SYS ADMIN STUFF

I suppose you could say that putting commands up here 
reveals which kind of system you are running
but if you can't figure that out from other means you probably can't
get in anyway.

Also these are things that any sysadmin might need on various systems.
Hopefully they will be helpful to those who need them.


Zombie Processes:

look for zombie processes
ps -xua | grep -w Z
ps -wuax| grep defunct |grep -v grep

kill zombie processes (might need reboot)
Look in the third column after ps -xua find the parent process
and restart it.


To see what your firewall has been doing with ipfw (freebsd):
ipfw -cat list
ipfw list


I don't know why anyone would want their server on some timezone
other than UTC, but if you do the command is:
tzsetup for freebsd


Sed is a very useful tool. Probably the most used and easiest command is as follows:
to replace text in a file globally

sed -i -e 's/searchfor/replacewith/g' filename

sed on BSD does not like to escape a new line replacement
This works:
sed -i .bak '1s/^/newfirstline'$'\n''/g'
under sh the the $\n is replaced before sed executes
sed in BSD does not escape control characters
sed in BSD also requires the extension after the -i

dd is great for making images and backups
block sizes of greater than 32k don't seem to make too much of a speed 
difference.

dd -if=/dev/sdx -of=[backupfilename] -bs=64K status=progress

floating point calculations in shell
sh and bash do not do floating point calculations.
so you have to send your calculations to bc (basic calc)

() [] (()) in shell script
[] is a boolean expansion
((xxxxx)) is a mathematical expansion.
$() dereferences the variable and returns the value

SSH
ssh-keygen -t ed25519 on your local machine
ssh-copy-id login@address on your local machine.

or scp the public key to the server into the .ssh directory.
then cat the public key into the authorized keys file.
cat ed25519.pub >> authorized_keys


STRIP HTML from a file
This is a very useful little sed script:
However, if you have multiline html it misses it.  each
tag must exist on a single line (it is sed afterall)
sed 's/<[^>]*>//g ; /^$/d' htmlpage.html > htmlpage.txt


BATCH rename files
ls | grep \.JPG$ | sed 'p;s/\.JPG/\.jpg/' | xargs -n2 mv

How to convert HEIC files to jpg
apt install libheif-examples
find . -iname "*.heic" -exec heif-convert -q 100 {} {}.jpg \;
for f in *.HEIC; do heif-convert -q 100 $f $f.jpg; done

Bash and Echo
when putting line feeds in a variable and sending them to echo:
1 you can't just put \n in the string "hello \n there"
   it must be "hello"$'\n'"there"
2 using a variable, i.e. myvar="hello"$'\n'"there";echo $myvar
   but ooops that does not work it removes the line feed when
   bash dereferences the variable before passing it to echo.
   so you must use double quotes ie echo "$myvar" to keep the line feed