Linux-Filesystem ... VFS(Virtual File System) Concepts

This article contains information related to Linux File system, Virtual File System, Superblock, inode, dentry, fd concepts with examples.

I discussed about status command(stat) the last blog post. I also discussed using mount command to display all filesystem types. I strongly recommend you to read earlier blogs for better understanding . If you find something missing, Please provide feedback in comment box so that I can take corrective action.

Here is big picture of Linux File system working principle. The Linux VFS(Virtual File System) here depends on

Metadata of Filesystem i.e. Superblock created after file sytem mounting
Metadata of File object i.e inode Constructed when file is created
inode link to name & parent by dentry(Directory Entry)
File Descriptor i.e fd assigned when file is opened
File which contains a stream of bytes in a particular format

Block Diagram from Anatomy of the Linux File System

Both user and kernel works in their virtual address space. User application will make use of syscall to access a file. Here is another important link for FD.

Process Specific Information

As we noticed that Kernel mounts the root file system after booting at / . One of the important file sytem type is /proc which provides information related to processes(Active Programs) .

/proc/cpuinfo provides information similar to system in Windows

sanjay@sanjay-VirtualBox:~$ cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 142
model name : Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
stepping : 9
cpu MHz : 2711.998
cache size : 3072 KB

Whenever a process is created, By default it will have
- stdin(0),stdout(1),stderr(2),255 file descriptors. 255 is trick to keep copy of these during redirection.
You may use ps or echo $$ to display process ID

Can we analyze the process of FD allocation ?

sanjay@sanjay-VirtualBox:~$ ls /proc/$/fd
0 1 2 255

Here is one example of file opened using command exec . As per POSIX description of exec: execute commands and open, close, or copy file descriptors

Following will open the test6 file and assign fd automatically

sanjay@sanjay-VirtualBox:~/linux_day3$ exec {myfd}<>test6

sanjay@sanjay-VirtualBox:~/linux_day3$ echo $myfd 10

sanjay@sanjay-VirtualBox:~/linux_day3$ ls /proc/$/fd 0 1 10 2 255 //Close File , Will be automatically closed when script/shell exits

sanjay@sanjay-VirtualBox:~/linux_day3$ exec 10>&-

sanjay@sanjay-VirtualBox:~/linux_day3$ ls /proc/$/fd 0 1 2 255

How to check all open files by a process ?

lsof lists all the open files on the system and background processes . We can also list files open for a particular process using -p option

sanjay@sanjay-VirtualBox:~/linux_day3$ lsof -p $ COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME bash 10426 sanjay cwd DIR 8,1 4096 2257750 /home/sanjay/linux_day3 bash 10426 sanjay rtd DIR 8,1 4096 2 / bash 10426 sanjay txt REG 8,1 1037528 3407879 /bin/bash bash 10426 sanjay mem REG 8,1 47600 5785749 /lib/x86_64-linux-gnu/libnss_files-2.23.so bash 10426 sanjay mem REG 8,1 47648 5785753 /lib/x86_64-linux-gnu/libnss_nis-2.23.so :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: bash 10426 sanjay mem REG 8,1 26258 790384 /usr/lib/x86_64-linux-gnu/gconv/gconv-modules.cache bash 10426 sanjay 0u CHR 136,2 0t0 5 /dev/pts/2 bash 10426 sanjay 1u CHR 136,2 0t0 5 /dev/pts/2 bash 10426 sanjay 2u CHR 136,2 0t0 5 /dev/pts/2 bash 10426 sanjay 255u CHR 136,2 0t0 5 /dev/pts/2

cwd : Current working directory
rtd: Root directory
REG: Regular file
txt: Program text (code and Data)
mem: memory mapped file
u: read & write access
Further detail can be found by man lsof

Linux-Filesystem … VFS(Virtual File System) Concepts