Scripting with ed and here documents
------------------------------------
Last edited: $Date: 2018/08/20 18:27:50 $
Ed is the standard editor and although it is very old it is still a
very useful and powerful editor.
Ed is made to be used interactively, but we can also use it in
scripts. The use of here documents is one way to do this.
A simple example:
ed -s myfile << EOF
a
A new line
.
w
EOF
## Remove from the first line to line with some pattern
A handy way of removing the lines from the start of a text file to
some arbitrary line containing a certain pattern, is to use line
labels. In Ed, we can label a line with the `k' command, like this:
kx
This gives the line the label ``x'', which we address with 'x
(apostrophe, immediately followed by the label name).
So, if we want to remove the preamble of a Latex document this would
do the trick:
ed -s mylatexfile << EOF
H
/\\\begin{document}/
kx
1,'xd
w
EOF
The first line toggles the explanation flag (`H').
The second line searches for \begin{document}. We have to escape the
backslash with extra backslashes, because of the shell we run this
script from.
The line `kx' is used to label the line with the \begin{document}
pattern.
The line `1,'xd' deletes all the lines from the first to the line
with the \begin{document} pattern.
The last line `w' is to write the buffer back to disk.
## Replace from the first line to line with some pattern with other
text
If we want to replace the deleted preamble with a different one, we
can no give the command `1i', meaning "go to line 1 and insert
before that":
ed -s mylatexfile << EOF
H
/\\\begin{document}/
kx
1,'xd
1i
\documentclass [english,useplainpaper]{article}
....
....
last insert line
.
w
EOF
The text to insert can be arbitrary long, it does not have to be the
same number of lines as the number of deleted lines.
Don't forget to end the insert of lines with a line containing only
a single dot, to end the insert-mode.
## Direct addressing the line with the search pattern
A shorter way to remove from the first line to a line with some
pattern is to direct it directly, like this:
ed -s mylatexfile << EOF
H
1,/\\\begin{document}/d
w
EOF
Sometimes, using the first method, with adding a label to the
specific line, it is easier to understand.
## Learn Ed and prosper
Of course, this are some silly examples, Ed can do so much more :)
If you have not done yet, learn yourself some basic Ed and practice
until you get comfortable with it, and than start to write some
scripts with here-documents. The time you invest on this is well
worth it, you'll acquire a skill that can be extremely useful in the
future.
