Shell Programming and Scripting

I have been looking at the file structure:
1) All lines start with the " ", single space, character as an absolute minimum, are these indents and need to be preserved?
2) If so then line 1 is shifted 2 out of 3 " "'s, 2 spaces, leftwards on the output which implies "NO" to number 1)
3) If indentation IS required then you need to remove <!-- <- note the TRAILING space.
4) Similarly the other end --> <- again note the LEADING space.
5) And leave the 2 lost spaces intact in the first output line.
Just an observation that caught my attention...

Your explanation is awesome,...I understand now, what makes me confuse before was, I remember to do a backlash to any special character, But in the command below, theres none.

Thanks

Code:

 sed -i '/<!--.*<connections_per_host>1</{s/<!--//; s/-->//; s/>.*</>5</};' a

---------- Post updated at 08:52 AM ---------- Previous update was at 08:51 AM ----------

I like your explanation,. i understand the syantax because of you. But pelase educate me, when to use a backslash to negate a special character?
Thanks

The rule is: when you use a character that has a special meaning (to sed) and you want it to mean just the character itself you need to "escape" it - that is, prepend it with a backslash.

An example:

Code:

sed 's/abc./xyz/' /input/file

The dot (".") here doesn't mean a dot, but is a special character, meaning "any one character". If you want it to mean a real, literal dot and nothing else, you need to escape it:

Code:

sed 's/abc\./xyz/' /input/file

Notice that this escaping is implicit in some situations, for instance in "character-classes". Look at the following regular expression:

Code:

sed 's/a[bcd]e/xyz/' /input/file

This searches for an "a", followed by either an "b", a "c" or a "d", followed by an "e". It would match any of these strings:

abe
ace
ade

Inside the brackets all characters lose their special meaning. I.e. a[bc.]d would search for any of these strings:

abd
acd
a.d

If you would try to escape a character here the escaping backslash would be treated as a normal character too. The regexp a[bc\.]d would find these strings:

abd
acd
a\d
a.d

Notice that the characters you called special - "<", ">", etc. - don't have any special meaning in sed anyway and would have been safe to use without any escaping in first place.

I hope this helps.

bakunin

backslashes don't negate characters, they "escape" them, i.e. remove their special meaning, force them back to normal chars. Negate, BTW, is a difficult term with characters. You might mean "complement"?