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I have two files in a Linux / Bash environment:

# Example data
$ cat abc
2       a
1       b
3       c

$ cat bcd
5       c
2       b
1       d

I'm trying to join the two files on the first column. The following does not work because the input files must be sorted on the match field.

# Wrong: join on unsorted input does not work
$ join abc bcd

I can get around this by creating two temp files and joining them

$ sort abc > temp1
$ sort bcd > temp2
$ join temp1 temp2
1 b d
2 a b

But is there a way to do this without creating temp files?

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up vote 18 down vote accepted

The following will work in the bash shell:

# Join two files
$ join <(sort abc) <(sort bcd)
1 b d
2 a b

You can join on any column as long as you sort the input files on that column

# Join on the second field
$ join -j2 <(sort -k2 abc) <(sort -k2 bcd)
b 1 2
c 3 5

The -k2 argument to sort means sort on the second column. The -j2 argument to join means join on the second columns. Alternatively join -1 x -2 y file1 file2 will join on the xth column of file1 and the yth column of file2.

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Zsh answer:

join =(sort abc) =(sort bcd)
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This will work in bash shell:

# Join two files
$ sort abc | join - <(sort bcd)
1 b d
2 a b


# Join two files
$ sort bcd | join <(sort abc) -
1 b d
2 a b

Because join can read standard input by using '-'.

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