diff --git a/knowledge base/linux/acl.md b/knowledge base/linux/acl.md
index c0774f2..e7f6ac6 100644
--- a/knowledge base/linux/acl.md	
+++ b/knowledge base/linux/acl.md	
@@ -8,6 +8,10 @@
 ## TL;DR
 
 ```sh
+# Install the tool.
+apt install 'acl'
+dnf install 'acl'
+
 # Show acls of files.
 getfacl 'test/declarations.h'
 
diff --git a/knowledge base/linux/getfattr.md b/knowledge base/linux/getfattr.md
index 457e15b..98036c3 100644
--- a/knowledge base/linux/getfattr.md	
+++ b/knowledge base/linux/getfattr.md	
@@ -9,6 +9,10 @@
 ## TL;DR
 
 ```sh
+# Install the tool.
+apt install 'attr'
+dnf install 'attr'
+
 # Get values for specific extended attributes.
 getfattr -n 'name' 'path/to/file.1' … 'path/to/file.N'
 ```
diff --git a/knowledge base/linux/setfattr.md b/knowledge base/linux/setfattr.md
index ba86c52..1dabdd8 100644
--- a/knowledge base/linux/setfattr.md	
+++ b/knowledge base/linux/setfattr.md	
@@ -9,6 +9,10 @@
 ## TL;DR
 
 ```sh
+# Install the tool.
+apt install 'attr'
+dnf install 'attr'
+
 # Add extended attributes.
 setfattr -n 'name' -v 'value' 'path/to/file.1' … 'path/to/file.N'