Faster way to merge multiple files with unequal number of rows by column in bash

Question

I have a multiple files that I want to merge by column using shell script, let's say file a.txt and file b.txt . File a.txt contains sorted unique value and the first column will be used as a reference value.
example:

# cat a.txt 
001|johan
002|mike
003|adam

# cat b.txt
001|chu
001|stewart
002|lewis
002|jordan
003|lambert
003|johnson
003|smith
003|long

The both files will be combined to produce an output like below.

# cat c.txt
001|johan chu
001|johan stewart
002|mike lewis
002|mike jordan
003|adam lambert
003|adam johnson
003|adam smith
003|adam long

I tried simple iteration using while do .

while read line
do
  ids=`echo $line | awk -F"|" '{print $1}'`
  fn=`grep $ids a.txt`
  echo $fn"|"$line | awk -F"|" '{print $1"|"$2" "$4}'
done < b.txt > c.txt

But it takes a lot of time if I have million number of rows.
In MySQL we can easily achieve it using JOIN clause. But we need to load/insert them first.
Faster approach is probably using paste command, but as far as I know, the total rows of both files must be equal. Well I can adjust a.txt first. But still comsumes a lot of time when the script running.
Maybe someone has a better approach.

Answer 1

You could put all in one awk script:

awk -F'|' '{if (NR==FNR) a[$1]=$2; else print $1 "|" a[$1] " " $2}' a.txt b.txt 

001|johan chu
001|johan stewart
002|mike lewis
002|mike jordan
003|adam lambert
003|adam johnson
003|adam smith
003|adam long

Answer 2

Assumptions:

• both files are sorted by the 1st column
• ignore rows that do not have a match in the opposite file

One idea using join and sed (to remove the 2nd | ):

join -t'|' -o 1.1,1.2,2.2 a.txt b.txt | sed -E 's/\|([^|]*$)/ \1/'

One awk idea:

awk '
BEGIN   { FS=OFS="|" }
FNR==NR { a[$1]=$2; next }
$1 in a { print $1,a[$1] " " $2 }
' a.txt b.txt

Both of these generate:

001|johan chu
001|johan stewart
002|mike lewis
002|mike jordan
003|adam lambert
003|adam johnson
003|adam smith
003|adam long