File Descriptors and Redirections

What is a File Descriptor (FD)?#

A File Descriptor (FD) is an integer reference used by the operating system to track open files, devices, and input/output (I/O) resources. When a process opens a file or a resource, the system assigns a unique FD, which acts as an identifier for subsequent operations. Conceptually, it functions like a ticket number that allows the system to locate and manage the associated resource.

In Unix/Linux systems, three standard FDs are predefined:

STDIN (FD 0) – Standard input (where data is read from).
STDOUT (FD 1) – Standard output (where normal results are displayed).
STDERR (FD 2) – Standard error (where error messages are shown).

Examples of STDIN, STDOUT, and STDERR#

STDIN and STDOUT Interaction#

When a command such as cat is executed, it reads input from STDIN (typically the keyboard) and displays the output via STDOUT (the terminal). For example:

Input: SOME INPUT
Output: The same text is echoed back to the terminal.

STDERR for Error Handling#

If a command encounters an issue (e.g., insufficient permissions), the error message is sent to STDERR instead of STDOUT. For example:

Running find /etc/ -name shadow may produce a “Permission denied” error, which appears in the terminal via STDERR.

File Descriptor Redirection#

Redirecting STDERR to /dev/null#

To suppress error messages, STDERR can be redirected to /dev/null, a special device that discards data:

find /etc/ -name shadow 2>/dev/null

Redirecting STDOUT to a File#

Normal output can be saved to a file instead of displaying it on the terminal:

find /etc/ -name shadow > results.txt

Separating STDOUT and STDERR#

Output and errors can be directed to different files:

find /etc/ -name shadow 1> stdout.txt 2> stderr.txt

Redirecting STDIN from a File#

Commands can read input from a file instead of keyboard input:

cat < input.txt

Appending Output to a File#

Using >> prevents overwriting and appends new data to an existing file:

find /etc/ -name passwd >> output.log

Here Documents (<<) for Input Streams#

A block of text can be passed as input until a specified delimiter (e.g., EOF) is encountered:

cat << EOF > document.txt
This is a multi-line
input block.
EOF

Using Pipes (|)#

Pipes allow the output of one command to serve as input for another:

find /etc/ -name *.conf 2>/dev/null | grep systemd

Combining Multiple Commands with Pipes#

Pipes can chain several commands for complex operations:

find /etc/ -name *.conf 2>/dev/null | grep systemd | wc -l

This example:

Searches for .conf files in /etc/.
Filters results containing “systemd”.
Counts the number of matching files.