linux administration notes

docs/administration.md

@@ -0,0 +1,307 @@
+---
+title: Linux administration
+---
+
+## User administration
+
+For below examples, I am logged into a RHEL compatible linux distribution.
+
+### Create user
+
+In order to add new user we need to switch to `root` account. We can do that by:
+
+```bash
+su -
+# su stands for switch user
+# in some systems
+sudo su -
+```
+
+Once we are logged into the system as `root`, we can add new user:
+
+```bash
+useradd <username>
+useradd linuxfan
+```
+
+We can check the info of the newly added user by:
+
+```bash
+id <username>
+```
+
+And it will print the following:
+
+```bash
+uid=1001(linuxfan) gid=1001(linuxfan) groups=1001(linuxfan)
+```
+
+We see that along with the newly created user, a group with the same name is
+also created, and the new user is added to that group by default.
+
+We can also look for the user information in the `/etc/passwd` file:
+
+```bash
+cat /etc/passwd | grep linuxfan
+```
+
+That prints out:
+
+```bash
+linuxfan:x:1001:1001::/home/linuxfan:/bin/bash
+```
+
+We can provide additional options to `useradd`. Please see help for more
+details.
+
+```bash
+[root@host:~]# useradd --help
+Usage: useradd [options] LOGIN
+       useradd -D
+       useradd -D [options]
+
+Options:
+      --badname                 do not check for bad names
+  -b, --base-dir BASE_DIR       base directory for the home directory of the
+                                new account
+      --btrfs-subvolume-home    use BTRFS subvolume for home directory
+  -c, --comment COMMENT         GECOS field of the new account
+  -d, --home-dir HOME_DIR       home directory of the new account
+  -D, --defaults                print or change default useradd configuration
+  -e, --expiredate EXPIRE_DATE  expiration date of the new account
+  -f, --inactive INACTIVE       password inactivity period of the new account
+  -g, --gid GROUP               name or ID of the primary group of the new
+                                account
+  -G, --groups GROUPS           list of supplementary groups of the new
+                                account
+  -h, --help                    display this help message and exit
+  -k, --skel SKEL_DIR           use this alternative skeleton directory
+  -K, --key KEY=VALUE           override /etc/login.defs defaults
+  -l, --no-log-init             do not add the user to the lastlog and
+                                faillog databases
+  -m, --create-home             create the user's home directory
+  -M, --no-create-home          do not create the user's home directory
+  -N, --no-user-group           do not create a group with the same name as
+                                the user
+  -o, --non-unique              allow to create users with duplicate
+                                (non-unique) UID
+  -p, --password PASSWORD       encrypted password of the new account
+  -r, --system                  create a system account
+  -R, --root CHROOT_DIR         directory to chroot into
+  -P, --prefix PREFIX_DIR       prefix directory where are located the /etc/* files
+  -s, --shell SHELL             login shell of the new account
+  -u, --uid UID                 user ID of the new account
+  -U, --user-group              create a group with the same name as the user
+  -Z, --selinux-user SEUSER     use a specific SEUSER for the SELinux user mapping
+```
+
+Assign password for a user:
+
+```bash
+passwd <username>
+```
+
+Encrypted password info is saved in `/etc/shadow`.
+
+```bash
+cat /etc/shadow | grep linuxfan
+
+linuxfan:$6$YwIUOQqwKk6HwgU9$M.KOwYx9D.21U3kNREw79ToghJXx2tzVIGnjwr/lzR8YDveAeEF9U56fAU6mhVDM1W.Zt6TMzvgh.2z5OkOrU.:19845:0:99999:7:::
+```
+
+Change username:
+
+```bash
+usermod -l <new-username> <current-username>
+usermod -l codingfan linuxfan
+```
+
+Now if we look for information of `codingfan` in `/ect/passwd`:
+
+```bash
+[root@host:~]# cat /etc/passwd | grep codingfan
+codingfan:x:1001:1001::/home/linuxfan:/bin/bash
+```
+
+Notice that the home directory name is still the previous one.
+
+Delete a user:
+
+```bash
+userdel <username>
+```
+
+Lock a user:
+
+```bash
+usermod -L <username>
+```
+
+Unlock a user:
+
+```bash
+usermod -U <username>
+```
+
+### Create group
+
+```bash
+groupadd projectX
+```
+
+We can see group list in
+
+```bash
+[root@host:~]# cat /etc/group
+[snipped]
+linuxfan:x:1001:
+projectX:x:1002:
+```
+
+Add a user to a new group.
+
+```bash
+usermod -G projectX linuxfan
+
+[root@host:~]# cat /etc/group | grep projectX
+projectX:x:1002:linuxfan
+
+[root@host:~]# id linuxfan
+uid=1001(linuxfan) gid=1001(linuxfan) groups=1001(linuxfan),1002(projectX)
+```
+
+Delete a group:
+
+```bash
+groupdel <groupname>
+```
+
+### System info
+
+```bash
+uname -a
+```
+
+```bash
+arch
+```
+
+```bash
+hostname
+```
+
+```bash
+dmidecode
+```
+
+```bash
+cat /etc/os-release
+```
+
+## System monitoring
+
+```bash
+who
+```
+
+```bash
+w
+```
+
+```bash
+last
+```
+
+```bash
+tail -f /var/log/secure
+```
+
+```bash
+systemctl list-units --type=service
+
+systemctl status sshd
+systemctl stop sshd
+systemctl start sshd
+```
+
+## Resource management
+
+```bash
+top
+```
+
+Exit `top` screen by pressing `q` key in the keyboard.
+
+```bash
+free
+```
+
+```bash
+df
+df -h
+```
+
+```bash
+du /path
+du -hs /path
+du -h /path | sort -nr
+```
+
+```bash
+uptime
+```
+
+```bash
+dmesg
+dmesg | more
+```
+
+```bash
+ps
+```
+
+```bash
+cat /var/log/messages
+```
+
+## Networking
+
+```bash
+ping google.com
+```
+
+```bash
+ifconfig
+
+# newer replacement
+ip
+ip addr show
+ip route show
+ip link set eth0 up
+```
+
+## System update
+
+Update the full system:
+```bash
+yum update
+
+# newer dnf is replacing yum
+dnf update
+```
+
+```bash
+dnf list installed
+dnf list installed | wc -l
+
+dnf repoinfo
+
+dnf search openssh
+dnf install openssh
+
+dnf list installed | grep openssl
+rpm -qa | grep openssl
+rpm -ql openssl
+
+dnf remove openssh
+```

docs/index.md

@@ -26,9 +26,10 @@ to get a quick summary of various Linux/ UNIX commands.
 + [Shell scripting](shell-scripting.mdx)
 + [Regular Expression](regex.md)
 + [Miscellaneous](/category/misc)
+    - [Setting up Linux](setup.md)
+    - [Linux administration](administration.md)
     - [PGP encryption](pgp.md)
     - [GNU Plot](gnuplot.md)
-    - [Setting up Linux](setup.md)
     - [Filesystem](filesystem.md)
     - [Network setup](network-setup.md)
     - [Firewall](firewall.md)

sidebars.js

@@ -16,6 +16,7 @@ module.exports = {
       },
       items: [
         "setup",
+        "administration",
         "pgp",
         "gnuplot",
         "filesystem",