File: gawk.info,  Node: Command Line Field Separator,  Next: Full Line Fields,  Prev: Single Character Fields,  Up: Field Separators

4.5.4 Setting 'FS' from the Command Line
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'FS' can be set on the command line.  Use the '-F' option to do so.  For
example:

     awk -F, 'PROGRAM' INPUT-FILES

sets 'FS' to the ',' character.  Notice that the option uses an
uppercase 'F' instead of a lowercase 'f'.  The latter option ('-f')
specifies a file containing an 'awk' program.

   The value used for the argument to '-F' is processed in exactly the
same way as assignments to the predefined variable 'FS'.  Any special
characters in the field separator must be escaped appropriately.  For
example, to use a '\' as the field separator on the command line, you
would have to type:

     # same as FS = "\\"
     awk -F\\\\ '...' files ...

Because '\' is used for quoting in the shell, 'awk' sees '-F\\'.  Then
'awk' processes the '\\' for escape characters (*note Escape
Sequences::), finally yielding a single '\' to use for the field
separator.

   As a special case, in compatibility mode (*note Options::), if the
argument to '-F' is 't', then 'FS' is set to the TAB character.  If you
type '-F\t' at the shell, without any quotes, the '\' gets deleted, so
'awk' figures that you really want your fields to be separated with TABs
and not 't's.  Use '-v FS="t"' or '-F"[t]"' on the command line if you
really do want to separate your fields with 't's.  Use '-F '\t'' when
not in compatibility mode to specify that TABs separate fields.

   As an example, let's use an 'awk' program file called 'edu.awk' that
contains the pattern '/edu/' and the action 'print $1':

     /edu/   { print $1 }

   Let's also set 'FS' to be the '-' character and run the program on
the file 'mail-list'.  The following command prints a list of the names
of the people that work at or attend a university, and the first three
digits of their phone numbers:

     $ awk -F- -f edu.awk mail-list
     -| Fabius       555
     -| Samuel       555
     -| Jean

Note the third line of output.  The third line in the original file
looked like this:

     Jean-Paul    555-2127     jeanpaul.campanorum@nyu.edu     R

   The '-' as part of the person's name was used as the field separator,
instead of the '-' in the phone number that was originally intended.
This demonstrates why you have to be careful in choosing your field and
record separators.

   Perhaps the most common use of a single character as the field
separator occurs when processing the Unix system password file.  On many
Unix systems, each user has a separate entry in the system password
file, with one line per user.  The information in these lines is
separated by colons.  The first field is the user's login name and the
second is the user's encrypted or shadow password.  (A shadow password
is indicated by the presence of a single 'x' in the second field.)  A
password file entry might look like this:

     arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash

   The following program searches the system password file and prints
the entries for users whose full name is not indicated:

     awk -F: '$5 == ""' /etc/passwd