join command in UNIX is a command line utility for joining lines of two files on a common field. It can be used to join two files by selecting fields within the line and joining the files on them. The result is written to standard output.
To join two files using the
join command files must have identical join fields. The default join field is the first field delimited by blanks. For the following example there are two files
cat foodtypes.txt 1 Protein 2 Carbohydrate 3 Fat cat foods.txt 1 Cheese 2 Potato 3 Butter
These files share a join field as the first field and can be joined.
join foodtypes foods.txt 1 Protein Cheese 2 Carbohydrate Potato 3 Fat Butter
To join files using different fields the
-2 options can be passed to
join. In the following example there are two files
cat wine.txt Red Beaunes France White Reisling Germany Red Riocha Spain cat reviews.txt Beaunes Great! Reisling Terrible! Riocha Meh
These files can be joined by specifying the fields that should be used to join the files. Common to both files is the name of the wine. In
wine.txt this is the second field. In
reviews.txt this is the first field. The files can be joined using
-2 by specifying these fields.
join -1 2 -2 1 wine.txt reviews.txt Beaunes Red France Great! Reisling White Germany Terrible! Riocha Red Spain Meh
Join expects that files will be sorted before joining. For this example suppose there are two files from the previous example are not sorted.
cat wine.txt White Reisling Germany Red Riocha Spain Red Beaunes France cat reviews.txt Riocha Meh Beaunes Great! Reisling Terrible!
join on these files results in an error becuase the files are not sorted.
join -1 2 -2 1 wine.txt reviews.txt join: wine.txt:3: is not sorted: Red Beaunes France join: reviews.txt:2: is not sorted: Beaunes Great! Riocha Red Spain Meh Beaunes Red France Great!
The sort command can sort the files before passing to join.
join -1 2 -2 1 <(sort -k 2 wine.txt) <(sort reviews.txt) Beaunes Red France Great! Reisling White Germany Terrible! Riocha Red Spain Meh
To specify a field separator for joining using the
join command use the
-t option. An example is a CSV file where the separator is
,. In the following example there are two files
cat names.csv 1,John Smith,London 2,Arthur Dent, Newcastle 3,Sophie Smith,London cat transactions.csv £1234,Deposit,John Smith £4534,Withdrawal,Arthur Dent £4675,Deposit,Sophie Smith
-t option the comma can set as the delimiter.
join -1 2 -2 3 -t , names.csv transactions.csv John Smith,1,London,£1234,Deposit Arthur Dent,2, Newcastle,£4534,Withdrawal Sophie Smith,3,London,£4675,Deposit
To specify the output format of join use the
-o option. This allows the order of fields that will be shown in the output to be defined, or for only certain fields to be shown.
In the previous example the output we as follows.
To specify the order the list of fields are passed to
-o. For this example this is
-o 1.1,1.2,1.3,2.2,2.1. This formats the output in the order desired.
join -1 2 -2 3 -t , -o 1.1,1.2,1.3,2.2,2.1 names.csv transactions.csv 1,John Smith,London,Deposit,£1234 2,Arthur Dent, Newcastle,Withdrawal,£4534 3,Sophie Smith,London,Deposit,£4675
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