littlebat wrote:
> Hi,
> I am learning LFS BOOK:
> http://www.linuxfromscratch.org/lfs/view/6.6/chapter05/adjusting.html
>
> Below is a sed syntax I can't understand and haven't found a place to
> learn it.
> <code>
> sed -e "/^\*cpp:$/{n;s,$, -isystem /tools/include,}"
> </code>
You already got a good answer, perhaps a little more detail helps...
I'm no sed expert, but this is the way I read that command.
sed the command
-e means "execute this little program which follows"
" the quotes are necessary to keep the shell from
trying to do stuff with what's here, and to make
what follows "all one argument" to the program
/ sed looks at the first character, and takes that
to be the "delimeter". So, everything from here to
the next "/" is the "address" sed will use to select
lines from the file; the program gets executed on lines
which match this pattern, all other lines pass through
unchanged
^ this indicates that the pattern must start at
the beginning of the line
\* we have to "escape" the "*", or the shell will try to
put file names in there, hence the "\" to make this
a literal "*"
cpp: more string to look for
$ this says that when we've matched what went before,
we must next find end of line, so, the entire line
must be "*cpp:", so the command gets executed only
on lines which contain "*cpp:" and nothing else
/ here's the other delimeter "/" which ends the "address"
{ this tells sed that what is contained is the script to
execute, when we find a matching line; we do so up to
the closing "}"
n Read/append the next line of input into the pattern space
IOW, print what has been matched so far ("*cpp:") and
then work on the next line
; end of "n" command, so all we print is just "*cpp:"
we use ";" to put multiple commands together, so this
separates the "n" command from the "s" command
s now we start a "substitute" command
, this is taken by sed to be the delimter of the string
to substitute for; this could be any character, like
the "/" above; the "s" command wants
s<delim><string to find><delim><string to sub><delim>
where <delim> may be any character you like, but all three
must be the same. In this case, ","
$ the pattern we are going to substitute for is end of line...
, ... and nothing else, the second "," matches the one above
and ends the search string
-isystem /tools/include
this is the string to substitute at end of line
, here's the third delimeter
} this marks end-of-command
" this is the matching quote for the shell to see
HTH
Mike
--
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