Some useful links to explain the concepts of Environment Variables
- https://www.guru99.com/linux-environment-variables.html
- https://www.digitalocean.com/community/tutorials/how-to-read-and-set-environmental-and-shell-variables-on-a-linux-vps
- https://www.slashroot.in/difference-between-bashrc-and-bashprofile
- https://www.cyberciti.biz/faq/linux-list-all-environment-variables-env-command/
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/
- https://www.pantz.org/software/cron/croninfo.html
- http://cronchecker.net/
- http://crontab-generator.org/
- http://www.cronmaker.com/
- http://htmlminifiers.com/cron-maker.php
- http://blog.endpoint.com/2008/12/best-practices-for-cron.html
Some useful links to cover the working of Grep
- https://www.linode.com/docs/tools-reference/search-and-filter-text-with-grep
- https://danielmiessler.com/study/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://www.digitalocean.com/community/tutorials/using-grep-regular-expressions-to-search-for-text-patterns-in-linux
- https://alvinalexander.com/unix/edu/examples/grep.shtml
- https://stackoverflow.com/questions/2427913/how-can-i-grep-for-a-string-that-begins-with-a-dash-hyphen
- https://stackoverflow.com/questions/221921/use-grep-exclude-include-syntax-to-not-grep-through-certain-files
- https://stackoverflow.com/questions/25853722/how-to-suppress-binary-file-matching-results-in-grep
- 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
Some useful links to cover the working of Find
- https://danielmiessler.com/study/find/
- https://www.digitalocean.com/community/tutorials/how-to-use-find-and-locate-to-search-for-files-on-a-linux-vps
- http://tecadmin.net/delete-files-older-x-days/
- 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
- https://www.linode.com/docs/tools-reference/linux-users-and-groups
- https://linux-audit.com/unused-linux-users-delete-or-keep/
- https://askubuntu.com/questions/515103/how-can-i-display-all-users-and-groups-with-a-command
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
- http://www.linuxnix.com/chmod-command-explained-linuxunix/
- https://unix.stackexchange.com/questions/21251/execute-vs-read-bit-how-do-directory-permissions-in-linux-work
- http://www.grymoire.com/Unix/Permissions.html
- http://www.thegeekstuff.com/2010/06/chmod-command-examples/
- https://danielmiessler.com/study/unixlinux_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
- http://cavepopo.hd.free.fr/wordpress/linux/sudo-command-sudoers-file-concepts-and-practical-examples/
- http://serverfault.com/questions/364334/test-whether-a-user-has-sudo-privileges-without-requiring-user-input
- https://www.digitalocean.com/community/tutorials/how-to-add-delete-and-grant-sudo-privileges-to-users-on-a-debian-vps
- http://www.thegeekstuff.com/2010/09/sudo-command-examples/
- 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
- http://www.tecmint.com/usermod-command-examples/
- http://linoxide.com/linux-command/linux-usermod-command-to-modify-user-details/
- http://crybit.com/15-switches-of-usermod-command-with-example-unixlinux/
- https://muffinresearch.co.uk/linux-changing-uids-and-gids-for-user/
- http://www.htpcbeginner.com/safely-change-primary-group-group-in-linux/
- http://www.cyberciti.biz/faq/howto-linux-add-user-to-group/
- http://www.tutonics.com/2012/12/linux-file-permissions-chmod-umask.html
- https://www.slashroot.in/suid-and-sgid-linux-explained-examples
- https://www.thegeekstuff.com/2013/02/sticky-bit/
- http://www.linuxnix.com/suid-set-suid-linuxunix/
- https://unix.stackexchange.com/questions/28363/whats-the-difference-between-s-and-s-in-ls-la
- → 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
- https://www.computerhope.com/unix/uumask.htm
- http://www.cyberciti.biz/tips/understanding-linux-unix-umask-value-usage.html
- https://www.digitalocean.com/community/tutorials/linux-permissions-basics-and-how-to-use-umask-on-a-vps
Some useful links to cover the working of SymLinks
- http://www.cyberciti.biz/tips/understanding-unixlinux-symbolic-soft-and-hard-links.html
- http://www.thegeekstuff.com/2010/10/linux-ln-command-examples/
- http://bencane.com/2013/10/10/symlinks-vs-hardlinks-and-how-to-create-them/
- http://geek-university.com/linux/symbolic-links/
- http://geek-university.com/linux/hard-links/
- http://unix.worldiswelcome.com/what-is-the-difference-between-exec-and-xargs
- https://danielmiessler.com/blog/linux-xargs-vs-exec/
- http://www.differencebetween.co.in/technology/difference-between-xargs-and-exec/
- https://stackoverflow.com/questions/16758525/make-xargs-handle-filenames-that-contain-spaces
- http://support.ntp.org/bin/view/Support/TroubleshootingNTP
- https://support.rackspace.com/how-to/using-ntp-to-sync-time/
- https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.aix.cmds4/ntpdate.htm
- 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
- https://www.linux.com/blog/linux-shell-tip-remove-files-names-contains-spaces-and-special-characters-such
- 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
- https://www.if-not-true-then-false.com/2011/linux-display-show-file-contents-tabs-line-breaks-non-printing-characters/
- http://computers.tutsplus.com/tutorials/speed-up-your-terminal-workflow-with-command-aliases-and-profile--mac-30515
- http://blog.taylormcgann.com/2012/06/13/customize-your-shell-command-prompt/
- http://aarvik.dk/blacklist-check-unix-linux-utility/
- 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/study/tar/
- https://danielmiessler.com/blog/collection-of-less-commonly-used-unix-commands/
- http://www.commandlinefu.com/commands/browse
- https://distrochooser.de/en
- https://www.cyberciti.biz/open-source/command-line-hacks/compgen-linux-command/
- https://www.dynacont.net/documentation/linux/Useful_SystemD_commands/
Some useful links to cover the working of SysCTL
- https://rtcamp.com/tutorials/linux/sysctl-conf
- https://www.cyberciti.biz/faq/linux-kernel-etcsysctl-conf-security-hardening
- https://www.slashroot.in/linux-network-tcp-performance-tuning-sysctl
- https://www.linux.com/news/kernel-tuning-sysctl
- https://danielmiessler.com/study/lsof/
- http://www.catonmat.net/blog/unix-utilities-lsof/
- http://www.thegeekstuff.com/2012/08/lsof-command-examples/
- https://www.ibm.com/developerworks/aix/library/au-lsof.html
- https://unix.stackexchange.com/questions/253321/how-to-display-size-human-readable-in-lsof-grep-var