Well, sed is so simple, I taught myself. I find there are two flavors of script, loopers and filters. Loopers read most or all lines but the first using N, so they can see multiple lines in the buffer, and loop using branch features. Filters just do things on each line, as one mainly thinks of sed doing.

Awk has variables, arrays, hash tables, a sense of fields and records divorced from the line feed, which sed lacks, but is complex enough that one might just as well learn PERL, which is more able and orthogonal. Adding to the pifalls of awk is that there was an old awk and new awk, sometimes nawk, so examples may vary in dialect.

Most of sed is also usable in grep, vi, ex, and on the command line of ksh and bash when in 'set -o vi' mode, my favorite. I started on regex line editors with qedx in MULTICs, then used them in GCOS with University of Waterloo FRED the friendly editor, before I arrived in UNIX/vi land.

Regex are extended in several flavors, so what works in sed is more than basic grep but less that egrep/"grep -E" and that is different from awk.

You can make executable pure sed or awk files if the first line is "#!/bin/sed -f". I usually write sed right on the command line, barefoot inside ', like this extra blank (all white space) line remover, a classic looper (\t is a real tab, [f a real form feed):

Code:

sed '
  s/[ \t\f]*$//
  :loop
  $b
  N
  s/[ \t\f]*$//
  s/^\n$//
  t loop
  P
  s/.*\n//
  t loop
 '

The '$b' ensures that N is not run at EOF, losing a line on some early, defective sed implementations. Early proprietary sed was fixed buffer but faster than GNU and later sed's with realloc()-able indirect buffers. I often kept both around for script compatibility, renaming GNU sed 'gsed'. In one app, I turned whole pages into lines using 'tr' to swap line feeds and form feeds, so I could mark up the pages in sed to insert them into the database as big strings. I am not shy about running multiple sed's in a row, as looper sed and filter sed are not friends, and get some multiprocessing in the deal:

Code:

sed '
  s/[ \t\f]*$//
 ' | sed '
  :loop
  $b
  N
  s/^\n$//
  t loop
  P
  s/.*\n//
  t loop
 '