Using getline from a File

Use `getline < file' to read the next record from the file file. Here file is a string-valued expression that specifies the file name. `< file' is called a redirection since it directs input to come from a different place.

For example, the following program reads its input record from the file `secondary.input' when it encounters a first field with a value equal to 10 in the current input file.

awk '{
    if ($1 == 10) {
         getline < "secondary.input"
         print
    } else
         print
}'

Since the main input stream is not used, the values of NR and FNR are not changed. But the record read is split into fields in the normal manner, so the values of $0 and other fields are changed. So is the value of NF.

According to POSIX, `getline < expression' is ambiguous if expression contains unparenthesized operators other than `$'; for example, `getline < dir "/" file' is ambiguous because the concatenation operator is not parenthesized, and you should write it as `getline < (dir "/" file)' if you want your program to be portable to other awk implementations.

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