A JBang script to edit your /etc/hosts file (add and remove entries)

HostEdit.java

///usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS info.picocli:picocli:4.7.4

import java.io.File;
import java.io.IOException;
import java.lang.System.Logger.Level;
import java.nio.file.Files;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;

import picocli.CommandLine;

public class HostEdit implements Callable<Integer> {

    private static final String SECTION_ENTRY = "# Host Edit Section";
    private static final String SECTION_EXIT = "# End of Host Edit Section";

    private static final System.Logger LOGGER = System.getLogger("Host Edit");

    @CommandLine.Option(names = { "-h","--host-file" }, 
        description = "The path to the host file", defaultValue = "/etc/hosts")
    private File hosts;

    @CommandLine.Option(names = { "--ip" }, description = "The ip to add to the hosts file") 
    private String ip;

    @CommandLine.Option(names = { "--host" }, description = "The host name to add or remove", required = true) 
    private String host;

    @CommandLine.Option(names = { "--dry-run" }, description = "Do not write the file, just print the outcome", defaultValue="false") 
    private boolean dryRun;

    public static void main(String... args) {
        var exitCode = new CommandLine(new HostEdit()).execute(args);
        System.exit(exitCode);
    }

    @Override
    public Integer call() throws Exception {
        if (!ensureWritable()) {
            return 1;
        }

        var lines = read();
        if (ip != null) {
            LOGGER.log(Level.INFO, "Adding `{0} {1}` to the hosts file", ip, host);
            lines = insert(lines, ip, host);
        } else if (host != null) {
            LOGGER.log(Level.INFO, "Removeing line with `{0}` from the hosts file", host);
            lines = remove(lines, host);
        }

        if (dryRun) {
            LOGGER.log(Level.INFO, "Resulting {0} file", hosts);
            System.out.println();
            lines.forEach(l -> {
                System.out.println("\t" + l);
            });
        } else {
            Files.write(hosts.toPath(), lines);
        }

        return 0;
    }

    private List<String> insert(List<String> lines, String ip, String host) {
        boolean inSection = false;
        boolean endOfSection = false;
        List<String> output = new ArrayList<>();
        for (String line : lines) {
            if (line.trim().equals(SECTION_ENTRY)) {
                inSection = true;
                output.add(line);
            } else if (inSection  && line.trim().equals(SECTION_EXIT)) { 
                // End of section
                output.add(ip + " " + host);
                endOfSection = true;
                output.add(line);
            } else {
                output.add(line);
            }
        }
        if (inSection && ! endOfSection) {
            output.add(SECTION_EXIT);
        }
        if (! inSection) {
            output.add(SECTION_ENTRY);
            output.add(ip + " " + host);
            output.add(SECTION_EXIT);
        }
        return output;
    }

    private List<String> remove(List<String> lines, String host) {
        boolean inSection = false;
        boolean endOfSection = false;
        boolean found = false;
        String removed = null;
        int count = 0;
        List<String> output = new ArrayList<>();
        for (String line : lines) {
            if (line.trim().equals(SECTION_ENTRY)) {
                inSection = true;
                output.add(line);
            } else if (inSection  && line.trim().equals(SECTION_EXIT)) { 
                // End of section
                endOfSection = true;
                output.add(line);
            } else if (inSection  && ! endOfSection  && line.contains(host)) {
                // skip line
                found = true;
                removed = line;
            }else if (inSection  && !endOfSection) {
                output.add(line);
                count = count + 1;
            } else {
                output.add(line);
            }
        }

        if (! found) {
            LOGGER.log(Level.ERROR, "Unable to find the line with `{0}` in the hosts file", host);
        } else {
            LOGGER.log(Level.INFO, "The line `{0}` has been removed", removed);
            if (count == 0) {
                LOGGER.log(Level.INFO, "Removing section");
                output = removeSection(lines);
            }
        }

        return output;
    }

    private List<String> removeSection(List<String> lines) {
        List<String> output = new ArrayList<>();
        boolean inSection = false;
        boolean endOfSection = false;
        for(String line : lines) {
            if (line.trim().equals(SECTION_ENTRY)) {
                // Skip
                inSection = true;
            } else if (inSection  && ! endOfSection  && line.trim().equals(SECTION_EXIT)) {
                endOfSection = true;
            } else if (! inSection  && ! endOfSection) {
                output.add(line);
            }
        }
        return output;
    }

    private List<String> read() throws IOException {
        return Files.readAllLines(hosts.toPath());
    }

    private boolean ensureWritable() {
        if (!hosts.isFile()) {
            LOGGER.log(Level.ERROR, "The host file ({0}) does not exist", hosts.getAbsolutePath());
            return false;
        }

        if (!hosts.canWrite()) {
            LOGGER.log(Level.ERROR, "The host file ({0}) cannot be written, try running with elevated privileges.",
                    hosts.getAbsolutePath());
            if (dryRun) {
                LOGGER.log(Level.WARNING, "The host file ({0}) is not writable, but dry-run is enabled, so continuing...", hosts.getAbsolutePath());
                return true;
            }
            return false;
        }
        return true;

    }

}