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Chapter 6: I/O Redirection

Unix-like systems treat input and output as streams of data. By default, these streams are connected to your terminal, but the real power of the shell comes from your ability to reconnect these streams to files or other programs.

I. The Three Standard File Descriptors

Every process in Unix starts with three open "files" (streams) represented by integer IDs.

IDNameDescriptionDefault Source/Target
0stdinStandard InputYour keyboard
1stdoutStandard OutputYour terminal screen
2stderrStandard ErrorYour terminal screen (for errors)

II. Basic Redirection (>, >>, <)

1. Output Redirection

  • Overwrite (>): ls > files.txt (Creates or overwrites files.txt).
  • Append (>>): echo "log entry" >> system.log (Adds to the end of the file).

2. Input Redirection

  • Read from file (<): sort < names.txt (Feeds the content of names.txt into the sort command).

III. Handling Errors (stderr)

By default, both stdout and stderr are printed to the screen. You can redirect them separately to keep your logs clean.

# Redirect only errors to a file
ls /root 2> error.log

# Redirect stdout to one file and stderr to another
command > output.log 2> error.log

# Merge stderr into stdout (The "2>&1" idiom)
# This sends both to the same file.
command > all_output.log 2>&1

IV. The Pipeline (|)

A pipe takes the stdout of the left command and connects it to the stdin of the right command. This allows you to build complex data processing "assembly lines."

cat data.csvgrep "Active"awk '{print $1}'sort

V. Advanced: Here Documents and Strings

1. Here Documents (<<)

Allows you to pass multi-line strings into a command. This is great for generating config files within a script.

cat << EOF > config.xml
<settings>
  <user>$USER</user>
  <mode>production</mode>
</settings>
EOF

2. Here Strings (<<<)

A shorthand for passing a single variable or string into a command's stdin.

# Instead of: echo "$VAR" | grep "search"
# Use:
grep "search" <<< "$VAR"

VI. The /dev/null Black Hole

If you want to run a command but don't care about its output or errors, you can redirect them to the "null device"—a special file that discards everything written to it.

# Silence all output and errors
command > /dev/null 2>&1

In the next chapter, we'll learn how to add logic to our scripts using Conditionals.

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