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# grep -b -A10 pattern file perl -ne '$c=11if/pattern/;$d=$c<11?"-":":";$t||=0;$c--,print"$t$d$_"i +f$c;$t=tell' file
perl -lne 'if(/pattern/){print"--"if!$c&&$t;$c=11};$d=$c<11?"-":":";$t +||=0;$c--,print"$t$d$_"if$c;$t=tell' file