A sed syntax
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
I read man sed and info sed and googled but can't find this kind of
sed usage, can you give me a reference link on the internet or tell
me which section in info sed?
I knew ^ indicates the begin of a line and $ indicates end of a
line, so ^\*cpp:$ indicates a line includes and only includes a
string *cpp:. And, I also knew the effect of this command is append
string -isystem /tools/include to the end of the next line of line
*cpp: by a test. But what's meaning of the every part in {n;s, $,
-isystem /tools/include,} and how this command to achieve this
effect?
Thanks,
littlebat
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Re: A sed syntax
On Wednesday 23 June 2010 02:24:51 littlebat wrote:
Hi,
I am learning LFS BOOK:
http://www.linuxfromscratch.org/lfs/view/6.6/chapter05/adjusting.html
Below is a sed syntax I can't understand and haven't found a place to
learn it.
code
sed -e /^\*cpp:$/{n;s,$, -isystem /tools/include,}
/code
Basically, '/^\*cpp:$/' is the address that matches lines that contain
exactly '*cpp:'. The braces indicate a 'compound command'. 'n;s' means
execute those two commands (n: print the pattern space; s: substitute the EOL
with the 'option'). The effect is, as you know, to append the option to the
end of all lines with just '*cpp:' on them.
The commas are an unusual selection, but perfectly valid, since the s command
allows pretty much any character to delimit the match and replace phrases.
This could be rewritten as
sed -e 's=^\(*cpp:\)=\1 -s system /tools/include/='
which would be a little more grokable.
The only thing you need to find in the manual is the definition and use of the
braces.
Good enough?
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Re: A sed syntax
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
sdelimstring to finddelimstring to subdelim
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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Re: A sed syntax
On Wed, 23 Jun 2010 02:57:44 -0400
Neal Murphy neal.p.mur...@alum.wpi.edu wrote:
On Wednesday 23 June 2010 02:24:51 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
Basically, '/^\*cpp:$/' is the address that matches lines that
contain exactly '*cpp:'. The braces indicate a 'compound command'.
'n;s' means execute those two commands (n: print the pattern space;
s: substitute the EOL with the 'option'). The effect is, as you know,
to append the option to the end of all lines with just '*cpp:' on
them.
The commas are an unusual selection, but perfectly valid, since the s
command allows pretty much any character to delimit the match and
replace phrases.
This could be rewritten as
sed -e 's=^\(*cpp:\)=\1 -s system /tools/include/='
which would be a little more grokable.
The only thing you need to find in the manual is the definition and
use of the braces.
Good enough?
--
http://linuxfromscratch.org/mailman/listinfo/lfs-support
FAQ: http://www.linuxfromscratch.org/lfs/faq.html
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Thanks. Good answer. Save me much time, with your help.
Always hate read the whole man page (info page) of those big program :-)
Got at least three from this:
1, Sed syntax: sed -e /address/command
2, command can be a 'compound command', it is {}
3, just forgot delimiter can be any character
sed -e 's=^\(*cpp:\)=\1 -s system /tools/include/='
I modified it as below but can't achieve the effect:
sed -e s=^\(*cpp:\)$=\1 -isystem /tools/include=
It only append -isystem /tools/include to the end of the line which
only include string *cpp:. The book needs append the string to the
next line of the line only include string *cpp:.
But, at least I have known more on sed. Thanks.
--
littlebat
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Re: A sed syntax
On Wed, 23 Jun 2010 05:22:35 -0500
Mike McCarty mike.mcca...@sbcglobal.net wrote:
So much detailed answer, I can post it into my blog as a detailed
manual. too many thanks.
littlebat wrote:
Hi,
I am learning LFS BOOK:
http://www.linuxfromscratch.org/lfs/view/6.6/chapter05/adjusting.html
Below is a sed syntax I can't understand and haven't found a place
to learn it.
code
sed -e /^\*cpp:$/{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
-emeans 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
sdelimstring to finddelimstring to subdelim
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
--
p=p=%c%s%c;main(){printf(p,34,p,34);};main(){printf(p,34,p,34);}
Oppose globalization and One World Governments like the UN.
This message made from 100% recycled bits.
You have found the bank of Larn.
I speak only for myself, and I am unanimous in that!
--
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--
littlebat
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Re: A sed syntax
On Wed, 23 Jun 2010 09:23:49 -0500 Mike McCarty mike.mcca...@sbcglobal.net wrote: littlebat wrote: On Wed, 23 Jun 2010 05:22:35 -0500 Mike McCarty mike.mcca...@sbcglobal.net wrote: So much detailed answer, I can post it into my blog as a detailed manual. too many thanks. It's considered polite to ask the copyright holder before reproducing his work :-) Mike Of course, I will comment the original author is Mike McCarty Mike.McCarty at sbcglobal.net and original link is http://linuxfromscratch.org/pipermail/lfs-support/2010-June/039030.html :-) -- littlebat -- http://linuxfromscratch.org/mailman/listinfo/lfs-support FAQ: http://www.linuxfromscratch.org/lfs/faq.html Unsubscribe: See the above information page
