Neither one nor Many

May 5 2018

Some random snippets

Potentiometer example / Analog reading/writing / mapping

const int analogInPin = A0; // Analog input pin that the potentiometer is attached to const int analogOutPin = 9; // Analog output pin that the LED is attached to int sensorValue = 0; // value read from the pot int outputValue = 0; // value output to the PWM (analog out) sensorValue = analogRead(analogInPin); outputValue = map(sensorValue, 0, 1023, 0, 255); analogWrite(analogOutPin, outputValue);

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December 28 2017

Some electronics notes


L7805CV - 5V - Voltage regulator

  • Pins from left to right: {input, ground, output}
  • The top part is also ground.
  • Can attach heat sink if needed
  • Quiescent current is not so nice: 6 mA
  • "Not really suitable for usage in batteries, consider instead: Low drop out regulator"
  • Info on the part:

Ultra-Low Quiescent Current LDO Regulator

IRFZ44N Mosfet

Not a "logic-level" mosfet, so I bought the wrong thing,


"If you want to drive this from an Arduino, which only outputs 5V, you will need a "logic-level" MOSFET. For this type of MOSFET, Vgs=5V is enough to turn it on. For example, the ST STP55NF06L would be appropriate and is comparable to the IRFZ44N, except the STP55NF06L is logic-level.

You should also have a resistor in series with the Arduino output to limit the current, since the MOSFET gate is highly capacitive and can draw a big instantaneous current when you try to turn it on (or off). 220 ohms or so is appropriate."

Logic-level alternatives: * FQP30N06L * STP55NF06

  • Pins from left to right: {gate, drain, source}
  • Enabling "gate" will enable electricity to flow from source to drain.

Difference between N or P-channel mosfet:

MOSFETs come in two polarities, P channel and N channel, where “P” stands for positive and “N” stands for negative.

P Channel To turn a P channel MOSFET on, you apply a negative voltage to the gate. This voltage is negative relative to ground. In a circuit, you connect the P channel MOSFET’s source terminal to a positive voltage supply and the drain to a resistor connected to ground; the resistor limits the current flowing through the transistor. The circuit diagram symbol for a P channel MOSFET has an arrow pointing away from the gate.

N Channel An N channel MOSFET turns on when you apply a positive voltage at its gate terminal. The voltage is greater than the positive voltage supply at the drain terminal. A resistor between the positive supply and the drain limits current; for an N channel MOSFET, the source terminal connects to ground. The circuit symbol for an N channel MOSFET has an arrow pointing toward the device’s gate.

DHT22 - Temperature and Humidity sensor.

  • Pins from left to right: { VCC DATA NC GND } (NC = No connection )
  • Info on the part:

PIR 5Pcs/Lot SR501 HC-SR501 Adjust IR Pyroelectric Infrared PIR module Motion Sensor Detector Module for Arduino

(jpeg-image pir)

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September 22 2017

Launch debug shell in some arbitrary piece of code

import code

Or using IPython:

import IPython


name params with ?P<X>,'

In [18]: output = "x:3372 y:1280 screen:0 window:29360137"
In [19]: reg = re.compile(r'x:(?P<X>\d*) y:(?P<Y>\d*)')
In [20]: reg.match(output).group('X')
Out[20]: '3372'

match non-greedy by adding ? suffix

read json from file


import json
from pprint import pprint

with open('data.json') as f:
    data = json.load(f)


Edit for Python3:

with open('data.json', encoding='utf-8') as data_file:
    data = json.loads(

yapf formatter

There is a plugin for pycharm. Installed it, and also pip install yapf.

Then mv /usr/local/bin/yapf /usr/local/bin/yapf.x and created this bash script to tweak some values:

/usr/local/bin/yapf.x --style='{based_on_style: google, column_limit: 100}' "$@" 

file writing

file = open("testfile.txt","w") 
file.write("Hello World") 


with exec_helpers:

with exec_helpers.Subprocess() as executor:
    apiserver = executor.check_call('module load kubernetes && kubectl config view -o ' \
    if apiserver:
        return apiserver.stdout_str
raise AbortException("Could not retrieve the Kubernetes API server address")


subprocess.check_output(["echo", "Hello World!"])
subprocess.check_output("exit 1", shell=True)
subprocess.check_output("ls non_existent_file; exit 0", stderr=subprocess.STDOUT, shell=True)
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August 14 2017

Place to keep commands I tend to google more than once for some reason.


