gawk.1

早期freebsd实现

.PP
.RS
.ft B
.nf
if (val in array)
	print array[val]
.fi
.ft
.RE
.PP
If the array has multiple subscripts, use
.BR "(i, j) in array" .
.PP
The
.B in
construct may also be used in a
.B for
loop to iterate over all the elements of an array.
.PP
An element may be deleted from an array using the
.B delete
statement.
.SS Variable Typing And Conversion
.PP
Variables and fields
may be (floating point) numbers, or strings, or both. How the
value of a variable is interpreted depends upon its context. If used in
a numeric expression, it will be treated as a number, if used as a string
it will be treated as a string.
.PP
To force a variable to be treated as a number, add 0 to it; to force it
to be treated as a string, concatenate it with the null string.
.PP
When a string must be converted to a number, the conversion is accomplished
using
.IR atof (3).
A number is converted to a string by using the value of
.B CONVFMT
as a format string for
.IR sprintf (3),
with the numeric value of the variable as the argument.
However, even though all numbers in AWK are floating-point,
integral values are
.I always
converted as integers.  Thus, given
.PP
.RS
.ft B
.nf
CONVFMT = "%2.2f"
a = 12
b = a ""
.fi
.ft R
.RE
.PP
the variable
.B b
has a value of \fB"12"\fR and not \fB"12.00"\fR.
.PP
.I Gawk
performs comparisons as follows:
If two variables are numeric, they are compared numerically.
If one value is numeric and the other has a string value that is a
``numeric string,'' then comparisons are also done numerically.
Otherwise, the numeric value is converted to a string and a string
comparison is performed.
Two strings are compared, of course, as strings.
According to the \*(PX standard, even if two strings are
numeric strings, a numeric comparison is performed.  However, this is
clearly incorrect, and
.I gawk
does not do this.
.PP
Uninitialized variables have the numeric value 0 and the string value ""
(the null, or empty, string).
.SH PATTERNS AND ACTIONS
AWK is a line oriented language. The pattern comes first, and then the
action. Action statements are enclosed in
.B {
and
.BR } .
Either the pattern may be missing, or the action may be missing, but,
of course, not both. If the pattern is missing, the action will be
executed for every single line of input.
A missing action is equivalent to
.RS
.PP
.B "{ print }"
.RE
.PP
which prints the entire line.
.PP
Comments begin with the ``#'' character, and continue until the
end of the line.
Blank lines may be used to separate statements.
Normally, a statement ends with a newline, however, this is not the
case for lines ending in
a ``,'', ``{'', ``?'', ``:'', ``&&'', or ``||''.
Lines ending in
.B do
or
.B else
also have their statements automatically continued on the following line.
In other cases, a line can be continued by ending it with a ``\e'',
in which case the newline will be ignored.
.PP
Multiple statements may
be put on one line by separating them with a ``;''.
This applies to both the statements within the action part of a
pattern-action pair (the usual case),
and to the pattern-action statements themselves.
.SS Patterns
AWK patterns may be one of the following:
.PP
.RS
.nf
.B BEGIN
.B END
.BI / "regular expression" /
.I "relational expression"
.IB pattern " && " pattern
.IB pattern " || " pattern
.IB pattern " ? " pattern " : " pattern
.BI ( pattern )
.BI ! " pattern"
.IB pattern1 ", " pattern2
.fi
.RE
.PP
.B BEGIN
and
.B END
are two special kinds of patterns which are not tested against
the input.
The action parts of all
.B BEGIN
patterns are merged as if all the statements had
been written in a single
.B BEGIN
block. They are executed before any
of the input is read. Similarly, all the
.B END
blocks are merged,
and executed when all the input is exhausted (or when an
.B exit
statement is executed).
.B BEGIN
and
.B END
patterns cannot be combined with other patterns in pattern expressions.
.B BEGIN
and
.B END
patterns cannot have missing action parts.
.PP
For
.BI / "regular expression" /
patterns, the associated statement is executed for each input line that matches
the regular expression.
Regular expressions are the same as those in
.IR egrep (1),
and are summarized below.
.PP
A
.I "relational expression"
may use any of the operators defined below in the section on actions.
These generally test whether certain fields match certain regular expressions.
.PP
The
.BR && ,
.BR || ,
and
.B !
operators are logical AND, logical OR, and logical NOT, respectively, as in C.
They do short-circuit evaluation, also as in C, and are used for combining
more primitive pattern expressions. As in most languages, parentheses
may be used to change the order of evaluation.
.PP
The
.B ?\^:
operator is like the same operator in C. If the first pattern is true
then the pattern used for testing is the second pattern, otherwise it is
the third. Only one of the second and third patterns is evaluated.
