diff --git a/content/notes/15-file-systems.md b/content/notes/15-file-systems.md
index 7717f574e..22d5eff4c 100644
--- a/content/notes/15-file-systems.md
+++ b/content/notes/15-file-systems.md
@@ -6,7 +6,7 @@ tags:
 - lecture
 ---
 
-# what is a file
+# what is a file (LINUX)
 an interface of OS to users
 
 os manages files just as files - it doesn't care what type of file it is - they are all bytes arranges logically in sequence.
@@ -21,8 +21,47 @@ files need to be interpreted. THis is done by applications. e.g., powerpoint, ms
 	- the operating system maps files onto regions of the storage
 	- files are really an artifact of the dialogue between the user and the OS
 
-# how to define it
-- a colle
+## how to define it
+- a collection of related data
+- e.g., the set of lines in a program
+
+structure:
+- a byte stream or a sequence of bytes, text file is a stream of ascii characgers
+
+what can the user do
+- create, write, read, reposition, delete, truncate (through system calls)
+
+## attributes
+- name
+- owner
+	- Uid and Gid
+- file type
+- location of data
+- size of data
+- permission
+- "housekeeping" information
+	- time and created modified etc
+- and more
+
+## where is the data stored
+- in an index node (inode) pointed by an entry of a directory. (you can think of a directory as just a set of <name, inode> entries. )
+
+internal fragmentation
+- dont know exactly how many bytes to allocate to a file
+- extra space is lost
+
+external fragmentaion TODO
+
+these issues have been solved using block allocaiton instead of continuous allocation
+
+## system calls for file systems
+- open close, read, write, ioctl, etc
+
+`ssize_t write(int fd, const, void *buf, size_t count)`
+	- fd - file descriptor
+	- write up to count butes from the buffer pointed buf to the file referred to by the file descriptor
+	- returns -1 if error. and errno is set
+	- should return number of bytes written if ok
 
 # file access pattersns
 # directory structure