1.6 awk Statements Versus Lines

Most often, each line in an awk program is a separate statement or separate rule, like this:

awk '/12/  { print $0 }
     /21/  { print $0 }' mail-list inventory-shipped

However, gawk ignores newlines after any of the following symbols and keywords:

,    {    ?    :    ||    &&    do    else

A newline at any other point is considered the end of the statement.⁹

If you would like to split a single statement into two lines at a point where a newline would terminate it, you can continue it by ending the first line with a backslash character (‘\’). The backslash must be the final character on the line in order to be recognized as a continuation character. A backslash followed by a newline is allowed anywhere in the statement, even in the middle of a string or regular expression. For example:

awk '/This regular expression is too long, so continue it\
 on the next line/ { print $1 }'

We have generally not used backslash continuation in our sample programs. gawk places no limit on the length of a line, so backslash continuation is never strictly necessary; it just makes programs more readable. For this same reason, as well as for clarity, we have kept most statements short in the programs presented throughout the Web page.

Backslash continuation is most useful when your awk program is in a separate source file instead of entered from the command line. You should also note that many awk implementations are more particular about where you may use backslash continuation. For example, they may not allow you to split a string constant using backslash continuation. Thus, for maximum portability of your awk programs, it is best not to split your lines in the middle of a regular expression or a string.

CAUTION: Backslash continuation does not work as described with the C shell. It works for awk programs in files and for one-shot programs, provided you are using a POSIX-compliant shell, such as the Unix Bourne shell or Bash. But the C shell behaves differently! There you must use two backslashes in a row, followed by a newline. Note also that when using the C shell, every newline in your awk program must be escaped with a backslash. To illustrate:

% awk 'BEGIN { \
?   print \\
?       "hello, world" \
? }'
-| hello, world

Here, the ‘%’ and ‘?’ are the C shell’s primary and secondary prompts, analogous to the standard shell’s ‘$’ and ‘>’.

Compare the previous example to how it is done with a POSIX-compliant shell:

$ awk 'BEGIN {
>   print \
>       "hello, world"
> }'
-| hello, world

awk is a line-oriented language. Each rule’s action has to begin on the same line as the pattern. To have the pattern and action on separate lines, you must use backslash continuation; there is no other option.

Another thing to keep in mind is that backslash continuation and comments do not mix. As soon as awk sees the ‘#’ that starts a comment, it ignores everything on the rest of the line. For example:

$ gawk 'BEGIN { print "dont panic" # a friendly \
>                                    BEGIN rule
> }'
error→ gawk: cmd. line:2:                BEGIN rule
error→ gawk: cmd. line:2:                ^ syntax error

In this case, it looks like the backslash would continue the comment onto the next line. However, the backslash-newline combination is never even noticed because it is “hidden” inside the comment. Thus, the BEGIN is noted as a syntax error.

When awk statements within one rule are short, you might want to put more than one of them on a line. This is accomplished by separating the statements with a semicolon (‘;’). This also applies to the rules themselves. Thus, the program shown at the start of this section could also be written this way:

/12/ { print $0 } ; /21/ { print $0 }

NOTE: The requirement that states that rules on the same line must be separated with a semicolon was not in the original awk language; it was added for consistency with the treatment of statements within an action.