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gawk: Comments
gawk: Running gawk
1.1.6 Shell Quoting Issues --------------------------
· DOS Quoting Quoting in Windows Batch Files.
For short to medium-length 'awk' programs, it is most convenient to
enter the program on the 'awk' command line. This is best done by
enclosing the entire program in single quotes. This is true whether you
are entering the program interactively at the shell prompt, or writing
it as part of a larger shell script:
awk 'PROGRAM TEXT' INPUT-FILE1 INPUT-FILE2 ...
Once you are working with the shell, it is helpful to have a basic
knowledge of shell quoting rules. The following rules apply only to
POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again
Shell). If you use the C shell, you're on your own.
Before diving into the rules, we introduce a concept that appears
throughout this Info file, which is that of the "null", or empty,
string.
The null string is character data that has no value. In other words,
it is empty. It is written in 'awk' programs like this: '""'. In the
shell, it can be written using single or double quotes: '""' or ''''.
Although the null string has no characters in it, it does exist. For
example, consider this command:
$ echo ""
Here, the 'echo' utility receives a single argument, even though that
argument has no characters in it. In the rest of this Info file, we use
the terms "null string" and "empty string" interchangeably. Now, on to
the quoting rules:
* Quoted items can be concatenated with nonquoted items as well as
with other quoted items. The shell turns everything into one
argument for the command.
* Preceding any single character with a backslash ('\') quotes that
character. The shell removes the backslash and passes the quoted
character on to the command.
* Single quotes protect everything between the opening and closing
quotes. The shell does no interpretation of the quoted text,
passing it on verbatim to the command. It is _impossible_ to embed
a single quote inside single-quoted text. Refer back to 
Comments for an example of what happens if you try.
* Double quotes protect most things between the opening and closing
quotes. The shell does at least variable and command substitution
on the quoted text. Different shells may do additional kinds of
processing on double-quoted text.
Because certain characters within double-quoted text are processed
by the shell, they must be "escaped" within the text. Of note are
the characters '$', '`', '\', and '"', all of which must be
preceded by a backslash within double-quoted text if they are to be
passed on literally to the program. (The leading backslash is
stripped first.) Thus, the example seen in Read Terminal:
awk 'BEGIN { print "Don\47t Panic!" }'
could instead be written this way:
$ awk "BEGIN { print \"Don't Panic!\" }"
-| Don't Panic!
Note that the single quote is not special within double quotes.
* Null strings are removed when they occur as part of a non-null
command-line argument, while explicit null objects are kept. For
example, to specify that the field separator 'FS' should be set to
the null string, use:
awk -F "" 'PROGRAM' FILES # correct
Don't use this:
awk -F"" 'PROGRAM' FILES # wrong!
In the second case, 'awk' attempts to use the text of the program
as the value of 'FS', and the first file name as the text of the
program! This results in syntax errors at best, and confusing
behavior at worst.
Mixing single and double quotes is difficult. You have to resort to
shell quoting tricks, like this:
$ awk 'BEGIN { print "Here is a single quote <'"'"'>" }'
-| Here is a single quote <'>
This program consists of three concatenated quoted strings. The first
and the third are single-quoted, and the second is double-quoted.
This can be "simplified" to:
$ awk 'BEGIN { print "Here is a single quote <'\''>" }'
-| Here is a single quote <'>
Judge for yourself which of these two is the more readable.
Another option is to use double quotes, escaping the embedded,
'awk'-level double quotes:
$ awk "BEGIN { print \"Here is a single quote <'>\" }"
-| Here is a single quote <'>
This option is also painful, because double quotes, backslashes, and
dollar signs are very common in more advanced 'awk' programs.
A third option is to use the octal escape sequence equivalents (
Escape Sequences) for the single- and double-quote characters, like
so:
$ awk 'BEGIN { print "Here is a single quote <\47>" }'
-| Here is a single quote <'>
$ awk 'BEGIN { print "Here is a double quote <\42>" }'
-| Here is a double quote <">
This works nicely, but you should comment clearly what the escapes mean.
A fourth option is to use command-line variable assignment, like
this:
$ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }'
-| Here is a single quote <'>
(Here, the two string constants and the value of 'sq' are
concatenated into a single string that is printed by 'print'.)
If you really need both single and double quotes in your 'awk'
program, it is probably best to move it into a separate file, where the
shell won't be part of the picture and you can say what you mean.