I have two files where I want to perform union operation based on 1st column:
file1.txt
foo 1 bar 2 qux 3
file2.txt
foo x qux y boo z
The result I hope to get is like this:
foo 1 x bar 2 - qux 3 y boo - z
where the empty fields of column 1 is padded with “-“.
But why this join command doesn’t work as I expected?
$ join -a1 -a2 -e"-" file1.txt file2.txt
What’s the right way to do it?
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Answer
“Important: FILE1 and FILE2 must be sorted on the join fields.” (from this online manpage).
This problem #1. Problem #2 is worse: option -e is badly documented — only works in conjunction with -o, so for example:
$ join -a 1 -a 2 -e'-' -o '0,1.2,2.2' sfile1.txt sfile2.txt bar 2 - boo - z foo 1 x qux 3 y
where the s prefix name indicated files that I’ve sorted beforehand.
Edit: man join explains the -o switch (so does the online manpage I point to above). It specifies the fields to output (1.2 means 2nd field from file 1, &c), or 0 to mean the join field, and is a comma-separated list. (I didn’t remember the 0 value, actually, so had originally given a clumsier solution requiring awk post-processing, but the current solution is better… and no awk needed!).