Start cmd.exe as Administrator

powercfg.exe /hibernate off|on

Some file in C:\ hiberfil.sys (IIRC) will consume quite a bit of disk space which may be a reason for not using the hibernate feature.

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December 19 2016

Assuming it's already flashed etc., (I forgot the steps because it's too long ago!)

What I do is connect the Edison via both micro USB's, one will be used as power, the other can be used for connecting to it.

In case the edison is booted you can open the terminal with screen:

sudo screen /dev/ttyUSB0 1152000 115200

Configure the WiFi via configure_edison --wifi

One caveat if you are also on the same network (i.e., your laptop is also on the same network) you may get a conflict via the USB wire. You can see with ifconfig two IP addresses one for the wlan0 and one for usb0, which is probably causing a conflict somehow. Solution is:

ifconfig usb0 down
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August 26 2016

Some stuff I keep here as a cheetsheat for myself

TODO process:

mass replace recursively

NOTE: do not run in case you have .git directory, it can corrupt it.


mv .git /tmp/git_tmp
find . -type f -print0 | xargs -0 sed -i 's/subdomainA\.example\.com/'
mv /tmp/git_tmp .git 

return value

$? return last exit code

buffer control with stdbuf

time make -j24 "$@"  2>&1 | stdbuf -i0 -o0 -e0 tee ./mm.log

sets buffer to zero, everything is piped immediately, bad for throughput, good for latency

< <(command)

gdb on program that works on stream, with mkfifo to make this a combined example (you can issue the bash command instead of the tail -f in this case)

mkfifo /tmp/foo
bash > /tmp/foo
gdb <application>
run <program args> < <(tail -f /tmp/foo)

This syntax is also useful for stuff like:

while read -d ':' line; do
    echo "$line"
done < <(hbase classpath)

A pipe | creates a new subshell, this is also a nice way to avoid this (subshells cannot modify variables of their parent)!

EDIT: This is called process substitution, see

echoing multiple lines to a file

cat <<EOT >> greetings.txt
line 1
line 2


if [[ $foo =~ foo ]]; then

Note that you DO NOT use asterisks like *foo*!

processing stdin

A pattern I use quite a bit is the following.

function func
    while read line; do
        echo $line
    done < /dev/stdin
cat foo.txt | func

subshells / pids

Sometimes subshells are useful.

echo mypid = $!

( sleep 10; ) &
typeset pid=$!
sleep 1
kill $pid # kill if still running..

Note that you can use exit in a subshell without destroying the parent.

trap CTRL+C

function myfunc
    echo Do some cleanup here..
    exit 0
trap "myfunc" 2
print "Press CTRL+C to abort 10 seconds of waiting.."
sleep 10


First learned about this in kornshell, but seems to work for bash as well.

function func {
    local -n test_ref=$1
    test_ref="Hello world"

func test # note omitted $-sign

echo $test

If you want kornshell compatibility use typeset -n instead of local -n.

The same example, but with an array, you have to first explicitly declare the array before passing it to func.

declare -A array
function func {
    local -n array_ref=$1
    array_ref=([one]=aaa [two]=bbb [three]=ccc)
func array

(If you want kornshell compatibility again, use typeset -A instead of declare -A (and typeset -n for local -n)).


Continuing the previous paragraph, some useful examples regarding arrays.

echo ${array[@]}  # aaa bbb ccc
echo ${!array[@]} # one two three
echo ${#array[*]} # 3

for key in ${!array[@]}; do
    local value=${array[$key]}
    echo $key = $value # one = aaa, ... 

