I have a file parse.txt
parse.txt contains the following
remo/hello/1.0,remo/hello2/2.0,remo/hello3/3.0,whitney/hello/1.0,julie/hello/2.0,julie/hello/3.0
and I want the output.txt file as (to reverse the order from last to first)using parse.txt
julie/hello/3.0,julie/hello/2.0,whitney/hello/1.0,remo/hello3/3.0,remo/hello2/2.0,remo/hello/1.0
I have tried the following code:
tail -r parse.txt
You can use the surprisingly helpful tac from GNU Coreutils.
tac -s "," parse.txt > newparse.txt
tac by default will "cat" the file to standard out, reversing the lines. By specifying the separator using the -s flag, you can simply reverse your fields as desired.
(You may need to do a post-processing step to get the commas to work out correctly, which can be another step in your pipeline.)
rev to reverse linesrev is not applicable to this particular problem. printf "%s\n" abc def ghi | tac versus printf "%s\n" abc def ghi | revI like the tac solution; it's tight and elegant, but as Micah pointed out, tac is part of GNU Coreutils, which means that it's not available by default in FreeBSD, OSX, Solaris, etc.
This can be done in pure bash, no external tools required.
#!/usr/bin/env bash
unset comma
read foo < parse.txt
bar=(${foo//,/ })
for (( count="${#bar[@]}"; --count >= 0; )); do
printf "%s%s" "$comma" "${bar[$count]}"
comma=","
done
This obviously only handles one line, per your sample input. You can wrap it in something if you need to handle multiple lines of input.
The logic here is that we can convert the input into an array by replacing commas with spaces. Of course, if our input data included spaces, this would have to be adjusted. Once we have the array, we simply step backwards through it, printing each record.
Note that this does not include a terminating newline. If you want one, you can add it with:
printf '\n'
as a final line.
printf '\n' with echo
read and bar= lines with IFS=, read -a bar < parse.txt to directly read into an array, separating on commasecho behaves consistently as long as you're in bash, but years ago I got out of the habit of using it because of inconsistencies in other shells and operating systems, and I've stuck with printf ever since. Luckily, both echo and printf are built-ins in bash, so there isn't a serious performance hit. :)perl -F, -lane 'print join ",", reverse @F' parse.txt > output.txt
You can use this awk command:
awk -v RS=, '{a[++i]=$1} END{for (k=i; k>=1; k--) printf a[k] (k>1?RS:ORS)}' parse.txt
julie/hello/3.0,julie/hello/2.0,whitney/hello/1.0,remo/hello3/3.0,remo/hello2/2.0,remo/hello/1.0
The question is tagged unix and you have mentioned tail -r which suggests you might not be using Linux (with full GNU toolchain), but instead some "real" Unix (BSD variant), e.g. osx.
As such, the tac command is not available, but as mentioned in the question, tail -r is. So you can use the following:
$ tr ',' '\n' < parse.txt | tail -r | tr '\n' ',' | sed 's/,$//'
julie/hello/3.0,julie/hello/2.0,whitney/hello/1.0,remo/hello3/3.0,remo/hello2/2.0,remo/hello/1.0
$
Notes:
tr.sed is to remove a trailing comma, that was converted from a trailing newline inserted by tailEmulating tac with sed:
tr , '\n' <parse.txt | sed '1!G; h; $!d' | paste -sd ,
Alternatively, if you don't have paste:
tr , '\n' <parse.txt | sed '1!G; h; $!d' | tr '\n' , | sed 's/,$//'
Output:
julie/hello/3.0,julie/hello/2.0,whitney/hello/1.0,remo/hello3/3.0,remo/hello2/2.0,remo/hello/1.0
You can use any language to do that
xargs ruby -e "puts ARGV[0].split(',').reverse.join(',')" < parse.txt
chomp, and xargs seems needlessly complicated: ruby -ne 'puts $_.chomp.split(",").reverse.join(",")' parse.txtReverse can be done by tac (from cat). As commented this will reverse the lines not what the OP asked for.
tac filename
You can still you tac if you provide line by line and reverse by not linefeed delimiter but the field separator, here ,.
echo "a,b,c" | tr '\n' ',' | tac -s "," | sed 's/,$/\n/'
