If you've worked with databases, you'll probably know what to do with the UNIX join command; see your online manual page. If you don't have a database (as far as you know!), you'll still probably have a use for join : combining or "joining" two column-format files. join searches certain columns in the files; when it finds columns that match one another, it "glues the lines together" at that column. This is easiest to show with an example.
I needed to summarize the information in thousands of email messages under the MH mail system. MH made that easy: it has one command (
scan
) that gave me almost all the information I wanted about each message in the format I wanted. But I also had to use
wc -l
(
29.6
)
to count the number of lines in each message. I ended up with two files, one with
scan
output and the other with
wc
output. One field in both lines was the message number; I used
sort
(
36.1
)
to sort the files on that field. I used
awk '{print $1 "," $2}'
to massage
wc
output into comma-separated fields. Then I used
join
to "glue" the two lines together on the message-number field. (Next I fed the file to a PC running
dBASE
, but that's another story.)
Here's the file that I told
scan
to output. The columns (message number, email address, comment, name, and date sent) are separated with commas (
,
):
0001,[email protected],,Andy Ernbaum,19901219 0002,[email protected],,Zoe Doan,19910104 0003,[email protected],,Head Honcho,19910105 ...
Here's the file from wc and awk with the message number and number of lines:
0001,11 0002,5 0003,187 ...
Then, this
join
command joined the two files at their first columns (
-t,
tells
join
that the fields are comma-separated):
%join -t, scanfile wcfile
The output file looked like:
0001,[email protected],,Andy Ernbaum,19901219,11 0002,[email protected],,Zoe Doan,19910104,5 0003,[email protected],,Head Honcho,19910105,187 ...
Of course, join can do a lot more than this simple example shows. See your online manual page.
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