Shell Programming and Scripting

cat file1
1 a b c
2 d e f
3 a f r

cat file2
a c e

1
3

awk 'NR=FNR {A[$1]=$1;next} {print $1}' file2 file1

1
2
3

1
2
3

awk 'NR=FNR {A[$1]=$1;next} # for file2 assign $1 to A[] and print nothing and move to the next record
{print $1} # now we're in file1, print the first field (1, then 2, then 3)
' file2 file1

$ cat file1 
1 a b c
2 d e f
3 a f r

$ cat file2 
a c e

# file1 first and file2 second, 1 = sign (assignment)
[user@host ~]$ awk 'NR=FNR {A[$1]=$1;next} {print $1}' file1 file2


# file1 first and file2 second, 2 = sign (comparison
[user@host ~]$ awk 'NR==FNR {A[$1]=$1;next} {print $1}' file1 file2
a
 
 # file2 first and file1 second, 1 = sign (assignment)
[user@host ~]$ awk 'NR=FNR {A[$1]=$1;next} {print $1}' file2 file1 

# file2 first and file1 second, 2 = sign (comparison) 
[user@host ~]$ awk 'NR==FNR {A[$1]=$1;next} {print $1}' file2 file1
1
2
3

1
3