www.linux.com/learn/understanding-linux-file-permissions
https://www.linux.com/learn/how-easily-back-and-restore-linux-file-permissions
https://unix.stackexchange.com/questions/102286/ls-gives-no-such-file-or-directory-message
Why sudo cd <path-to-dir> doesn’t work
Sudoers
https://www.garron.me/en/linux/visudo-command-sudoers-file-sudo-default-editor.html
https://www.digitalocean.com/community/tutorials/how-to-edit-the-sudoers-file-on-ubuntu-and-centos
https://askubuntu.com/questions/73864/how-to-modify-an-invalid-etc-sudoers-file
UserMod Command
SetUID and SetGID
UMask Values
→ UMask contains default permissions for newly created files / directories based on base permissions for those
→ Base permissions for files is 666 (read-write permissions for owner-group-other : execute permissions are excluded by for files as security measure)
→ Similarly, the base permissions for directories are 777 (read-write-execute permissions for owner-group-other)
→ The UMask values are inverse to regular file permissions → i.e: chmod
→ UMask values = Base permissions - required permissions All UMasks
http://www.cyberciti.biz/tips/understanding-linux-unix-umask-value-usage.html
SymLinks
Some useful links to cover the working of SymLinks
Others
XArgs vs Exec
http://unix.worldiswelcome.com/what-is-the-difference-between-exec-and-xargs
http://www.differencebetween.co.in/technology/difference-between-xargs-and-exec/
https://stackoverflow.com/questions/16758525/make-xargs-handle-filenames-that-contain-spaces
NTP
Renaming / Deleting multiple files
http://www.cyberciti.biz/tips/renaming-multiple-files-at-a-shell-prompt.html
http://tips.webdesign10.com/how-to-bulk-rename-files-in-linux-in-the-terminal
http://www.cyberciti.biz/faq/linux-bash-delete-all-files-in-directory-except-few/
https://www.tecmint.com/delete-all-files-in-directory-except-one-few-file-extensions/
https://askubuntu.com/questions/470134/how-to-find-the-creation-time-of-a-file
Rename multiple files with different extensions
Command Prompt Tweaks
Random Tools
http://www.tecmint.com/progress-monitor-check-progress-of-linux-commands/
http://www.tecmint.com/screen-command-examples-to-manage-linux-terminals/
http://www.thegeekstuff.com/2009/04/chage-linux-password-expiration-and-aging/
https://danielmiessler.com/blog/collection-of-less-commonly-used-unix-commands/
https://www.cyberciti.biz/open-source/command-line-hacks/compgen-linux-command/
https://www.dynacont.net/documentation/linux/Useful_SystemD_commands/
SysCTL
Some useful links to cover the working of SysCTL