Neither one nor Many

August 26 2016

Some stuff I keep here as a cheetsheat for myself

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;

clipboard fu

There is a difference in clipboards I've noticed, which confuses me bigtime.

Apparently one is the "X clipboard":

echo Hello world | xclip

Whenever pasting doesn't work try the middle mouse button.. Or use xsel instead:

echo Hello world | xsel --clipboard
echo Hello world | xsel -b # same

Some params for xsel:

Selection options
    -p, --primary         Operate on the PRIMARY selection (default)
    -s, --secondary       Operate on the SECONDARY selection
    -b, --clipboard       Operate on the CLIPBOARD selection

resize images in directory to smaller size (recursively)

find . -name '*.jpg' -execdir mogrify -resize 1024x {} \;

installing trusted certificates in system

  • chromium has some way to also import authorities you might also try it
  • get certs with openssl s_client -showcerts -connect
  • I put the certs for example 3 different .crt files in /usr/local/share/ca-certificates/
  • do not use .pem or they won't be recognized.
  • you have to be root to put them there.
  • sudo update-ca-certificates should pick them up

git change author for bunch of commits

git rebase -i HEAD~2
mark commits as edit, then use this over and over:

git commit --amend --author="Ray Burgemeestre <>"
git rebase --continue


socat TCP-LISTEN:8081,fork TCP:

git stuff


find files less then a day old

find ./ -name '*.meta' -type f -mtime -1;

# then delete
find ./ -name '*.meta' -type f -mtime -1 -exec rm -f {} \;


get rid of rpm without dependencies example


# rpm -qa | grep "php-sqlite2"
# rpm -e --nodeps "php-sqlite2-5.1.6-200705230937"

determine octal representation of permissions

Sometimes it's annoying, to reverse engineering rwx -> 7, r-x -> 5 and r-x -> 5. For example, I misread the group part for -xr the first time.

shell# ls -althrst /var/lib|grep etcd
0 drwxr-xr-x  2 etcd          etcd             6 Nov  5 14:19 etcd

Thanks to google and stackoverflow (

shell# stat -c "%a %n"  /var/lib/etcd/
755 /var/lib/etcd/

Much easier!

print dates for bunch of dirs

cat /tmp/dirs | xargs -n 1 stat -c '%y %n'


sometimes you have some disconnected session that still forces some pane to be non-full size. you can force a full resize again using: ctrl+b, :detach-client -a.


Running an X application in docker I usually google for the right parameters, but sometimes they forget the X authority file. Now I use this:

xhost +
docker run --name=foobar --privileged -it -v $PWD/../:/articlemanager --workdir /articlemanager \
       -e DISPLAY \
       -v /tmp/.X11-unix:/tmp/.X11-unix \
       -v $HOME/.Xauthority:/root/.Xauthority \
       --net=host \
    foobar:latest xeyes

If you want to do it in a more correct/secure way, I found this blog post:

Also found some useful info here: (In my above example the net=host was needed too, but it's annoying port 80 is already in use for example.)

bash if statements and defaults

There are some interesting snippets here that I need to extract:

add user to group

usermod -a -G docker ray

if you are added to a group, you can do newgrp docker or login again.

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Ray Burgemeestre
february 23th, 1984

C++, Linux, Webdev

Other interests:
Music, Art, Zen