Unix tool tip

'The enjoyment of one's tools is an essential ingredient of successful work.' -- Donald E. Knuth

List lines that do NOT match a regular expression: grep -v

Linux pros and cons

Display a file with line numbers: cat -n or nl.

Count how many lines of a text file begin with each letter. cut -c1 file.txt | sort | uniq -c | sort -rn

Making a search 20,000x faster by tolerating a chance of error johndcook.com/blog/2026/07/1…

Sed FAQ pement.org/sed/sedfaq.html

Editing by semantic units johndcook.com/blog/2012/07/2…

Remove duplicate lines: awk '!a[$0]++' from catonmat.net/blog/awk-book/

Gnu Emacs reference card damtp.cam.ac.uk/user/sje30/ess… [pdf]

The sed command r inserts a file. For example, insert foo.h after 'INCLUDE': sed '/INCLUDE/ r foo.h'

Show line endings in a file: cat -E

The option -i or --ignore-case tells diff to do a case-insensitive comparison.

List files containing a match for a regular expression: grep -l

The substitute command in sed can take a 'w' option to specify an output file. e.g. sed s/foo/bar/w myfile.

Negative space in operating systems johndcook.com/blog/2008/12/2…

Awk script to count the number of words in a file: awk '{ total = total + NF }; END { print total+0 }'

wc -L shows the length of the longest line in a file.

wc returns the number of lines, words, and bytes in a file. Can use -l, -w, -c to get just one of these.

@AlgebraFact Blog post on how silver rectangles relate to counting the number of paths a king can take diagonally across a chessboard. johndcook.com/blog/2026/06/3…

@AlgebraFact If you stick two squares on the longer side of a silver rectangle, you get another silver rectangle.

If you stick a square on the longer side of a golden reactangle, you get another golden rectangle.