.PP
The 
.IB pattern1 ", " pattern2
form of an expression is called a range pattern.
It matches all input records starting with a line that matches
.IR pattern1 ,
and continuing until a record that matches
.IR pattern2 ,
inclusive. It does not combine with any other sort of pattern expression.
.SS Regular Expressions
Regular expressions are the extended kind found in
.IR egrep .
They are composed of characters as follows:
.TP \w'\fB[^\fIabc...\fB]\fR'u+2n
.I c
matches the non-metacharacter
.IR c .
.TP
.I \ec
matches the literal character
.IR c .
.TP
.B .
matches any character except newline.
.TP
.B ^
matches the beginning of a line or a string.
.TP
.B $
matches the end of a line or a string.
.TP
.BI [ abc... ]
character class, matches any of the characters
.IR abc... .
.TP
.BI [^ abc... ]
negated character class, matches any character except
.I abc...
and newline.
.TP
.IB r1 | r2
alternation: matches either
.I r1
or
.IR r2 .
.TP
.I r1r2
concatenation: matches
.IR r1 ,
and then
.IR r2 .
.TP
.IB r +
matches one or more
.IR r 's. 
.TP
.IB r *
matches zero or more
.IR r 's. 
.TP
.IB r ?
matches zero or one
.IR r 's. 
.TP
.BI ( r )
grouping: matches
.IR r .
.PP
The escape sequences that are valid in string constants (see below)
are also legal in regular expressions.
.SS Actions
Action statements are enclosed in braces,
.B {
and
.BR } .
Action statements consist of the usual assignment, conditional, and looping
statements found in most languages. The operators, control statements,
and input/output statements
available are patterned after those in C.
.SS Operators
.PP
The operators in AWK, in order of increasing precedence, are
.PP
.TP "\w'\fB*= /= %= ^=\fR'u+1n"
.PD 0
.B "= += \-="
.TP
.PD
.B "*= /= %= ^="
Assignment. Both absolute assignment
.BI ( var " = " value )
and operator-assignment (the other forms) are supported.
.TP
.B ?:
The C conditional expression. This has the form
.IB expr1 " ? " expr2 " : " expr3\c
\&. If
.I expr1
is true, the value of the expression is
.IR expr2 ,
otherwise it is
.IR expr3 .
Only one of
.I expr2
and
.I expr3
is evaluated.
.TP
.B ||
Logical OR.
.TP
.B &&
Logical AND.
.TP
.B "~ !~"
Regular expression match, negated match.
.B NOTE:
Do not use a constant regular expression
.RB ( /foo/ )
on the left-hand side of a
.B ~
or
.BR !~ .
Only use one on the right-hand side.  The expression
.BI "/foo/ ~ " exp
has the same meaning as \fB(($0 ~ /foo/) ~ \fIexp\fB)\fR.
This is usually
.I not
what was intended.
.TP
.PD 0
.B "< >"
.TP
.PD 0
.B "<= >="
.TP
.PD
.B "!= =="
The regular relational operators.
.TP
.I blank
String concatenation.
.TP
.B "+ \-"
Addition and subtraction.
.TP
.B "* / %"
Multiplication, division, and modulus.
.TP
.B "+ \- !"
Unary plus, unary minus, and logical negation.
.TP
.B ^
Exponentiation (\fB**\fR may also be used, and \fB**=\fR for
the assignment operator).
.TP
.B "++ \-\^\-"
Increment and decrement, both prefix and postfix.
.TP
.B $
Field reference.
.SS Control Statements
.PP
The control statements are
as follows:
.PP
.RS
.nf
\fBif (\fIcondition\fB) \fIstatement\fR [ \fBelse\fI statement \fR]
\fBwhile (\fIcondition\fB) \fIstatement \fR
\fBdo \fIstatement \fBwhile (\fIcondition\fB)\fR
\fBfor (\fIexpr1\fB; \fIexpr2\fB; \fIexpr3\fB) \fIstatement\fR
\fBfor (\fIvar \fBin\fI array\fB) \fIstatement\fR
\fBbreak\fR
\fBcontinue\fR
\fBdelete \fIarray\^\fB[\^\fIindex\^\fB]\fR
\fBexit\fR [ \fIexpression\fR ]
\fB{ \fIstatements \fB}
.fi
.RE
.SS "I/O Statements"
.PP
The input/output statements are as follows:
.PP
.TP "\w'\fBprintf \fIfmt, expr-list\fR'u+1n"
.BI close( filename )
Close file (or pipe, see below).
.TP
.B getline
Set
.B $0
from next input record; set
.BR NF ,
.BR NR ,
.BR FNR .
.TP
.BI "getline <" file
Set
.B $0
from next record of
.IR file ;
set
.BR NF .
