Linux: Find Out How Many File Descriptors Are Being Used

轉載自 https://www.cyberciti.biz/tips/linux-procfs-file-descriptors.html

While administrating a box, you may wanted to find out what a processes is doing and find out how many file descriptors (fd) are being used. You will surprised to find out that process does open all sort of files:
=> Actual log file

=> /dev files

=> UNIX Sockets

=> Network sockets

=> Library files /lib /lib64

=> Executables and other programs etc

In this quick post, I will explain how to to count how many file descriptors are currently in use on your Linux server system.

Step # 1 Find Out PID

To find out PID for mysqld process, enter:
# ps aux | grep mysqld
OR
# pidof mysqld

Step # 2 List File Opened By a PID # 28290

Use the lsof command or /proc/$PID/ file system to display open fds (file descriptors), run:
# lsof -p 28290 # lsof -a -p 28290
OR
# cd /proc/28290/fd # ls -l | less
You can count open file, enter:
# ls -l | wc -l

Tip: Count All Open File Handles

To count the number of open file handles of any sort, type the following command:
# lsof | wc -l
Sample outputs:

List File Descriptors in Kernel Memory

Type the following command:
# sysctl fs.file-nr
Sample outputs:

fs.file-nr = 1020 0 70000

Where,

1020 The number of allocated file handles.
0 The number of unused-but-allocated file handles.
70000 The system-wide maximum number of file handles.

You can use the following to find out or set the system-wide maximum number of file handles:
# sysctl fs.file-max
Sample outputs:

fs.file-max = 70000

See how to set the system-wide maximum number of file handles under Linux for more information.

More about /proc/PID/file & procfs File System

/proc (or procfs) is a pseudo-file system that it is dynamically generated after each reboot. It is used to access kernel information. procfs is also used by Solaris, BSD, AIX and other UNIX like operating systems. Now, you know how many file descriptors are being used by a process. You will find more interesting stuff in /proc/$PID/file directory:

/proc/PID/cmdline : process arguments
/proc/PID/cwd : process current working directory (symlink)
/proc/PID/exe : path to actual process executable file (symlink)
/proc/PID/environ : environment used by process
/proc/PID/root : the root path as seen by the process. For most processes this will be a link to / unless the process is running in a chroot jail.
/proc/PID/status : basic information about a process including its run state and memory usage.
/proc/PID/task : hard links to any tasks that have been started by this (the parent) process.