Node:Getline/File, Next:Getline/Variable/File, Previous:Getline/Variable, Up:Getline

Using getline from a File

Use getline < file to read the next record from file. Here file is a string-valued expression that specifies the file name. < file is called a redirection because 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:

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

Because the main input stream is not used, the values of NR and FNR are not changed. However, the record it reads is split into fields in the normal manner, so the values of $0 and the other fields are changed, resulting in a new 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. You should write it as getline < (dir "/" file) if you want your program to be portable to other awk implementations. (It happens that gawk gets it right, but you should not rely on this. Parentheses make it easier to read.)