Linux Sysadmin Grymoire

Random tidbits that I need to reference every now and then

bash

rewrite previous command

helps you edit the current command in an editor
- ctrl-x ctrl-e

windows vs dos line endings

dos2unix is your command
cat -v ${file} | grep ^M
- this command will find any of the CRLF line endings in your file

git

git branch -a
git reset ...
git log --oneline --graph --decorate --all

diff goodness

git diff --cached
- shows the difference between index and current commit
git diff
- shows the difference between index and working directory
git diff HEAD
- shows the difference between current commit and working directory

regular workflow

git pull
git rebase --onto origin/master

networking

iptables

iptables-restore {iptables_file} restore iptables from this file
iptables -L lists iptable rules
iptables -F drops all the iptables

process management

htop

ps

tar

# Serialize to tar format
tar -cvf <archive>.tar <directory>
# Serialize to tar and then compress with gzip
tar -zcvf <archive>.tar.gz> <directory>

vmstat

vmstat 1 # prints report every second
vmstat -d 1 # prints report on disk stats, so new columns

Most useful columns imo are:

Procs:r/b
- r: runqueue, either running or waiting to run
- b: this is number of processes in uninterruptible sleep
IO:bi/bo
- this is the number of memory swapped in and out /s
Memory:free
- amount of idle memory

memory management

ncdu

jq

example.json

{
	"meow" : "nyan",
	"woof" : [
	{ "color" : "red" },
	{ "color" : "blue" },
	{ "color" : "green" }
	       ]
}

basic jq usage

Print the whole struct

cat example.json | jq '.'
jq '.' example.json

Getting a key

jq '.meow'
jw '.[meow]'

listing all keys of an object

jq '. | keys' example.json

length of an object

jq '.woof | length'

getting a key from every element in an array

jq '.woof[] | .color'

Explanation of Piping and Iteration

From my understanding, piping is used to separate function calls and field accesses. Looping and iteration are also usually using pipes. ‘.’ is the input value at each stage of a pipeline, so as an example:

jq '.woof[].color'
jq '.woof | .[] | .color'
jq '.woof | .color' # does not work because the .color filter is trying to access the color field of an array, which doesn't make sense

getting multiple keys

jq '.meow,.woof'

Outputting csv and maps

csv takes arrays maps will apply the function to all the elements in an array if a single array is passed, or all the elements if multiple elements are passed. Feels suspiciously like the Python list function

jq -r '.woof | map(.color) | @csv'

Getting the key and value

jq '. | to_entries[] | ".key,.value"'

Anything more complicated? PLEASE USE A REAL PROGRAMMING LANGUAGE

printf

it is very similar to printf c strings

printf “%s” “hello world”

find

# find and delete by inum
find . -type f -inum ${INUM} -delete

# find and delete by inum by another way
find . -type f -inum ${INUM} -exec rm {} \;

# find by case insensitive name
find . -iname '*.jpg'

# list everything
find .

xargs

input.txt

123
456
789

# equivalent of running echo 123 456 789
$ cat input.txt | xargs echo
123 456 789

# equivalent of running echo 3 times for each line
$ cat input.txt | xargs -I {} echo
123
456
789

# running xargs in parallel
$ cat input.txt | xargs -P 10 -I {} echo
123
456
789