command line / bash variable in awk command

 This is how we use a command line / bash variable (named num) in a awk command. We define an awk  variable 'var' with -v option. This also shows use of BEGIN block.

awk -F, -v var=$num 'BEGIN{srand(var)} {print $1","$2","rand()}' file_name 

Change nameserver entries in resolve.conf

 We can directly change the contents of the file but they dont seem to be in effect. So I added Google's nameserver in the file as follows : 

$ sudo cat /etc/resolvconf/resolv.conf.d/base 

nameserver 8.8.8.8

nameserver 8.8.4.4

And then updated the file as follows : 

$ sudo resolvconf -u 

we can also just update /etc/resolve.conf and it will be in effect immediately.

$ sudo sed -i '1 i\nameserver 8.8.8.8\nnameserver 8.8.4.4' /etc/resolv.conf 

To remove the changes :

$ sudo sed -i '1,2d' /etc/resolv.conf 

renaming all the files in a folder

 IF we want to rename all the files with a part of the file we can do something following:

for file in $(ls); do nn=$(echo $file | awk -F'_' '{print($2)}'); mv $file $nn; done

shell loop through dates

 Easy command to loop through the dates : 

for i in 2022-01-{01..31} 2022-02-{01..15}; do echo $i; done

More comprehensive command :

d=2015-01-01
while [ "$d" != 2015-02-20 ]; do 
  echo $d
  d=$(date -I -d "$d + 1 day")
done