Custom file descriptors

A file descriptor is an abstract indicator for accessing a file. Each file access is associated with a special number called a file descriptor. 0, 1, and 2 are reserved descriptor numbers for stdin, stdout, and stderr.

The exec command can create new file descriptors. If you are familiar with file access in other programming languages, you may be familiar with the modes for opening files. These three modes are commonly used:

• Read mode
• Write with append mode
• Write with truncate mode

The < operator reads from the file to stdin. The > operator writes to a file with truncation (data is written to the target file after truncating the contents). The >> operator writes to a file by appending (data is appended to the existing file contents and the contents of the target file will not be lost). File descriptors are created with one of the three modes.

Create a file descriptor for reading a file:

$ exec 3<input.txt # open for reading with descriptor number 3

We can use it in the following way:

$ echo this is a test line > input.txt
$ exec 3<input.txt

Now you can use file descriptor 3 with commands. For example, we will use cat<&3:

$ cat<&3
this is a test line

If a second read is required, we cannot reuse the file descriptor 3. We must create a new file descriptor (perhaps 4) with exec to read from another file or re-read from the first file.

Create a file descriptor for writing (truncate mode):

$ exec 4>output.txt # open for writing

Consider this example:

$ exec 4>output.txt
$ echo newline >&4
$ cat output.txt
newline

Now create a file descriptor for writing (append mode):

$ exec 5>>input.txt

Consider the following example:

$ exec 5>>input.txt
$ echo appended line >&5
$ cat input.txt
newline
appended line