Tricks for the CLI

Mostly bash and java and OS X

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Hand-crafted artisanal HTTP responses via nc -kl <port>

Not only can you listen for incoming HTTP Requests with netcat, you can also be a good citizen and respond to them. The client making a HTTP request will typically wait for the server to respond; although, it should eventually time out.

Run nc -kl $PORT and when you see an incoming HTTP connection, paste or type in the properly formatted HTTP response you want to provide when you see incoming HTTP connections. This is useful for debugging clients that post updates over a callback URI.

Many clients' APIs are cool with an empty response as long as it has status code 200:

HTTP/1.1 200 OK
Content-Length: 0

(note that for a proper HTTP response, you need two line feeds)

There is probably a simple way of piping netcat the response periodically, without manually pasting it in; however, On OS X, while true; do echo -e $HTTP_RESPONSE | nc -l $PORT; done would often result in an unexpected EOF error in my client from time to time.


Rename files and folders based on pattern

This loop will echo commands that would change the first substring old to new for any matching files or folders under your working directory.

for i in `find . -path ./skipme -prune -o -name "*old*"`; do
    newname=`echo $i | sed 's/old/new/g'`
    echo mv $i $newname

If you are happy with the result, remove the word echo and rerun the loop so that the mv command is executed instead of being echoed out. Note that this loop only matches the first instance of old in any filepath, so you will need to run it more than once if any of the intended files or folders have more than one substring matching old (e.g. my/old/directory/is/old.txt).

-path ./skipme -prune -o means don't check the filenames within the skipme directory. This isn't perfect, since it does not exclude the skipme directory itself from being renamed if -name's pattern matches it.


Monitor detailed java garbage collection with jstat
$ /usr/java/jdk1.7.0_10/bin/jstat -gc -t -h 20 <PID> 5s | tee ~/jstat.out

The meaning of the column headers is detailed in

Also interesting to note is that, by default, Java 7 seems to shrink the Eden space over time; at least, when it can do so. This causes an increase in the rate of the incremental garbage collection; however, this is nothing to be alarmed at as long as full garbage collection rate does not increase.

Listen to HTTP Requests with nc -kl <port>

This is not the best way of listening for HTTP requests, but netcat (nc) is often available on linux servers and can be used to listen to an HTTP request on a specific port. I found this useful for very basic verification of HTTP POSTs initiated as part of a callback.


Increase concurrency in Bash scripts with wait
$ help wait
wait: wait [n]
    Wait for the specified process and report its termination status.  If
    N is not given, all currently active child processes are waited for,
    and the return code is zero.  N may be a process ID or a job
    specification; if a job spec is given, all processes in the job's
    pipeline are waited for.
echo 5 concurrent sleep cycles later
for i in {1..5}; do 
  { # perform this block of commands in the background 
  sleep 1; echo -n '.'
  } &
echo "we are up"


Run a java (main) class using maven

exec-maven-plugin is a quick way to kick off Java scripts in a project that is already using Maven.

mvn clean compile exec:java -Dexec.mainClass=""

Usually this class is going to live in the src/test directory instead of src/main. In this case exec:java will need the classpathScope="test" as well.

mvn clean compile exec:java -Dexec.mainClass=" -Dexec.classpathScope="test"


Undo a release made by maven-release-plugin and git

This command is a heuristic so it doesn't really care about the maven-release plugin, basically it's supposed to check out the latest master branch, delete the local and remote tags, and undo 2 commits. It assumes that no one has pushed any commits in since you made a release with maven-release-plugin.

function git-undo-mvn-release { TAG_NAME=$1 && echo "  >>> undoing last two commits and removing tag named '${TAG_NAME}' <<<" && echo "  getting and checking out the latest copy of the the master branch" && git fetch && git checkout master && git pull origin master && git status && echo "  removing local and remote tag ${TAG_NAME} if both exist" && git tag -d ${TAG_NAME} && git push origin :${TAG_NAME} && echo "  git reset HEAD^^ --hard" && git reset HEAD^^ --hard && git status && git log --oneline | head && echo '  >>> If you are satisfied, run `git push origin master --force` <<<'; }

Here is a sample of how it should work:

$ git log --oneline | head -n 1
b747131 a real commit... the one you want to get back to.

$ mvn release:prepare 
# ...

$ git log --oneline | head -n 3
db09eda [maven-release-plugin] prepare for next development iteration
00024fe [maven-release-plugin] prepare release test/1.0
b747131 a real commit... the one you want to get back to.

