Notes On Linux System Administration

Check avaiable resources

• Avaliable memory: in /proc/meminfo

 free -h
#              total        used        free      shared  buff/cache   available
#Mem:            30G        5.3G         18G        120M        7.4G         25G
#Swap:          8.0G        2.0G        6.0G

Daemon process

• Configuration files of systemd
  • Not modify
    • /usr/lib/systemd/system
    • /lib/systemd/system
  • Can be customized
    • /etc/systemd/system
    • /run/systemd/system
• List avaiable service

systemctl list-units --type=service
systemctl list-unit-files --type=service

• Show logging

journalctl

• https://askubuntu.com/questions/903354/difference-between-systemctl-and-service-commands

ssh login without typing password

• Append local public key ~/.ssh/id_rsa.pub to ~/.ssh/authorized_keys in remote machine should work

crontab

• Configuration: minute(s) hour(s) day(s) month(s) weekday(s) command(s)
• /var/spool/cron/crontabs
• See https://kb.iu.edu/d/afiz

Account management

• Add account
  • adduser: a system command, not create home directory
  • useradd: a perl wrapper for adduser, create home directory
• Remove account

userdel -r {user.name}

• Change user group

 usermod -a -G {group.name} {user.name}

Network file system (NFS)

NIS, LDAP and single sign on

• LDAP (Lightweight Directory Access Protocol)

• PAM (Pluggable Authentication Module system)

• https://blog.csdn.net/developerinit/article/details/76141065

• https://www.cnblogs.com/lfdblog/p/9803276.html

• Show all user with access

getent passwd

• Show password specific to LDAP

getent passwd --service=ldap

Security related topics

• /var/log/secure provides useful loggings

• Do not log in as root

• https://www.liquidweb.com/kb/how-do-i-set-up-setuid-setgid-and-sticky-bits-on-linux/

• modify timestamp

touch -t 202001010000.00  test.txt
# use -t -h for symlink