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$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

Explanation:

awk implicitly loops through each file, one line at a time. Since we gave it file2 as the first argument, it is read first. file1 is read second.

• FNR==NR{a[$1]=$2;next}

  NR is the number of lines that awk has read so far and FNR is the number of lines that awk has read so far from the current file. Thus, if FNR==NR, we are still reading the first named file: file2. For every line in file2, we assign a[$1]=$2.
  a[$1]=$2 means saving second column while indexed based on the first column (of file2)

  next tells awk to skip the rest of the commands and start over with the next line.

• ($1 in a) {print $1,a[$1],$2}

  If we get here, that means that we are reading the second file: file1. If we saw the first field of the line in file2, as determined by the contents of array a, then we print out a line with the values of field 2 from both files.

$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

Explanation:

awk implicitly loops through each file, one line at a time. Since we gave it file2 as the first argument, it is read first. file1 is read second.

• FNR==NR{a[$1]=$2;next}

  NR is the number of lines that awk has read so far and FNR is the number of lines that awk has read so far from the current file. Thus, if FNR==NR, we are still reading the first named file: file2. For every line in file2, we assign a[$1]=$2.

  Here, a is an associative array and a[$1]=$2 means saving file2's second column, denoted $2, as a value in array a using file2's first column, $1, as the key.

  next tells awk to skip the rest of the commands and start over with the next line.

• ($1 in a) {print $1,a[$1],$2}

  If we get here, that means that we are reading the second file: file1. If we saw the first field of the line in file2, as determined by the contents of array a, then we print out a line with the values of field 2 from both files.

$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

Explanation:

awk implicitly loops through each file, one line at a time. Since we gave it file2 as the first argument, it is read first. file1 is read second.

• FNR==NR{a[$1]=$2;next}

  NR is the number of lines that awk has read so far and FNR is the number of lines that awk has read so far from the current file. Thus, if FNR==NR, we are still reading the first named file: file2. For every line in file2, we assign a[$1]=$2.

  next tells awk to skip the rest of the commands and start over with the next line.

• ($1 in a) {print $1,a[$1],$2}

  If we get here, that means that we are reading the second file: file1. If we saw the first field of the line in file2, as determined by the contents of array a, then we print out a line with the values of field 2 from both files.

$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

Explanation:

awk implicitly loops through each file, one line at a time. Since we gave it file2 as the first argument, it is read first. file1 is read second.

• FNR==NR{a[$1]=$2;next}

  NR is the number of lines that awk has read so far and FNR is the number of lines that awk has read so far from the current file. Thus, if FNR==NR, we are still reading the first named file: file2. For every line in file2, we assign a[$1]=$2.
  a[$1]=$2 means saving second column while indexed based on the first column (of file2)

  next tells awk to skip the rest of the commands and start over with the next line.

• ($1 in a) {print $1,a[$1],$2}

  If we get here, that means that we are reading the second file: file1. If we saw the first field of the line in file2, as determined by the contents of array a, then we print out a line with the values of field 2 from both files.

$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

$ awk 'FNR==NR{a[$1]=$2;next} ($1 in a) {print $1,a[$1],$2}' file2 file1
aa 45 32
bb 31 15
cc 50 78

Explanation:

awk implicitly loops through each file, one line at a time. Since we gave it file2 as the first argument, it is read first. file1 is read second.

• FNR==NR{a[$1]=$2;next}

  NR is the number of lines that awk has read so far and FNR is the number of lines that awk has read so far from the current file. Thus, if FNR==NR, we are still reading the first named file: file2. For every line in file2, we assign a[$1]=$2.

  next tells awk to skip the rest of the commands and start over with the next line.

• ($1 in a) {print $1,a[$1],$2}

  If we get here, that means that we are reading the second file: file1. If we saw the first field of the line in file2, as determined by the contents of array a, then we print out a line with the values of field 2 from both files.