Instead of initializing with the array=([foo]=bar [baz]=ban) syntax, assignment syntax is array[foo]=bar.

bash substrings

typeset hostname=$(hostname -s)
hostname=${hostname:0:10}  # first 10 chars..

NFS caching

Trick to use cachefilesd on NFS mounts. Useful if you work remote on a mounted NFS share. btrfs is not supported, so I create an ext4 mount instead like this:

dd if=/dev/zero of=/tmp/fscache.txt bs=1M count=4000
# be careful with choosing /dev/loopN
losetup /dev/loop10 /tmp/fscache.txt
mkfs.ext4 /dev/loop10
mount /dev/loop10 /var/cache/fscache

Very useful other commands can be found here: (i.e.: cat /proc/fs/nfsfs/*)

Benchmark I/O

start iperf -s server, then iperf -c

Show Network I/O per process

atop can do this, which comes provided by your package manager probably, but chances are you need to compile it yourself for a newer version. The new version makes it possible to compile a kernel module, which (after reboot) will make network counters available in atop. Quite awesome!

simulate slow network

export interface=enx9cebe8349fdc
sudo tc qdisc add dev $interface root netem delay 10000ms
sudo tc qdisc del dev $interface root netem 


Prints files count per directory for current directory level:

du -a | cut -d/ -f2 | sort | uniq -c | sort -nr

UDP traffic through SSH tunnel

Using iptables to redirect ip address

iptables -t nat -A OUTPUT -d -j DNAT --to-destination

sslh i.e.,

WHAT IS IT? sslh accepts connections on specified ports, and forwards them further based on tests performed on the first data packet sent by the remote client.


trigen@zenbook:~> echo "Hello world (y)" | sed 's/(y)/:-)/'
Hello world :-)
trigen@zenbook:~> echo "Hello world (y)" | sed 's/.*\((y)\)/smiley: \1/'
smiley: (y)

trigen@zenbook:~> sed -i.bak 's/H/J/g' test.txt
trigen@zenbook:~> cat test.txt
Jello world
trigen@zenbook:~> cat test.txt.bak
Hello world

check if port is open

nc -z -v -w1 80

openssl check certificates

openssl x509 -in <INPUT> -text

openssl s_client -showcerts -connect </dev/null
# in case of SNI (combined domains in certificate)
openssl s_client -showcerts -servername -connect </dev/null

set boot target graphical or not


systemctl enable multi-user
systemctl set-default multi-user
systemctl enable graphical
systemctl set-default graphical

commandline json prettify

echo '{"foo": "lorem", "bar": "ipsum"}' | python -m json.tool

sed extract piece of text

openstack server list | grep ...  | sed -n 's/.*\(10\.2[^;]*\);.*/\1/p'

kpartx dd img

kpartx -rav some.dd

readonly add partition devmappings verbose

mount /dev/mapper/loop0p5 /mnt/loop0p5/

umount /mnt/loop0p5

kpartx -d some.dd or kpartx -d /dev/loop0

see losetup -a for available loops

note losetup -D doesn't work for some reason when the partitions were created with kpartx



"after you reattach a ctrl-a F runs the "fit" command to resize the current window. if you reattach using the -A option it should resize all windows when you reattach. "


Assuming the hash of the commit you want is c5f567 (source):

git checkout c5f567 -- file1/to/restore file2/to/restore

The git checkout man page gives more information. If you want to revert to the commit before c5f567, append ~1 (works with any number):

git checkout c5f567~1 -- file1/to/restore file2/to/restore

turn .a (thin) archive into "normal" one. (Source)

After some additional research, ar -t can be used to enumerate the object files in an archive, so after that it's just a matter of providing that list to ar as you usually would when creating an archive.

The following script handled this for all of the libraries at once:

for lib in `find -name '*.a'`;
    do ar -t $lib | xargs ar rvs $ && mv -v $ $lib;
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July 16 2016

C++ streams


 std::ifstream fi;"all.csv");
 std::string line;
 while (getline(fi, line, '\n')) {
     std::cout << "line = " << line << std::endl;


#include <fstream>
std::ofstream log_file(
    "log_file.txt", std::ios_base::out | std::ios_base::app );
log_file << text << std::endl;


Initialize N elements at once with: vector<int> vec_out(vec_in.size(), 0);.

