sed

sed -i '' '/null/d' filename.txt — removes all strings containing null from a file filename.txt

wc

wc -l filename.txt — prints the number of lines in the file

searching for text in files (in current directory)

ack "text-to-find" .

grep -r "text-to-find" .

counting the number of matching lines

ack -c "text-to-find" .

grep -cr "text-to-find" .

NB: ack is generally much faster than grep

Renaming many files

Easy with rename: brew install rename

Rename files like output_01_01_2017.txt to output_2017_01_01.txt:

rename 's/output_(\d\d)_(\d\d)_(\d\d\d\d)\.txt/output_$3_$2_$1\.txt/' *

Simple HTTP server

python -m SimpleHTTPServer [port]

Starts a webserver on port port (default is 8000).