LPIC-1 (102): Manage User Accounts

Study Notes

User account information is stored in /etc/passwd and /etc/group files.
/etc/shadow and /etc/gshadow files store encrypted passwords and are readable only by root and shadow group members.
When shadow passwords are used, an x appears in the second field of /etc/passwd and /etc/group files.
The ls -l /etc/passwd /etc/group /etc/shadow /etc/gshadow command shows file permissions.

The useradd -m command creates a new user account with a home directory.
The usermod command is used to modify user account properties.
The getent command is used to review password aging information for a user account.

Cron and at jobs can be used to schedule system administration tasks.
The /etc/cron.{d,daily,hourly,monthly,weekly}/ directories contain cron job files.
The /etc/at.deny and /etc/at.allow files control access to at services.
The crontab command is used to manage cron jobs.
The at command is used to schedule at jobs.
The atq and atrm commands are used to manage at jobs.
Systemd timer units can be used to automate system administration tasks.
The systemctl and systemd-run commands are used to manage systemd timer units.

The crontab -e command is used to edit or create a crontab file.
The default editor is specified by the VISUAL or EDITOR environment variables.
In some distributions, the editor can be chosen from a list when crontab is run for the first time.
The format of a crontab entry is: minute hour day month day_of_week command.
Example: 0 10 * * * /home/frank/foo.sh runs the foo.sh script every day at 10:00 am.

System crontabs are updated using an editor, not the crontab command.
The /etc/crontab file and files in /etc/cron.d must be edited manually.
The user account that will run the cron job must be specified in the crontab entry.
Example: 30 01 * * * root /root/barfoo.sh >>/root/output.log 2>>/root/error.log

The /etc/cron.allow and /etc/cron.deny files are used to set crontab restrictions.
If /etc/cron.allow exists, only non-root users listed within it can schedule cron jobs.
If /etc/cron.deny exists, users listed within it cannot schedule cron jobs.

systemd timer units can be used as an alternative to cron.
Timers are logged to the systemd journal.
The systemctl command is used to control systemd timer units.
Example: systemctl enable foobar.timer and systemctl start foobar.timer to enable and start a timer.

The crontab command has options to display, remove, and specify the user whose crontab needs to be modified.
Example: crontab -l displays the current crontab, crontab -r removes the current crontab, and crontab -u user specifies the user whose crontab needs to be modified.