Explanation for a specific command in a awk - awk

Question

Can you explain me what doing this command ' dateA="$dateA" '?
awk 'FNR>1 && dateA<=$5' FS='|' dateA="$dateA" "$infile"

Answer 1

awk 'FNR > 1 && dateA <= $5 ' FS='|' dateA="$dateA" "$infile"
FNR is variable gives you total number of records, related to current file, don't get confused with variable NR, FNR and NR value will be same as long as awk reads first file, for second file, variable FNR will reset, whereas NR doesn't.
This is how FNR and NR works in awk
$ seq 1 5 >file1
$ seq 1 3 >file2
$ cat file1
1
2
3
4
5
$ cat file2
1
2
3
$ awk '{print "Current line : "$0,"File: "FILENAME,"FNR : ",FNR,"NR : ",NR}' file1 file2
Current line : 1 File: file1 FNR : 1 NR : 1
Current line : 2 File: file1 FNR : 2 NR : 2
Current line : 3 File: file1 FNR : 3 NR : 3
Current line : 4 File: file1 FNR : 4 NR : 4
Current line : 5 File: file1 FNR : 5 NR : 5
Current line : 1 File: file2 FNR : 1 NR : 6
Current line : 2 File: file2 FNR : 2 NR : 7
Current line : 3 File: file2 FNR : 3 NR : 8
FNR > 1 && dateA <= $5 If no of records read is greater than 1 and variable dateA is less than or equal to 5th field/column, we get boolean true state, so such line will be printed
FS='|' FS is input field separator, you can also set it like
awk -F'|' '{ .... }' OR
awk -v FS='|' '{ .... }' OR
awk 'BEGIN{FS="|"}{ .... }'
dateA="$dateA" dateA is awk variable whose value is taken from your shell variable $dateA, similarly you can set it like
awk -v dateA="$dateA" '{ .... }'
Your above command can be rewritten like below also
awk -F'|' -v dateA="$dateA" 'FNR>1 && dateA <= $5' "$infile"
and some people prefer awk 'condition{action}' for better reading, so you can also write it as
awk -F'|' -v dateA="$dateA" 'FNR>1 && dateA <= $5{ print }' "$infile"
^ ^
| |
If this condition is true |
|
Action is to print line,
print or print $0 is same

Answer 2

Please go through the following explanation and let me know if this helps you.
Explanation: Kindly don't run following awk, it is expanded for explanation purposes only.
awk '
FNR>1 && dateA<=$5 ##FNR denotes the number of current line in awk so here 2 conditions with AND conditions are being checked.
##1st is if current line number is greater than 1 and second is variable named dateA value should be lesser
##and equal to 5.
##So let me explain here awk works on method of condition and then action, so if any condition is TRUE then action
##will happen, here condition is there but NO action defined, so by default print action will happen. print of
##current line.
'
FS='|' ##FS denotes the field separator, in awk we could define the field separator by ourselves too, so making it here as |
dateA="$dateA" ##creating variable named dateA whose value is equal to shell variable named dateA. In awk if we have to assign
##shell variable values to awk variables we have to create an awk variable and then assign shell variable value to
##it.
"$infile" ##Mentioning the Input_file name here which awk has to go through. Point to be noted here the "$infile" means
##it is a shell variable (as we all know to print shell variable value we have to use "$infile")

Answer 3

AWK allows assigning internal variable in the arguments with the form var=value. Since AWK does not have access to shell variables dateA="$dateA" is used to "export" dateA to the AWK script.
Note that assignment arguments occur during file processing, after BEGIN, and can be used in-between files:
$ echo >file1; echo >file2
$ awk -vx=0 '
BEGIN {
print "BEGIN", x
}
{
print FILENAME, x
}
END {
print "END", x
}' x=1 file1 x=2 file2 x=3
BEGIN 0
file1 1
file2 2
END 3