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File Descriptor

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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 Link to original

Fundamentals

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File Descriptors - Fundamentals

What are File Descriptors?

File descriptors are non-negative integers that represent open files or input/output channels in Unix-like systems. Every process starts with three standard file descriptors:

  • 0 = stdin (standard input)
  • 1 = stdout (standard output)
  • 2 = stderr (standard error)

Basic Redirection

# Redirect stdout to a file
echo "Hello" > output.txt        # Same as: echo "Hello" 1> output.txt
 
# Redirect stdin from a file
cat < input.txt                  # Same as: cat 0< input.txt
 
# Redirect stderr to a file
ls /nonexistent 2> errors.txt
 
# Append to a file instead of overwriting
echo "More text" >> output.txt

Testing File Descriptors

Use the -t test to check if a file descriptor is connected to a terminal:

# Check if stdout is a terminal
if [ -t 1 ]; then
    echo "Output is going to terminal"
else
    echo "Output is redirected/piped"
fi
 
# Check stdin
[ -t 0 ] && echo "Input is from terminal"
 
# Check stderr
[ -t 2 ] && echo "Errors go to terminal"

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