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I have a list of filenames as id-datetime.txt, one each line, where id is always the same and datetime is ordered.

I need first and last datetime so having a sed_script in a variable $script that grabs it I can do

sed -nr "1 $script p; \$ $script'

I get

datetime (oldest)
datetime (newest)

also working with a single-file list.

Now what if I'd like to get also another line with also the id like

datetime (oldest)
datetime (newest)

is there an (easy) way of editing line 1 twice so it gives 2 separate lines?

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2 Answers

So you want to extract some data from the first line and the last line? Just use a single command for each, printing two lines the first time.

sed -n -e '1s/^\(.*\)-\(.*\)\.txt$/\1\
\2/p' -e '$s/^\(.*\)-\(.*\)\.txt$/\2/p'

You might find awk clearer if wordier.

awk -F- '{gsub(/\.[^.]*$/,"");
         NR==1 {print $1; print $2}
         END {print dt}'

Or Perl.

perl -l -ne 's/\.[^.]*$//;
             /^(.*)-(.*)$/ or next;
             print $1 if $.==1;
             print $2 if $.==1 || eof'
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I'm not sure but isn't this suppose to parse the file twice, one for each -e? I was already using two different sed calls and wanted to optimize using a single pass, I found how to do it using hold buffer. +1 for awk and perl alternatives, I'll time them to see which is faster, thank you. –  neurino May 6 '11 at 14:04
@neurino: sed is a streaming editor, it never backs up in the input file. Multiple -e arguments are equivalent to specifying a multiline script. –  Gilles May 6 '11 at 14:07
@Gilles: I profiled on a dir with 5000+ files in it: time ls -1 | sed -nr -e '1 p' -e '1 p; $ p' takes 159ms, time ls -1 | sed -nr '1 {h; p; g; p}; $ p' 118ms (-25%). –  neurino May 6 '11 at 14:19
@neurino: I found the times to be effectively the same when I used your version (which still needs a second expression for the last line)... Gilles hasn't used two expressions for your {h; p; g; p}.. He too has used a single expression, with a newline char, to create the two lines you were after.... I did, howerver, need to add a backslash to the end of the first line, so that the newline created by pressing Enter was treated literally as '\n'. –  Peter.O May 8 '11 at 10:48
@fred: > I found the times to be effectively the same. Do you mean the same execution time for both statements or the same as I stated (-25% for using {h; p; g; p }) ? I repeated profiling several times and had constant results on my linux box. –  neurino May 8 '11 at 20:45
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up vote 4 down vote accepted

Ok, I got it

1 {
    #hold the line
    #extract id
    s|^([0-9]{6}).*|\1|; p
    #put line back again
    #get datetime

this part of the sed script will print (and edit) the first line twice

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+1 for "getting it", and utilizing sed beyong s/x/y/p :) ... It will be handy for other sed tasks, as it certainly works here, but for your specific example (above), you don't really need to use the hold space. You can simply insert \n between \1 (ID) and \2 (datetime)... as per Gilles example (where he uses the line ending as the \n) –  Peter.O May 9 '11 at 2:25
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