.TP
.BI getline " var"
Set
.I var
from next input record; set
.BR NF ,
.BR FNR .
.TP
.BI getline " var" " <" file
Set
.I var
from next record of
.IR file .
.TP
.B next
Stop processing the current input record. The next input record
is read and processing starts over with the first pattern in the
AWK program. If the end of the input data is reached, the
.B END
block(s), if any, are executed.
.TP
.B "next file"
Stop processing the current input file.  The next input record read
comes from the next input file.
.B FILENAME
is updated,
.B FNR
is reset to 1, and processing starts over with the first pattern in the
AWK program. If the end of the input data is reached, the
.B END
block(s), if any, are executed.
.TP
.B print
Prints the current record.
.TP
.BI print " expr-list"
Prints expressions.
.TP
.BI print " expr-list" " >" file
Prints expressions on
.IR file .
.TP
.BI printf " fmt, expr-list"
Format and print.
.TP
.BI printf " fmt, expr-list" " >" file
Format and print on
.IR file .
.TP
.BI system( cmd-line )
Execute the command
.IR cmd-line ,
and return the exit status.
(This may not be available on non-\*(PX systems.)
.PP
Other input/output redirections are also allowed. For
.B print
and
.BR printf ,
.BI >> file
appends output to the
.IR file ,
while
.BI | " command"
writes on a pipe.
In a similar fashion,
.IB command " | getline"
pipes into
.BR getline .
.BR Getline
will return 0 on end of file, and \-1 on an error.
.SS The \fIprintf\fP\^ Statement
.PP
The AWK versions of the
.B printf
statement and
.B sprintf()
function
(see below)
accept the following conversion specification formats:
.TP
.B %c
An \s-1ASCII\s+1 character.
If the argument used for
.B %c
is numeric, it is treated as a character and printed.
Otherwise, the argument is assumed to be a string, and the only first
character of that string is printed.
.TP
.B %d
A decimal number (the integer part).
.TP
.B %i
Just like
.BR %d .
.TP
.B %e
A floating point number of the form
.BR [\-]d.ddddddE[+\^\-]dd .
.TP
.B %f
A floating point number of the form
.BR [\-]ddd.dddddd .
.TP
.B %g
Use
.B e
or
.B f
conversion, whichever is shorter, with nonsignificant zeros suppressed.
.TP
.B %o
An unsigned octal number (again, an integer).
.TP
.B %s
A character string.
.TP
.B %x
An unsigned hexadecimal number (an integer).
.TP
.B %X
Like
.BR %x ,
but using
.B ABCDEF
instead of
.BR abcdef .
.TP
.B %%
A single
.B %
character; no argument is converted.
.PP
There are optional, additional parameters that may lie between the
.B %
and the control letter:
.TP
.B \-
The expression should be left-justified within its field.
.TP
.I width
The field should be padded to this width. If the number has a leading
zero, then the field will be padded with zeros.
Otherwise it is padded with blanks.
.TP
.BI . prec
A number indicating the maximum width of strings or digits to the right
of the decimal point.
.PP
The dynamic
.I width
and
.I prec
capabilities of the \*(AN C
.B printf()
routines are supported.
A
.B *
in place of either the
.B width
or
.B prec
specifications will cause their values to be taken from
the argument list to
.B printf
or
.BR sprintf() .
.SS Special File Names
.PP
When doing I/O redirection from either
.B print
or
.B printf
into a file,
or via
.B getline
from a file,
.I gawk
recognizes certain special filenames internally.  These filenames
allow access to open file descriptors inherited from
.IR gawk 's
parent process (usually the shell).
Other special filenames provide access information about the running
.B gawk
process.
The filenames are:
.TP \w'\fB/dev/stdout\fR'u+1n
.B /dev/pid
Reading this file returns the process ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/ppid
Reading this file returns the parent process ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/pgrpid
Reading this file returns the process group ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/user
Reading this file returns a single record terminated with a newline.
The fields are separated with blanks.
.B $1
is the value of the
.IR getuid (2)
system call,
.B $2
is the value of the
.IR geteuid (2)
system call,
.B $3
is the value of the
.IR getgid (2)
system call, and
.B $4
is the value of the
.IR getegid (2)
system call.
If there are any additional fields, they are the group IDs returned by
.IR getgroups (2).
(Multiple groups may not be supported on all systems.)
.TP
.B /dev/stdin
The standard input.
.TP
.B /dev/stdout
The standard output.
.TP
.B /dev/stderr
The standard error output.
.TP
.BI /dev/fd/\^ n
The file associated with the open file descriptor
.IR n .
.PP
These are particularly useful for error messages. For example:
.PP
.RS
.ft B