File Descriptor Image

Shell Process File Descriptors

SHELL PROCESS

FD 0: stdin FD 1: stdout FD 2: stderr

Keyboard (Terminal)

<!-- File input -->
<rect x="50" y="250" width="80" height="40" rx="5" fill="#f39c12" stroke="#e67e22" stroke-width="2"/>
<text x="90" y="265" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">File</text>
<text x="90" y="280" text-anchor="middle" font-family="monospace" font-size="9" fill="white">&lt; input.txt</text>

<!-- Pipe input -->
<rect x="50" y="310" width="80" height="40" rx="5" fill="#9b59b6" stroke="#8e44ad" stroke-width="2"/>
<text x="90" y="325" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">Pipe</text>
<text x="90" y="340" text-anchor="middle" font-family="monospace" font-size="9" fill="white">cmd1 | cmd2</text>

Terminal Display

<!-- File output -->
<rect x="670" y="220" width="80" height="40" rx="5" fill="#f39c12" stroke="#e67e22" stroke-width="2"/>
<text x="710" y="235" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">File</text>
<text x="710" y="250" text-anchor="middle" font-family="monospace" font-size="9" fill="white">&gt; output.txt</text>

<!-- Pipe output -->
<rect x="670" y="280" width="80" height="40" rx="5" fill="#9b59b6" stroke="#8e44ad" stroke-width="2"/>
<text x="710" y="295" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">Next Cmd</text>
<text x="710" y="310" text-anchor="middle" font-family="monospace" font-size="9" fill="white">cmd1 | cmd2</text>

Terminal (Errors)

<!-- Error file -->
<rect x="670" y="420" width="80" height="40" rx="5" fill="#e67e22" stroke="#d35400" stroke-width="2"/>
<text x="710" y="435" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">Error File</text>
<text x="710" y="450" text-anchor="middle" font-family="monospace" font-size="9" fill="white">2&gt; errors.txt</text>

<!-- /dev/null -->
<rect x="670" y="480" width="80" height="40" rx="5" fill="#34495e" stroke="#2c3e50" stroke-width="2"/>
<text x="710" y="495" text-anchor="middle" font-family="Arial, sans-serif" font-size="11" fill="white">/dev/null</text>
<text x="710" y="510" text-anchor="middle" font-family="monospace" font-size="9" fill="white">2&gt; /dev/null</text>

FD 0 FD 1 FD 2

Common Redirection Examples: echo "hello" > file.txt # stdout to file ls /bad 2> errors.txt # stderr to file sort < input.txt # stdin from file cmd 2> /dev/null # discard errors File Descriptor Numbers: 0 = stdin (input) 1 = stdout (normal output) 2 = stderr (error output) Always available in every process