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gawk: Using Variables Using Variables in a Program
 Variables let you give names to values and refer to them later.
 Variables have already been used in many of the examples.  The name of a
 variable must be a sequence of letters, digits, or underscores, and it
 may not begin with a digit.  Here, a "letter" is any one of the 52
 upper- and lowercase English letters.  Other characters that may be
 defined as letters in non-English locales are not valid in variable
 names.  Case is significant in variable names; 'a' and 'A' are distinct
    A variable name is a valid expression by itself; it represents the
 variable's current value.  Variables are given new values with
 "assignment operators", "increment operators", and "decrement operators"
 (⇒Assignment Ops).  In addition, the 'sub()' and 'gsub()'
 functions can change a variable's value, and the 'match()', 'split()',
 and 'patsplit()' functions can change the contents of their array
 parameters (⇒String Functions).
    A few variables have special built-in meanings, such as 'FS' (the
 field separator) and 'NF' (the number of fields in the current input
 record).  ⇒Built-in Variables for a list of the predefined
 variables.  These predefined variables can be used and assigned just
 like all other variables, but their values are also used or changed
 automatically by 'awk'.  All predefined variables' names are entirely
    Variables in 'awk' can be assigned either numeric or string values.
 The kind of value a variable holds can change over the life of a
 program.  By default, variables are initialized to the empty string,
 which is zero if converted to a number.  There is no need to explicitly
 initialize a variable in 'awk', which is what you would do in C and in
 most other traditional languages.
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