Check Open Port and Identify Process in Bash

#!/bin/bash

# --- Configuration ---
TARGET_PORT=${1:-8080} # Default to 8080 if no argument provided

# --- Check if lsof is installed ---
if ! command -v lsof &> /dev/null; then
    echo "Error: 'lsof' command not found. Please install it (e.g., 'sudo apt install lsof' or 'sudo yum install lsof')." >&2
    exit 1
fi

echo "Checking if port $TARGET_PORT is in use..."

# --- Use lsof to find processes listening on the target port ---
# -i : selects by internet address
# -P : suppresses port names conversion (e.g., "http" instead of "80")
# -n : suppresses hostnames conversion
# -s TCP:LISTEN : specifically look for TCP sockets in LISTEN state
LSOF_OUTPUT=$(lsof -iTCP:"$TARGET_PORT" -sTCP:LISTEN -P -n 2>/dev/null)

if [ -z "$LSOF_OUTPUT" ]; then
    echo "Port $TARGET_PORT is free."
    exit 0
else
    echo "Port $TARGET_PORT is in use. Details:"
    echo "-----------------------------------"
    echo "$LSOF_OUTPUT"
    echo "-----------------------------------"

    # --- Extract PID and Command for more user-friendly output ---
    # Get the header line
    HEADER=$(echo "$LSOF_OUTPUT" | head -n 1)
    echo ""
    echo "Summary:"
    echo "CMD     PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME"
    echo "---     --- ----   --   ---- ------ -------- ---- ----"

    # Loop through each line of output, skip header
    echo "$LSOF_OUTPUT" | tail -n +2 | while read -r line; do
        # Extract relevant columns: COMMAND, PID, USER
        CMD=$(echo "$line" | awk '{print $1}')
        PID=$(echo "$line" | awk '{print $2}')
        USER=$(echo "$line" | awk '{print $3}')
        PROTOCOL_STATE=$(echo "$line" | awk '{print $NF}') # Last column is typically NAME
        
        # Print formatted summary, showing only lines that are actually LISTENING for the specific port
        if [[ "$PROTOCOL_STATE" == *":$TARGET_PORT (LISTEN)"* ]]; then
             echo "$CMD    $PID $USER" # Only print the first three for a concise view. Full line is above.
        fi
    done
    exit 1 # Indicate that the port is in use
fi