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join Command

The join command is used to join the lines with the specified column content in two files. It first identifies the lines with the same content in the specified column in two files, merges them, and then outputs them to the standard output device.

Syntax

join [OPTION]... FILE1 FILE2

Options

  • -a FILENUM: Print unpairable lines from FILENUM, where FILENUM is 1 or 2, corresponding to FILE1 or FILE2.
  • -e EMPTY: Replace missing input fields with EMPTY.
  • -i, --ignore-case: Ignore differences in case when comparing fields.
  • -j FIELD: Equivalent to -1 FIELD -2 FIELD.
  • -o FORMAT: Construct the output line according to FORMAT.
  • -t CHAR: Use CHAR as the input and output field separator.
  • -v FILENUM: Similar to -a FILENUM, but do not join the output lines.
  • -1 FIELD: Join on this FIELD from file 1.
  • -2 FIELD: Join on this FIELD from file 2.
  • --check-order: Check that the input is correctly sorted, even if all input lines can be paired.
  • --nocheck-order: Do not check that the input is correctly sorted.
  • --header: Treat the first line of each file as field headers and print them without attempting to pair them.
  • --help: Display this help and exit.
  • --version: Output version information and exit.

Examples

Contents of file1.txt and file2.txt are as follows:

# file1.txt
1 AAYUSH
2 APAAR
3 HEMANT
4 KARTIK
5 TIM

# file2.txt
1 101
2 102
3 103
4 104

To merge files using the join command, the files must have some common fields, and in this case, the common fields in both files are labeled with numbers 1, 2....

join file1.txt file2.txt
# 1 AAYUSH 101
# 2 APAAR 102
# 3 HEMANT 103
# 4 KARTIK 104

Using the -a option to print the paired lines from FILE1 and also the unpaired ones.

join file1.txt file2.txt -a 1
# 1 AAYUSH 101
# 2 APAAR 102
# 3 HEMANT 103
# 4 KARTIK 104
# 5 TIM

Using the -v option to print the unpaired lines from FILE1.

join file1.txt file2.txt -v 1
# 5 TIM

join combines the lines of the files on the first common field, which is the default. However, the common value in the two files is not always in the first column, so join can use -1, -2 to specify the position of the common value. -1 and -2 denote the first and second files, and these options require a numerical argument that references the joining field of the corresponding file.

Contents of `file1.txt` and `file2.txt` are as follows:
# file1.txt
AAYUSH 1 
APAAR 2
HEMANT 3
KARTIK 4
TIM 5

# file2.txt
101 1
102 2
103 3
104 4

join -1 2 -2 2 file1.txt file2.txt
# 1 AAYUSH  101
# 2 APAAR 102
# 3 HEMANT 103
# 4 KARTIK 104

For the above example, we can also achieve the same using the -j parameter directly.

join -j 2 file1.txt file2.txt
# 1 AAYUSH  101
# 2 APAAR 102
# 3 HEMANT 103
# 4 KARTIK 104

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://www.computerhope.com/unix/ujoin.htm
https://www.runoob.com/linux/linux-comm-join.html
https://www.geeksforgeeks.org/join-command-linux/