diff --git a/knowledge base/useradd.md b/knowledge base/useradd.md
new file mode 100644
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--- /dev/null
+++ b/knowledge base/useradd.md	
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+# Useradd
+
+Create a new user.
+
+## TL;DR
+
+```shell
+# Create a new user.
+sudo useradd username
+sudo useradd -p encryptedPassword username
+
+# Create a new user with the specified user ID.
+sudo useradd --uid id username
+
+# Create a new user with the specified expiration date.
+sudo useradd -e 2022-10-10 username
+
+# Create a new user with the specified shell.
+sudo useradd --shell path/to/shell username
+
+# Create a new user belonging to additional groups.
+sudo useradd --groups group1,group2,... username
+
+# Create a new user with or without its default home directory.
+sudo useradd --create-home username
+sudo useradd --no-create-home username
+
+# Create a new user with the home directory filled by template directory files.
+sudo useradd --skel path/to/template_directory --create-home username
+
+# Create a new system user without the home directory.
+sudo useradd --system username
+```
+
+## Sources
+
+- [cheat.sh]
+
+[cheat.sh]: cheat.sh/useradd
diff --git a/knowledge base/userdel.md b/knowledge base/userdel.md
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+# Useradd
+
+Delete a user account and its related files.
+
+## TL;DR
+
+```shell
+# Remove a user.
+sudo userdel username
+
+# Remove a user in other root directory.
+sudo userdel --root path/to/other/root username
+
+# Remove a user along with the home directory and mail spool.
+sudo userdel --remove username
+```
+
+## Sources
+
+- [cheat.sh]
+
+[cheat.sh]: cheat.sh/userdel
diff --git a/knowledge base/usermod.md b/knowledge base/usermod.md
index 62da5d3..9f2aca2 100644
--- a/knowledge base/usermod.md	
+++ b/knowledge base/usermod.md	
@@ -3,6 +3,36 @@
 ## TL;DR
 
 ```shell
-# Add users to a group.
-sudo usermod --append --groups boinc user
+# Change a user's primary group.
+sudo usermod -g docker bob
+
+# Add/remove a user to/from supplementary groups.
+sudo usermod -aG wheel carly
+sudo usermod --append --groups kvm,video,audio alice
+sudo usermod -rG sudo,admin eve
+
+# Change a user's login name
+sudo usermod --login to-stephen from-micha
+
+# Change a user's ID.
+sudo usermod --uid 1001 hugo
+
+# Change a user's shell.
+sudo usermod --shell /usr/bin/zsh rick
+
+# Change a user's password.
+sudo usermod -p encryptedPassword john
+
+# Lock/unlock a user.
+sudo usermod -L damian
+sudo usermod -U luke
+
+# Change a user's home directory.
+sudo usermod --move-home --home path/to/new_home lonny
 ```
+
+## Sources
+
+- [cheat.sh]
+
+[cheat.sh]: cheat.sh/usermod