How do I print all records in file 1 but note the records that do not match file2?

Question

Is there an easy way to print all lines of a file1 (so that the output has the same number of lines as the input file1) but print a message such as NoMatch where the first entries of file1 does not match the first entries of file2?

file1:

Entry1 Entry2
a 2
b 3
c 4
d 5

file2:

a
b
b
a
d
d

Desired output:

Entry1
a 2
b 3
NoMatch 4
d 5

I am trying with

join -a1 -e "NoMatch" -11 -21 -o2.1 file1 file2

since I would like to keep the unpairable lines from file1 that do not match file2, and give a message for these cases as "no", but this keeps all of my records also in file2 (which contains duplicated records), what am I doing wrong? Could this be because my second file is tab delimited and my first file is space delimited?

Thanks so much for all the help...

Answer 1

With awk read file2 first and save $1 in seen[$1] then read file1 and if $1 wasn't "seen" replace it with NoMatch:

awk 'FNR==NR{seen[$1]++; next}
{(FNR==1 || ($1 in seen)) || $1="NoMatch"};1' file2 file1

If you prefer join, you need sorted input. You'll have to extract the header from file1 first, sort the remaining lines then join the result with the sorted file2:

{ head -n1; sort | join -j1 -a1 -e "NoMatch" -o 2.1 1.2 - <(sort file2); } <file1

and if needed, pipe everything to uniq to avoid duplicate lines:

{ head -n 1; sort | \
join -j1 -a1 -e "NoMatch" -o 2.1 1.2 - <(sort file2) | \
uniq; } <file1