File: gawk.info,  Node: Concatenation,  Next: Assignment Ops,  Prev: Arithmetic Ops,  Up: All Operators

6.2.2 String Concatenation
--------------------------

     It seemed like a good idea at the time.
                         -- _Brian Kernighan_

   There is only one string operation: concatenation.  It does not have
a specific operator to represent it.  Instead, concatenation is
performed by writing expressions next to one another, with no operator.
For example:

     $ awk '{ print "Field number one: " $1 }' mail-list
     -| Field number one: Amelia
     -| Field number one: Anthony
     ...

   Without the space in the string constant after the ':', the line runs
together.  For example:

     $ awk '{ print "Field number one:" $1 }' mail-list
     -| Field number one:Amelia
     -| Field number one:Anthony
     ...

   Because string concatenation does not have an explicit operator, it
is often necessary to ensure that it happens at the right time by using
parentheses to enclose the items to concatenate.  For example, you might
expect that the following code fragment concatenates 'file' and 'name':

     file = "file"
     name = "name"
     print "something meaningful" > file name

This produces a syntax error with some versions of Unix 'awk'.(1)  It is
necessary to use the following:

     print "something meaningful" > (file name)

   Parentheses should be used around concatenation in all but the most
common contexts, such as on the righthand side of '='.  Be careful about
the kinds of expressions used in string concatenation.  In particular,
the order of evaluation of expressions used for concatenation is
undefined in the 'awk' language.  Consider this example:

     BEGIN {
         a = "don't"
         print (a " " (a = "panic"))
     }

It is not defined whether the second assignment to 'a' happens before or
after the value of 'a' is retrieved for producing the concatenated
value.  The result could be either 'don't panic', or 'panic panic'.

   The precedence of concatenation, when mixed with other operators, is
often counter-intuitive.  Consider this example:

     $ awk 'BEGIN { print -12 " " -24 }'
     -| -12-24

   This "obviously" is concatenating -12, a space, and -24.  But where
did the space disappear to?  The answer lies in the combination of
operator precedences and 'awk''s automatic conversion rules.  To get the
desired result, write the program this way:

     $ awk 'BEGIN { print -12 " " (-24) }'
     -| -12 -24

   This forces 'awk' to treat the '-' on the '-24' as unary.  Otherwise,
it's parsed as follows:

         -12 ('" "' - 24)
     => -12 (0 - 24)
     => -12 (-24)
     => -12-24

   As mentioned earlier, when mixing concatenation with other operators,
_parenthesize_.  Otherwise, you're never quite sure what you'll get.

   ---------- Footnotes ----------

   (1) It happens that BWK 'awk', 'gawk', and 'mawk' all "get it right,"
but you should not rely on this.