Many developers encounter this seemingly paradoxical situation where sudo works for most commands but fails when redirecting output to protected files. The root cause lies in how shell redirection works:

sudo echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf
# Fails because the redirection (>>) is performed by your current shell, not by sudo

In the command pipeline:

1. Your current shell (running as your user) processes the >> redirection first
2. Only the echo command runs with sudo privileges
3. The shell attempts to write to /etc/sysctl.conf with your regular user permissions

For automated deployment scenarios like AWS EC2 builds, consider these robust approaches:

# Method 1: Using tee with sudo
echo "net.ipv4.ip_forward = 1" | sudo tee -a /etc/sysctl.conf >/dev/null

# Method 2: Temporary file approach (better for multiple edits)
TMPFILE=$(mktemp)
echo "net.ipv4.ip_forward = 1" >> "$TMPFILE"
sudo mv "$TMPFILE" /etc/sysctl.conf

The tee command:

• Receives input through stdin
• Writes to specified files with its own permissions
• The -a flag appends rather than overwrites
• We redirect tee's stdout to /dev/null to avoid duplicate output

After modifying sysctl.conf, remember to activate changes:

sudo sysctl -p
# For immediate effect without reboot:
sudo sysctl -w net.ipv4.ip_forward=1

When automating system file modifications:

• Always validate input strings
• Consider file permissions (0600 for sensitive configs)
• Use temporary files with proper cleanup
• Implement error checking in deployment scripts

When you run sudo echo "text" >> /file, the permission denied error occurs because the redirection operation (>>) is actually handled by your current shell, not by sudo. The shell (running as your regular user) tries to open the file for writing before sudo even executes.

Here's what happens step-by-step:

1. Shell parses the entire command line
2. Shell prepares redirection (as current user)
3. Shell executes sudo with the remaining command
4. sudo runs echo with elevated privileges

The critical failure occurs at step 2 when your regular user lacks write permissions.

Here are several working approaches:

# Method 1: Use tee with sudo
echo "net.ipv4.ip_forward = 1" | sudo tee -a /etc/sysctl.conf

# Method 2: Use a here document
sudo bash -c 'cat >> /etc/sysctl.conf' << EOF
net.ipv4.ip_forward = 1
EOF

# Method 3: For AWS automation (CloudInit example)
write_files:
  - path: /etc/sysctl.d/99-ipforward.conf
    content: |
      net.ipv4.ip_forward = 1
    owner: root:root
    permissions: '0644'

The tee solution works because:

• The echo runs without redirection
• tee executes with sudo privileges
• -a flag handles the append operation

The bash -c method creates a sub-shell where all operations run under sudo.

For EC2 automation, consider these robust approaches:

#!/bin/bash
# AWS UserData script example
cat << 'EOF' | sudo tee /etc/sysctl.d/90-ipforward.conf >/dev/null
# Enable IP forwarding
net.ipv4.ip_forward = 1
EOF

# Apply changes immediately
sudo sysctl -p /etc/sysctl.d/90-ipforward.conf

This method is preferred because:

• Creates a dedicated config file in /etc/sysctl.d/
• Uses proper quoting to prevent variable expansion
• Immediately applies the setting
• Follows Linux config file best practices

If you still encounter issues:

# Check effective permissions
sudo ls -l /etc/sysctl.conf

# Verify the file system isn't read-only
mount | grep "\s/\s"

# Test with a temporary file first
echo "test" | sudo tee -a /tmp/testfile

For enterprise environments, consider AWS SSM:

aws ssm send-command \
  --document-name "AWS-RunShellScript" \
  --parameters 'commands=[
    "echo \"net.ipv4.ip_forward = 1\" >> /etc/sysctl.conf",
    "sysctl -p"
  ]' \
  --instance-ids i-1234567890abcdef0