Put last elem in front data[0] = data[data.size() - 1]; and erase last data.erase(data.begin() + (data.size() - 1), data.end());


Sum accumulate(count.begin(), count.end(), 0).

Rotate 1 2 3 -> 2 3 1 with rotate(A.begin(), A.begin() + K, A.end());, the other way around with rbegin() and rend() respectively.

Find std::find(std::begin(c), std::end(c), 1001) != std::end(c);

Erase + remove idiom: c.erase(std::remove(std::begin(c), std::end(c), 1001), std::end(c));


std::vector<Foo> foos;
for (int i = 0; i<10; i++) {

std::vector<int> xs;
std::transform(foos.begin(), foos.end(), xs.begin(), [](Foo f){return f.x;});


with ranges:

#include <iostream>
#include <range/v3/view.hpp>

struct Object {
    int x;
    float y;

int main() {
    std::vector<Object> objs = {{1, 4.2f}, {42, 5.1f}, {69, 6.9f}};
    std::vector<int> xs = objs | ranges::view::transform(&Object::x);
    for (auto x : xs) {
        std::cout << " " << x;
    std::cout << std::endl;

    for (auto y : objs | ranges::view::transform(&Object::y)) { // No new vector created.
        std::cout << " " << y;
    std::cout << std::endl;

join string boost so

#include <boost/algorithm/string/join.hpp>
#include <vector>
#include <iostream>

int main(int, char **)
    std::vector<std::string> list;

    std::string joined = boost::algorithm::join(list, ", ");
    std::cout << joined << std::endl;

join string non-boost

std::string s = std::accumulate(std::next(v.begin()), v.end(),
                                std::to_string(v[0]), // start with first element
                                [](std::string a, int b) {
                                    return a + '-' + std::to_string(b);



template <typename... Params>
void remove(Params &&... node) {
  static_assert(std::is_constructible<Type, Params...>::value, "parameters are invalid for constructing a Type");
  auto n = std::make_unique<value_type >(std::forward<Params>(node)...);
  remove(static_cast<const value_type &>(*n.get()));


#include <sstream>
stringstream ss;
ss << "hello world" << 123 << endl;
ss.str(); // ...
ss.str("");  // re-use.

binary search

std::sort (v.begin(), v.end());                       // 10 10 10 20 20 20 30 30
auto low = std::lower_bound (v.begin(), v.end(), 20); //          ^        ^
auto up  = std::upper_bound (v.begin(), v.end(), 20); //                   ^


// "if the number is less.. goes down the pile"
priority_queue<int, vector<int>, std::less<int>> pq; 
for (int i : {1, 6, 3}) 
    pq.emplace(i);; // 6
pq.pop();; // 3

Exposes it's container as protected member variable. In order to access, you have to extend priority_queue. Underlying data structure is a heap.


bitset<8> bs; 
bs.set(2); // 00000101


Underlying container type is a binary tree. So most stuff is log n. Note that a binary tree is also sorted, so the first element *the_set.cbegin() is the min() element!


unique_ptr<foo> p{nullptr};
p = make_unique<foo>();

Perfect forwarding variable template args

namespace std {
  template<typename T, typename ...Args>
  std::unique_ptr<T> make_unique(Args &&...args) {
    return std::unique_ptr<T>(new T(std::forward<Args>(args)...));




#include <iterator>
copy(tokens.begin(), tokens.end(), ostream_iterator<string>(cout, "\n"));

readable input parameters

void split(string in, vector<string> *tokens_ptr, string delim = " ") {
    vector<string> &tokens = *tokens_ptr;

split(input, &tokens, " ");


// find
string s("aa b c");
s.find(" "); // finds 2
s.find(" ", 3); // finds 4 (starting search from the "b")
// substr()
s.substr(0, 2); // start + length!
s.substr(pos, pos - previous); // typical

tokenizer with getline()

really nice implementation using getline() from this stackoverflow answer.

