append data from one file to anoher file from right - shell

Question

I have two files with below formats:
file1:
Sub_amount , date/time
12 , 2018040412
78 , 2018040413
26 , 2018040414
file2:
Unsub_amount , date/time
76 , 2018040412
98 , 2018040413
56 , 2018040414
what I need is, append file2 to file1 from right. what I mean is:
Sub_amount, Unsub_amount , date/time
12 , 76 , 2018040412
78 , 98 , 2018040413
26 , 56 , 2018040414
At the end, what is needed to be shown is:
date/time , Unsub_amount , Sub_amount
2018040412, 76 , 12
2018040413, 98 , 78
2018040414, 26 , 56
I would be appreciate if anyone can support :)
Thanks.

Answer 1

I would use awk for this:
awk -F'[[:blank:]]*,[[:blank:]]*' -v OFS="," '
# remove leading and trailing blanks from the line
{ gsub(/^[[:blank:]]+|[[:blank:]]+$/, "") }
# skip empty lines
/^$/ { next }
# store the Sub values from file1
NR == FNR { sub_amt[$2] = $1; next }
# print the data from file2, matching the cached value from file1
{ print $2, $1, sub_amt[$2] }
' file1 file2
date/time,Unsub_amount,Sub_amount
2018040412,76,12
2018040413,98,78
2018040414,56,26

Answer 2

edit: Not as cool or well-working as the awk solution, but an alternative. Trims the columns (using sed) and extracts them (using cut), then joins them (using paste). Assumes that the rows match up. Happy coding!
#!/usr/bin/env sh
# copy this code
# pbpaste > csv-col-merge.sh # paste/create the file
# chmod u+x csv-col-merge.sh # make it executable
# ./csv-col-merge.sh --gen-example
# ./csv-col-merge.sh demo/subs.csv demo/unsubs.csv demo/combined.csv
#
# learn more:
# - "paste": https://askubuntu.com/questions/616166/how-can-i-merge-files-on-a-line-by-line-basis
# - "<<- EOF": https://stackoverflow.com/questions/2953081/how-can-i-write-a-heredoc-to-a-file-in-bash-script
# - "$_": https://unix.stackexchange.com/questions/271659/vs-last-argument-of-the-preceding-command-and-output-redirection
# - "cat -": https://stackoverflow.com/questions/14004756/read-stdin-in-function-in-bash-script
# - "cut -d ',' -f 1": split lines with ',' and take the first column, see "man cut"
# - "sed -E 's/find/replace/g'"; -E for extended regex, for ? (optional) support, see "man sed"
# -
# --gen-example
if [ "$1" = '--gen-example' ]; then
mkdir -p demo && cd $_
cat <<- EOF > subs.csv
sub_amount, date/time
12, 2018040412
78, 2018040413
26, 2018040414
EOF
cat <<- EOF > unsubs.csv
unsub_amount, date/time
76, 2018040412
98, 2018040413
56, 2018040414
EOF
exit
fi
# load
trim () { cat - | sed -E 's/ ?, ?/,/g'; }
subs="$(cat "$1" | trim)"
unsubs="$(cat "$2" | trim)"
combined=$3
# intermediate
getcol () { cut -d ',' -f $1; }
col_subs="$(echo "$subs" | getcol 1)"
col_unsubs="$(echo "$unsubs" | getcol 1)"
col_subs_date="$(echo "$subs" | getcol 2)"
col_unsubs_date="$(echo "$unsubs" | getcol 2)"
if [ ! "$col_subs_date" = "$col_unsubs_date" ]; then echo 'Make sure date col match up'; exit; fi
# process
mkdir tmp
echo "$col_subs_date" > tmp/a
echo "$col_unsubs" > tmp/b
echo "$col_subs" > tmp/c
paste -d ',' < tmp/a tmp/b tmp/c
rm -rf tmp