commands

To know what is inside file1.txt but not in file2.txt
awk 'NR==FNR{a[$0]=1;next}!a[$0]' file2 file1

To delete everything in a dir except file.txt
find . ! -name 'file.txt' -type f -exec rm -f {} +

Remove first n character from bunch of file names with Cut
rename -n 's/(.{5})(.*)$/$2/' *.*
The -n is for simulating; remove it to get the actual result.

To sort all lines in a file.txt
sort file.txt -o f file.txt

get md5sum of all files in a folder tree
find . -type f 2>/dev/null -exec md5sum {} \; >checksum.txt

list all files in dirs and subdirs with full path
find . -type f -exec ls {} \;

find which lines are present in 2.txt and not in 1.txt
grep -v -f 1.txt 2.txt > 3.txt

Delete all files whose size is more than 100MB
find -type f -size +100M -exec rm -rf {} \;