#include <string>
#include <sstream>
#include <vector>

using namespace std;

void split(const string &s, char delim, vector<string> &elems) {
    stringstream ss(s);
    string item;
    while (getline(ss, item, delim)) {

vector<string> split(const string &s, char delim) {
    vector<string> elems;
    split(s, delim, elems);
    return elems;

iterate to stdout with ostream_iterator<T> and copy()

#include <iterator>
#include <algorithm>

// output even a newline after each element
copy(c.begin(), c.end(), ostream_iterator<string>(cout, "\n"));

convert binary file into c code

xxd -i favicon.ico > favicon.ico.h

sorting concatinating

template <typename T>
std::string join_keys(std::vector<T> &idsVec) {
    std::stringstream ids;

    // remove dupes
    std::sort(idsVec.begin(), idsVec.end());
    idsVec.erase(std::unique(idsVec.begin(), idsVec.end() ), idsVec.end());

    // join ids with ","
    std::copy(idsVec.begin(), idsVec.end(), std::ostream_iterator<T>(ids, ","));
    std::string ids_str = ids.str();

    // remove the trailing ", "
    ids_str.erase(ids_str.empty() ? 0 : ids_str.length() - 1);
    return ids_str;


c++17 will support it, in g++ version 7.0 it can be enabled with --std=c++1z

#include <optional>

std::optional<std::string> empty;
std::optional<std::string> filled{"hello"};

if (filled) {
    std::cout << "filled = " << *filled << std::endl;
std::cout << "empty = " << empty.value_or("nothing") << std::endl;

c++14 flag --std=c++1y in g++ 4.9 supports std::experimental::optional

#include <experimental/optional>

std::experimental::optional<int> empty;

Note there is also make_optional.

file reading

using namespace std;
ifstream ifile("webroot/"s + folder + "/" + file, ios::binary);
string s( (istreambuf_iterator<char>(ifile)),
          (istreambuf_iterator<char>()) );
cout << "read = " << s.size() << " & " << s.length() << endl;

multiline string literal

const char * long_test = R"( ... )";
const char * long_test = R"XXX( ... )XXX";

std::experimental::optional and std::reference_wrapper

Passing optional reference:

#include <iostream>
#include <experimental/optional>
#include <functional>

namespace std {
  using std::experimental::optional;

void foo(std::optional<std::reference_wrapper<bool>> flag = {}) {
    if (flag) {
        flag->get() = true;

int main() {
    bool flag = false;
    std::cout << "flag is now: " << std::boolalpha << flag << std::endl;

chrono stuff

/* sleeping */

/* timing */
auto last_received = std::chrono::high_resolution_clock::now();
auto current_time = std::chrono::high_resolution_clock::now();
std::chrono::duration<double, std::milli> idle = current_time - last_received;
if (idle.count() > 500.) {

    /* literals */
    using namespace std::literals;




#include <algorithm> // copy
#include <iterator> // back_inserter
std::copy(from_vector.begin(), from_vector.end(), std::back_inserter(to_vector));

boost reverse iter

void OpenStackStrategy::apply(CustomizationFileEntryPairList &customizations)
  size_t index = customizations.size();
  for (const auto &customization : boost::adaptors::reverse(customizations)) {
    const auto &file = customization.first;
    const auto &entry  = customization.second;

cout fu

std::cout << std::fixed << some_double_variable << std::endl;
std::cout << std::boolalpha << some_boolean_variable << std::endl;

vector or set difference

std::sort(ignore_domains.begin(), ignore_domains.end());
std::sort(search_domains.begin(), search_domains.end());

std::vector<std::string> result;
        search_domains.begin(), search_domains.end(),
        ignore_domains.begin(), ignore_domains.end(),
        std::back_inserter( result )


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Ray Burgemeestre
february 23th, 1984

C++, Linux, Webdev

Other interests:
Music, Art, Zen