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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

Output:
28290
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:
5436
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

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