4.10.4 Using getline into a Variable from a File

Use ‘getline var < file’ to read input from the file file, and put it in the variable var. As earlier, file is a string-valued expression that specifies the file from which to read.

In this version of getline, none of the predefined variables are changed and the record is not split into fields. The only variable changed is var.²⁴ For example, the following program copies all the input files to the output, except for records that say ‘@include filename’. Such a record is replaced by the contents of the file filename:

{
     if (NF == 2 && $1 == "@include") {
          while ((getline line < $2) > 0)
               print line
          close($2)
     } else
          print
}

Note here how the name of the extra input file is not built into the program; it is taken directly from the data, specifically from the second field on the @include line.

The close() function is called to ensure that if two identical @include lines appear in the input, the entire specified file is included twice. See section Closing Input and Output Redirections.

One deficiency of this program is that it does not process nested @include statements (i.e., @include statements in included files) the way a true macro preprocessor would. See section An Easy Way to Use Library Functions for a program that does handle nested @include statements.