A file descriptor is an abstraction of a file being read from/written to by a process. It is created and exposed by the.kernel purely for the convenience of the user, where the convenience is typically realized in its shorter length canonical path and/or the ability to script interactions even when its path is not supplied as an input.
Every process keeps a list of file descriptors, virtual files representing the open files the process is accessing. Note that many processes can make use of the same file descriptor. The kernel keeps track of the file descriptors.
Examination per process
You can take a peek at the file descriptors a process has open. First, get a process number with ps.
$ ps PID TTY TIME CMD … 14649 pts/8 00:00:00 bash 14675 pts/8 00:00:00 ps …
Then, you look through a special directory, , which keeps runtime information about processes. (It's not a directory representing your hard drive; Rather, it is a virtual directory representing kernel information.)
$ ls -l /proc/14649/fd total 0 lrwx------ 1 lion lion 64 Aug 16 07:32 0 -> /dev/pts/8 lrwx------ 1 lion lion 64 Aug 16 07:32 1 -> /dev/pts/8 lrwx------ 1 lion lion 64 Aug 16 07:32 2 -> /dev/pts/8
Why did we look at directory?
- is for "process information"
- bash shell is for "process #14649", the running
- is for "file descriptors", the list of the process' open file descriptors
Now we see that we have three open file descriptors:, and .
Opening and closing file descriptors
There are several ways to manually open and close file descriptors.
- In C, you can use the fcntl library to open and close file descriptors.
This explanation here goes by way of Bash scripting. To open a file descriptor, you write:
$ exec 3<> File
To close the same file descriptor, you write:
$ exec 3>&-
The working directory.means "file descriptor #3". The word File is the name of a file in the
When a process is forked, the forked process shares all file descriptors with its parent.