# Help a sed noob 11 messages from 2007-08-03 to 2007-08-04. Participants: Nguyen Thai Ngoc Duy, Junio C Hamano, René Scharfe, martin f krafft, Jeff King, Simon 'corecode' Schubert, David Kastrup. Thread: https://gitlist.dev/t/9372 ## Nguyen Thai Ngoc Duy, 2007-08-03 23:22 Subject: Help a sed noob Message-ID: URL: https://gitlist.dev/e/fcaeb9bf0708031622w25ab6e9ev61372169cfc0bd98%40mail.gmail.com ``` Hi, There is a construct like this in git-parse-remote.sh which makes busybox ash unhappy: sed -ne '/^URL: */{ s///p q }' "$GIT_DIR/remotes/$1" It complains about "no previous regexp" while gnu sed is ok. Can anyone explain to me what does "s///p" do? GNU Sed info page says nothing about empty regexp. If I replace it with "s/\(.*\)/\1/p" then I get "URL: " along with the remote path. By the way, can we use another construct instead? It would be less work for me ;-) -- Duy ``` ## Junio C Hamano, 2007-08-03 23:38 Subject: Re: Help a sed noob Message-ID: <7vhcngkx38.fsf@assigned-by-dhcp.cox.net> URL: https://gitlist.dev/e/7vhcngkx38.fsf%40assigned-by-dhcp.cox.net In-Reply-To: ``` "Nguyen Thai Ngoc Duy" writes: > There is a construct like this in git-parse-remote.sh which makes > busybox ash unhappy: > > sed -ne '/^URL: */{ > s///p > q > }' "$GIT_DIR/remotes/$1" > > It complains about "no previous regexp" while gnu sed is ok. Can > anyone explain to me what does "s///p" do? GNU Sed info page says > nothing about empty regexp. If I replace it with "s/\(.*\)/\1/p" then > I get "URL: " along with the remote path. Traditionally empty LHS regexp means "the same as last match". Replace it with "s/^URL: *//p" and you would be Ok. ``` ## René Scharfe, 2007-08-03 23:40 Subject: Re: Help a sed noob Message-ID: <46B3BCE7.3090509@lsrfire.ath.cx> URL: https://gitlist.dev/e/46B3BCE7.3090509%40lsrfire.ath.cx In-Reply-To: ``` Nguyen Thai Ngoc Duy schrieb: > Hi, > > There is a construct like this in git-parse-remote.sh which makes > busybox ash unhappy: > > sed -ne '/^URL: */{ > s///p > q > }' "$GIT_DIR/remotes/$1" > > It complains about "no previous regexp" while gnu sed is ok. Can > anyone explain to me what does "s///p" do? GNU Sed info page says > nothing about empty regexp. If I replace it with "s/\(.*\)/\1/p" then > I get "URL: " along with the remote path. > > By the way, can we use another construct instead? It would be less > work for me ;-) You could manually add the previous regular expression, like this: sed -ne '/^URL: */{ s/^URL: *//p q }' "$GIT_DIR/remotes/$1" René ``` ## Nguyen Thai Ngoc Duy, 2007-08-04 00:06 Subject: Re: Help a sed noob Message-ID: URL: https://gitlist.dev/e/fcaeb9bf0708031706o3e3f4839t39eca4b64b0aab5d%40mail.gmail.com In-Reply-To: <7vhcngkx38.fsf@assigned-by-dhcp.cox.net> ``` Great! Thank you both. On 8/3/07, Junio C Hamano wrote: > "Nguyen Thai Ngoc Duy" writes: > > > There is a construct like this in git-parse-remote.sh which makes > > busybox ash unhappy: > > > > sed -ne '/^URL: */{ > > s///p > > q > > }' "$GIT_DIR/remotes/$1" > > > > It complains about "no previous regexp" while gnu sed is ok. Can > > anyone explain to me what does "s///p" do? GNU Sed info page says > > nothing about empty regexp. If I replace it with "s/\(.*\)/\1/p" then > > I get "URL: " along with the remote path. > > Traditionally empty LHS regexp means "the same as last match". > Replace it with "s/^URL: *//p" and you would be Ok. > > -- Duy ``` ## martin f krafft, 2007-08-04 10:31 Subject: Re: Help a sed noob Message-ID: <20070804103126.GA28402@lapse.madduck.net> URL: https://gitlist.dev/e/20070804103126.GA28402%40lapse.madduck.net In-Reply-To: ``` also sprach Nguyen Thai Ngoc Duy [2007.08.04.0122 +0200]: > sed -ne '/^URL: */{ > s///p > q > }' "$GIT_DIR/remotes/$1" Looks like it prints the first matching line: grep '^URL: *' | head -1 -- martin; (greetings from the heart of the sun.) \____ echo mailto: !#^."<*>"|tr "<*> mailto:" net@madduck quantum mechanics: the dreams stuff is made of. spamtraps: madduck.bogus@madduck.net ``` ## Jeff King, 2007-08-04 11:50 Subject: Re: Help a sed noob Message-ID: <20070804115003.GA9716@coredump.intra.peff.net> URL: https://gitlist.dev/e/20070804115003.GA9716%40coredump.intra.peff.net In-Reply-To: <20070804103126.GA28402@lapse.madduck.net> ``` On Sat, Aug 04, 2007 at 12:31:26PM +0200, martin f krafft wrote: > also sprach Nguyen Thai Ngoc Duy [2007.08.04.0122 +0200]: > > sed -ne '/^URL: */{ > > s///p > > q > > }' "$GIT_DIR/remotes/$1" > > Looks like it prints the first matching line: > > grep '^URL: *' | head -1 No, it prints everything _except_ "URL: *" from the first matching line. -Peff ``` ## martin f krafft, 2007-08-04 12:18 Subject: Re: Help a sed noob Message-ID: <20070804121811.GA1605@lapse.madduck.net> URL: https://gitlist.dev/e/20070804121811.GA1605%40lapse.madduck.net In-Reply-To: <20070804115003.GA9716@coredump.intra.peff.net> ``` also sprach Jeff King [2007.08.04.1350 +0200]: > No, it prints everything _except_ "URL: *" from the first matching line. I am convinced we could submit this to the Obfuscated Code Contest. :) The behaviour is absolutely unclear from the manpage and defies my logic. Can you elaborate a bit, even though this is off-topic? -- martin; (greetings from the heart of the sun.) \____ echo mailto: !