File Descriptors and Redirection#

Overview#

A file descriptor (FD) is a kernel-managed handle that represents an open I/O resource (file, socket, pipe, etc.). It provides a uniform interface for reading and writing data streams.

Windows equivalent: file handle

Standard File Descriptors#

By convention, every process starts with three predefined descriptors:

Basic Redirection#

Redirection operators control where data flows between streams and files.

Redirect STDERR#

Suppress errors by redirecting STDERR (2) to /dev/null:

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

• 2> → redirect STDERR
• /dev/null → discard output

Redirect STDOUT to File#

find /etc/ -name networks > results.txt

• > overwrites file (creates if missing)

Redirect STDOUT and STDERR Separately#

find /etc/ -name networks 2>/dev/null 1>stdout.txt

• 1> → STDOUT
• 2> → STDERR

STDIN Redirection#

File as Input#

cat < stdout.txt

• < redirects file contents into STDIN

Here Document (Heredoc)#

Stream input inline using <<:

cat << EOF >> stream.txt
My
Input
Stream
EOF

• << EOF → begin input stream
• EOF → terminates input
• >> → append to file

Append vs Overwrite#

Pipes#

Pipes (|) connect STDOUT of one command to STDIN of another.

Example: Filter + Count#

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

Execution Flow#

1. find → emits file paths (STDOUT)
2. grep → filters lines containing "systemd"
3. wc -l → counts matching lines

Mental Model#

• Each process reads from FD 0 and writes to FD 1/2
• Redirection rewires these streams
• Pipes create dataflow pipelines between processes
• /dev/null acts as a sink (black hole)

Common Patterns#

Key Takeaways#

• File descriptors abstract all I/O
• Redirection enables precise control of data flow
• Pipes enable composition of simple tools into complex workflows