$ git-undo-mvn-release test/1.0
  >>> undoing last two commits and removing tag named 'test/1.0' <<<
  getting and checking out the latest copy of the the master branch
Already on 'master'
 * branch            master     -> FETCH_HEAD
Already up-to-date.
On branch master
nothing to commit, working directory clean
git reset HEAD^^ --hard
HEAD is now at b747131 a real commit... the one you want to get back to.
  removing local and remote tag test/1.0 if both exist
Deleted tag 'test/1.0' (was db09eda)
 - [deleted]         test/1.0
On branch master
nothing to commit, working directory clean
  >>> If you are satisfied, run `git push origin master --force` <<<

$ git log --oneline | head -n 1
b747131 a real commit... the one you want to get back to.

$ git push origin master # --force


Enable assertions on package tree in java

Easy to overlook, but you should include ellipses (...) after a package name, to also enable assertions on it and all its subpackages.

$ java -? 2>&1 |grep -A2 -e '-ea'
                  enable assertions with specified granularity

So, use to enable assertions on and all its subpackages.


Exit from a running script using Ctrl+C
    echo "Interrupted."
    # Kill the parent process of the script.
    kill $PPID
    exit 1
trap 'int_handler' INT

Get more info on trap with:

$ help trap

Update: You can also add set -e to the top of your script to "Exit immediately if a [subsequent] command exits with a non-zero status." That way if you won't have to rely on being there to hit Ctrl+C when something goes wrong and you won't need to rely on $PPID being set in int_handler() either.


Collate multiple log files, chronologically

Set up your logging so that your log statements start with a very simple datePattern, such as yyyyMMddHHmmssSSS. Then, you should be able to see a collated view using:

$ ls *.log | xargs sort -bsnm | less -S

Even log statements spanning multiple lines will be sorted together. For example, when logging an error's stacktrace, only the first line has the necessary datePattern. The above command should keep most of those multi-line statements together and sort them by using the datePattern of the first line.


git commands I commonly use, relevant ramblings, and configuration.


Switch between different versions of maven using homebrew

You can try to do this with something like the following:

$ brew tap homebrew/versions 
$ brew search maven
$ brew install maven30
$ brew unlink maven
$ brew link maven30

You can also do this using the deprecated versions command:

Assuming you have brew's maven v3.1.1 installed, leverage the brew versions command to install maven v3.0.5.

$ brew unlink maven
$ brew versions maven
$ cd `brew --prefix`
# b4725ca happens to be the SHA for maven v3.0.5
$ git checkout b4725ca Library/Formula/maven.rb
$ brew install maven

Then, you can switch between both versions at will with brew switch.

$ brew switch maven 3.1.1
$ brew switch maven 3.0.5

Adapted from


Format OS X Clipboard contents as JSON
pbpaste | python -m json.tool


Article called 'Week Of Unix Tools'

It'll kind of teach you to list what it will teach you:

$ echo 'Day 1: sed
Day 2: cut and paste
Day 3: awk
Day 4: data source tools
Day 5: xargs' | cut -d' ' -f 3- | paste -s -d, - | sed -e 's/,/, /g'
sed, cut and paste, awk, data source tools, xargs


Maven Dependency Analysis
mvn dependency:analyze | less

It will show you unused declared dependencies, and used undeclared dependencies so you can add or remove dependencies as appropriate. Just don't go crazy with it.

Update: (mis?)Using this introduced some problems into my code because I removed dependencies whose transitive dependencies were being used. 2014/10/10 Update: The thing to pay attention to really is the used undeclared dependencies. They list the landmines where you are using a transitive dependency directly.


Running a Debugger on an Application running in Tomcat using JPDA

Start Tomcat with catalina jpda start. It will start Tomcat so that a remote debugger can be connected to port 8000.

Set up your debugger to run with the following options when the JVM is started:

-Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n

In IDEA, this done in the Debug Configurations by setting up the "Remote" configuration.


Packaging a multi-module project with maven for the first time

If you have internal dependencies (e.g. multi-module project), you should run mvn install on that dependency before running mvn package on the multi- module project. At least the first time, to get the dependencies installed into your local repository (under ~/.m2/repository/).


Fixing indendation in poms
export XMLLINT_INDENT="   "; for filepath in `find . -name pom.xml |xargs`; do xmllint --format --output $filepath $filepath; done