Using cut and grep commands in unix

This prints lines from file1.txt only if the second field is not in file2:

$ awk -F, 'FNR==NR{a[$1]=1; next;} !a[$2]' file2 file1.txt
aaa,,,,,
aaa,10001782,,,,
bbb,10001783,,,,

How it works

This works by reading file2 and keeping track of all lines seen in an associative array a. Then, lines in file1.txt are printed only if its column 2 is not in a. In more detail:

• FNR==NR{a[$1]=1; next;}

  When reading file2, set a[$1] to 1 to signal that we have seen the value on this line. We then instruct awk to skip the rest of the commands and start over on the next line.

  This section is only run for file2 because file2 is listed first on the command line and FNR==NR only when we are reading the first file listed on the command line. This is because FNR is the number of lines read from the current file and NR is the total number of lines read so far. These two are equal only for the first file.

• !a[$2]

  When reading file1.txt, a[$2] evaluates to true if column 2 was seen in file2. Since ! is negation, !a[$2] evaluates to true when column 2 was not seen. When this evaluates to true, the line is printed.

Alternative

This is the same logic, expressed in a slightly different style, as suggested in the comments by Tom Fenech:

$ awk -F, 'FNR==NR{a[$1]; next;} !($2 in a)' file2 file1.txt
aaa,,,,,
aaa,10001782,,,,
bbb,10001783,,,,

Soulution with grep

$ grep -vf file2 file1.txt
aaa,,,,,
aaa,10001782,,,,
bbb,10001783,,,,

John1024's awk soulution would be faster for large files though.