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• 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 Web page, 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
Web page, we use the terms null string and empty string
interchangeably. Now, on to the quoting rules:
awk
Programs
for an example of what happens if you try.
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
previously
in Running awk
Without Input Files:
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.
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 (see section 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 escape sequences 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.
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