#^."<*>"|tr "<*> mailto:" net@madduck kill ugly radio -- frank zappa spamtraps: madduck.bogus@madduck.net ``` ## Simon 'corecode' Schubert, 2007-08-04 12:26 Subject: Re: Help a sed noob Message-ID: <46B4705D.2010208@fs.ei.tum.de> URL: https://gitlist.dev/e/46B4705D.2010208%40fs.ei.tum.de In-Reply-To: <20070804121811.GA1605@lapse.madduck.net> ``` martin f krafft wrote: > also sprach Jeff King [2007.08.04.1350 +0200]: >> No, it prints everything _except_ "URL: *" from the first matching line. > > I am convinced we could submit this to the Obfuscated Code Contest. > :) This is basic sed scripting. > The behaviour is absolutely unclear from the manpage and defies my > logic. Can you elaborate a bit, even though this is off-topic? -ne means "don't print, execute the following code". /.../ should be clear, { starts a "function list" s// is "match last regexp, so s/// is "match last regexp and replace with empty string. flag "p" to function "s" means "print if matched" q is "quit". that's really straight forward and all documented in my sed(1). cheers simon -- Serve - BSD +++ RENT this banner advert +++ ASCII Ribbon /"\ Work - Mac +++ space for low €€€ NOW!1 +++ Campaign \ / Party Enjoy Relax | http://dragonflybsd.org Against HTML \ Dude 2c 2 the max ! http://golden-apple.biz Mail + News / \ ``` ## Jeff King, 2007-08-04 12:40 Subject: Re: Help a sed noob Message-ID: <20070804124005.GA15334@sigill.intra.peff.net> URL: https://gitlist.dev/e/20070804124005.GA15334%40sigill.intra.peff.net In-Reply-To: <20070804121811.GA1605@lapse.madduck.net> ``` On Sat, Aug 04, 2007 at 02:18:11PM +0200, martin f krafft wrote: > The behaviour is absolutely unclear from the manpage and defies my > logic. Can you elaborate a bit, even though this is off-topic? The original sed code in question was: sed -ne '/^URL: */{ s///p q }' "$GIT_DIR/remotes/$1" There are a few things to note: 1. -n means "do not print lines by default" 2. sed addresses consist of an address (in this case a regex meaning "do this for lines that match the regex") and a command 3. The braces start a set of commands, so that for lines matching the address, we do all of the commands. 4. An empty matching portion for a regex means "use the last regex". So this script comes down to: - don't write any lines except the ones we match - find a line that starts with URL: - replace the URL: part with nothing - print the result - quit It could be more simply written as: sed -ne 's/^URL: *//pq' which uses the substitution as an address, but I don't know whether that was allowed in the original sed. -Peff ``` ## martin f krafft, 2007-08-04 13:56 Subject: Re: Help a sed noob Message-ID: <20070804135630.GC1605@lapse.madduck.net> URL: https://gitlist.dev/e/20070804135630.GC1605%40lapse.madduck.net In-Reply-To: <46B4705D.2010208@fs.ei.tum.de> ``` also sprach Simon 'corecode' Schubert [2007.08.04.1426 +0200]: > s// is "match last regexp, so s/// is "match last regexp and replace with > empty string. This fact is not in the manpage, AFAICT, at least not on Debian systems. -- martin; (greetings from the heart of the sun.) \____ echo mailto: !#^."<*>"|tr "<*> mailto:" net@madduck due to lack of interest tomorrow has been cancelled. spamtraps: madduck.bogus@madduck.net ``` ## David Kastrup, 2007-08-04 14:06 Subject: Re: Help a sed noob Message-ID: <85abt7gzs1.fsf@lola.goethe.zz> URL: https://gitlist.dev/e/85abt7gzs1.fsf%40lola.goethe.zz In-Reply-To: <20070804135630.GC1605@lapse.madduck.net> ``` martin f krafft writes: > also sprach Simon 'corecode' Schubert [2007.08.04.1426 +0200]: >> s// is "match last regexp, so s/// is "match last regexp and replace with >> empty string. > > This fact is not in the manpage, AFAICT, at least not on Debian > systems. IIRC, this is a traditional _ed_ feature. I don't think it has ever been a traditional part of _sed_, but maybe some implementations have adopted this usage. -- David Kastrup, Kriemhildstr. 15, 44793 Bochum ```