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Unix - Frequently Asked Questions (3/7) [Frequent posting]
Section - Is it possible to pass shell variable settings into an awk program?

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3.12) Is it possible to pass shell variable settings into an awk program?

      There are two different ways to do this.  The first involves
      simply expanding the variable where it is needed in the program.
      For example, to get a list of all ttys you're using:

        who | awk '/^'"$USER"'/ { print $2 }'                           (1)

      Single quotes are usually used to enclose awk programs because
      the character '$' is often used in them, and '$' will be
      interpreted by the shell if enclosed inside double quotes, but
      not if enclosed inside single quotes.  In this case, we *want*
      the '$' in "$USER" to be interpreted by the shell, so we close
      the single quotes and then put the "$USER" inside double quotes.
      Note that there are no spaces in any of that, so the shell will
      see it all as one argument.  Note, further, that the double
      quotes probably aren't necessary in this particular case (i.e. we
      could have done

        who | awk '/^'$USER'/ { print $2 }'                             (2)

      ), but they should be included nevertheless because they are
      necessary when the shell variable in question contains special
      characters or spaces.

      The second way to pass variable settings into awk is to use an
      often undocumented feature of awk which allows variable settings
      to be specified as "fake file names" on the command line.  For
      example:

        who | awk '$1 == user { print $2 }' user="$USER" -              (3)

      Variable settings take effect when they are encountered on the
      command line, so, for example, you could instruct awk on how to
      behave for different files using this technique.  For example:

        awk '{ program that depends on s }' s=1 file1 s=0 file2         (4)

      Note that some versions of awk will cause variable settings
      encountered before any real filenames to take effect before the
      BEGIN block is executed, but some won't so neither way should be
      relied upon.

      Note, further, that when you specify a variable setting, awk
      won't automatically read from stdin if no real files are
      specified, so you need to add a "-" argument to the end of your
      command, as I did at (3) above.

      A third option is to use a newer version of awk (nawk), which allows
      direct access to environment vairables.  Eg.

        nawk 'END { print "Your path variable is " ENVIRON["PATH"] }' /dev/null

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Top Document: Unix - Frequently Asked Questions (3/7) [Frequent posting]
Previous Document: How do I check the exit status of a remote command executed via "rsh"?
Next Document: How do I get rid of zombie processes that persevere?

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Last Update March 27 2014 @ 02:12 PM