Filter(s) Use
cat "Catenate" a file.
base64 Encode/decode "ASCII armor"
diff Take the difference in two files
grep/egrep/fgrep         Apply regular expressions to a bytesream
fmt/pr/nl Apply formatting to a file.
head/tail Show the initial/final lines of a bytestream.
less/more/pg Display one page at a time.
cut/paste Excerpt and join input.
sort Sort
tr Transform a bytestream.
uniq Find/Remove/Count unique lines in a bytestream.
spell Find problematic spellings (ispell is great for interactively fixing spelling problems.)
xargs Build and execute command lines from standard input
sed The stream editor; arbitrarily modify lines in a bytestream.