Created at: 2014-11-06

AWK notes

Tips

Using getline

getline can be used for several different purposes, like reading from a pipe. Its usage can be strange for those who come from other programming languages. For instance, program 1 and 2 differ from only one slash. Compare the output of both:

**Program 1**::

    {
        "ls /" | getline var
        print var
        "ls /" | getline var
        print var
    }

**Program 2**::

    {
        "ls /" | getline var
        print var
        "ls //" | getline var
        print var
    }

The output of the command awk -f program.awk /etc/fstab are:

**Program 1**::

    altroot
    bin
    boot
    boot.cfg
    dev
    disk
    etc
    export
    home
    ...

**Program 2**::

    altroot
    altroot
    bin
    bin
    boot
    boot
    boot.cfg
    boot.cfg
    dev
    dev
    ...

awk program memorizes the string you used to pass to pipe and constantly reads from it if you use it several times. It is a very powerful tool and can be used in a loop::

    while ((command | getline) > 0)
        ...

So, if you want to use different commands, you have to take care to use different strings or close the latest command::

    command = "ls /"
    command | getline var
    close(command)

This problem came up when I needed to use two temporary files, but the following code didn't work::

    command = "mktemp /tmp/file.XXXX"
    command | getline file1
    command | getline file2

Instead, I had to use::

    command = "mktemp /tmp/file.XXXX"
    command | getline file1
    close(command)
    command | getline file2
    close(command)