Redirections: stdin, stdout, and stderr

Every Unix process has three standard data streams. Understanding how to redirect them gives you powerful control over your scripts.

1. The Three Standard Streams

• stdin (0): Standard Input. Data the program reads from (usually the keyboard).
• stdout (1): Standard Output. Normal output printed by the program (usually the terminal screen).
• stderr (2): Standard Error. Error messages printed by the program (also usually the terminal screen).

2. Redirecting Output to Files

# Overwrite file with stdout (creates the file if it doesn't exist)
echo "Deploy complete" > deploy.log

# Append stdout to an existing file (does not overwrite)
echo "Step 2 complete" >> deploy.log

# Redirect stderr to a separate error file
ffmpeg -i input.mp4 output.webm 2> ffmpeg_errors.log

# Redirect both stdout and stderr to the same file
./build.sh > build.log 2>&1

3. Suppressing Output

To run a command silently, redirect both output streams to /dev/null (the system's "black hole"):

# Run command silently, suppressing all output
apt-get install -y nginx > /dev/null 2>&1

4. Reading stdin in Scripts

To accept piped input in a script using a while read loop:

#!/bin/bash
# Read each line from stdin
while IFS= read -r line; do
  echo "Processing: $line"
done