echo "hi,my,name,is" | sed 's/,/\n/g' >out.txt
yields:
hi
my
name
is
But in reverse:
sed 's/\n/,/g' out.txt
yields:
hi
my
name
is
Why doesn't sed revert the operation with an inversion of the replace operation?
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don_crissti
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alexei.net
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because sed/awk strips the record separator (newline in your case) and adds back while printing... perl/ruby wont.. so you'd get your inversion there.. – Sundeep Mar 09 '18 at 05:42
2 Answers
5
Try:
$ sed 'H;1h;$!d;x; s/\n/,/g' out.txt
hi,my,name,is
By default, sed reads in only one line at a time, processes it, and prints it. Since a single line never has a newline character, the command s/\n/,/g does nothing. The solution, as above, is to read the whole file in at once and then do s/\n/,/g.
The commands H;1h;$!d;x read the whole file in at once. It is probably
simplest to think of this as an idiom. If you really want to know
the gory details:
H- Append current line to hold space1h- If this is the first line, overwrite the hold space with it$!d- If this is not the last line, delete pattern space and jump to the next line.x- Exchange hold and pattern space to put whole file in pattern space
Alternative: awk
$ awk 'NR>1{printf ","} {printf "%s",$0} END{print""}' out.txt
hi,my,name,is
Or:
$ awk '{printf "%s%s",(NR==1?"":","),$0} END{print""}' out.txt
hi,my,name,is
John1024
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@Sparhawk Very true. But, as you saw in the title of this question, the OP knew about the `tr` approach. He was looking for a better understanding of `sed`. – John1024 Mar 09 '18 at 04:18
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Three more awk ways: `{printf "%s",t $0; t=","} END{print""}` or `{t=t "," $0} END{print substr(t,2)}` optionally with redundant `BEGIN{t=""}` or `-vt=` for clarity; or on condition none of the lines are empty `-vRS="" '{gsub(/\n/,","); print}'` (which is almost like the sed) – dave_thompson_085 Mar 09 '18 at 06:03
