Gawk/Input-Summary

Input is split into records based on the value of RS. The possibilities are as follows:

FNR indicates how many records have been read from the current input file; NR indicates how many records have been read in total.
gawk sets RT to the text matched by RS.
After splitting the input into records, awk further splits the records into individual fields, named $1, $2, and so on. $0 is the whole record, and NF indicates how many fields there are. The default way to split fields is between whitespace characters.
Fields may be referenced using a variable, as in $NF. Fields may also be assigned values, which causes the value of $0 to be recomputed when it is later referenced. Assigning to a field with a number greater than NF creates the field and rebuilds the record, using OFS to separate the fields. Incrementing NF does the same thing. Decrementing NF throws away fields and rebuilds the record.

Field splitting is more complicated than record splitting:

Field separator value	Fields are split …	`awk` / `gawk`
`FS == " "`	On runs of whitespace	`awk`
`FS == any single character`	On that character	`awk`
`FS == regexp`	On text matching the regexp	`awk`
`FS == ""`	Such that each individual character is a separate field	`gawk`
`FIELDWIDTHS == list of columns`	Based on character position	`gawk`
`FPAT == regexp`	On the text surrounding text matching the regexp	`gawk`

Using ‘FS = "\n"’ causes the entire record to be a single field (assuming that newlines separate records).
FS may be set from the command line using the -F option. This can also be done using command-line variable assignment.
Use PROCINFO["FS"] to see how fields are being split.
Use getline in its various forms to read additional records from the default input stream, from a file, or from a pipe or coprocess.
Use PROCINFO[file, "READ_TIMEOUT"] to cause reads to time out for file.
Directories on the command line are fatal for standard awk; gawk ignores them if not in POSIX mode.