Removing consecutive newlines with ed

Question

Input file:

1 line1\
2 line2\
3 line3\
4 \
5 line4\
6 \
7 \
8 line5\

Desired output:

1 line1
2 line2
3 line3
4
5 line4
6
7 line5

Is it possible with POSIX ed?

Removing the trailing backslashes is easy, but how can I also handle consecutive blackslashes as expected output?

Answer 1

Do not know how to do this in one go. But if the contents is well known one could add a unique string to double lines.

# Add XX to lines that should be deleted
g/^$/+1s/^$/XX/
# Delete them
g/XX/d

Not very nice tho.

Answer 2

With sed, you could do:

sed 's/\\$//;/./,/^$/!d'

One of the main problems with ed (beside its syntax from another age) is error handling in script.

Basically, you have to think of it as ed returns success if it successfully edited the file. But to be a successful edit, you have to have each commands address to be valid, each command to be successful (for instance, for the s command, that means at least one substitution done).

So, if you want to check that the edit was successful in this case, it can be tricky.

To build up on @Sukminder's solution and attempt to cover the corner cases, one could write:

printf '%s\n' a a . 'g/\\$/s///' ',s/^/a/' 'g/^a$/+1,/./s/^a$//' \
  'v/./d' 'g/^a/s///' '$d' w | ed -s the-file

The g command would fail if the file was empty (because the default address range (1,$) would be invalid, so we need first to add a line at the end of the file (with the a command). ,s/\\$// would fail if there was no line ending in \ so we need to run it only if there are such lines (g/\\$/s///).

Here, instead of inserting a "xx" assuming it's not to be found in the input, we prepend "a" to each line and replace the lines to be removed with an empty line which we remove afterwards.

Hopefully, that ed script will only return a non-zero exit status if it fails to write the file (w command failing). Unfortunately, I've seen ed implementations returning success when failing to write a file when the FS was full.

IMO, ed is best avoided. Nowadays, using perl -pi.back is as portable and more reliable.