printf.html

unix 下的C开发手册,还用详细的例程。

<li>
If the leading character is a single- or double-quote, the value
will be the numeric
value in the underlying codeset of the character following the single- or
double-quote.
<p>
</ul>
<p>
If an argument operand cannot be completely converted into an internal
value appropriate to the corresponding conversion specification, a diagnostic
message will be written to standard error and the utility
will not exit with a zero exit status,
but will continue processing any remaining operands and
will write the value accumulated at the time the error was
detected to standard output.
</blockquote><h4><a name = "tag_001_014_1777">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>Successful completion.

<dt>&gt;0<dd>An error occurred.

</dl>
</blockquote><h4><a name = "tag_001_014_1778">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
Default.
</blockquote><h4><a name = "tag_001_014_1779">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The floating-point formatting conversion specifications of
<i><a href="../xsh/printf.html">printf()</a></i>
are not required
because all arithmetic in the shell is integer arithmetic.
The
<i><a href="awk.html">awk</a></i>
utility performs floating-point calculations and provides its own
<b>printf</b>
function.
The
<i><a href="bc.html">bc</a></i>
utility can perform arbitrary-precision floating-point
arithmetic, but does not provide extensive formatting capabilities.
(This
<i>printf</i>
utility cannot really be used to format
<i><a href="bc.html">bc</a></i>
output; it does not support arbitrary precision.)
Implementations are encouraged to support
the floating-point conversions as an extension.
<p>
Note that this
<i>printf</i>
utility, like the <b>XSH</b> specification
<i><a href="../xsh/printf.html">printf()</a></i>
function
on which it is based, makes no special provision for dealing with multi-byte
characters when using the
<b>%c</b>
conversion specification or when a precision
is specified in a
<b>%b</b>
or
<b>%s</b>
conversion specification.
Applications
should be extremely cautious using either of these features when there
are multi-byte characters in the character set.
<p>
Field widths and precisions cannot be specified as "*" since the "*"
can be replaced directly in the
<i>format</i>
operand using shell variable substitution.
Implementations can also provide this feature as an extension if
they so choose.
<p>
Hexadecimal character constants as defined in the ISO&nbsp;C standard are not
recognised in the
<i>format</i>
operand because there is no consistent way to
detect the end of the constant.
Octal character constants are limited
to, at most, three octal digits, but hexadecimal character constants are only
terminated by a non-hex-digit character.
In the ISO&nbsp;C standard, the
##
concatenation operator can be used to terminate a constant and follow it
with a hexadecimal character to be written.
In the shell, concatenation occurs
before the
<i>printf</i>
utility has a chance to parse the end of the hexadecimal constant.
<p>
The
<b>%b</b>
conversion specification is not part of the ISO&nbsp;C standard; it has
been added here as a portable way to process backslash escapes expanded
in string operands as provided by the
<i><a href="echo.html">echo</a></i>
utility.
See also the APPLICATION USAGE section of
<i><a href="echo.html">echo</a></i>
for ways to use
<i>printf</i>
as a replacement for all of the traditional versions of the
<i><a href="echo.html">echo</a></i>
utility.
<p>
If an argument cannot be parsed correctly for the corresponding conversion
specification, the
<i>printf</i>
utility is required to report an error.
Thus, overflow and extraneous characters at the end of an argument being
used for a numeric conversion are to be reported as errors.
<p>
It is not considered an error if an argument
operand is not completely used for a
c
or
s
conversion or if a string
operand's first or second character is used to get the numeric value of a
character.
</blockquote><h4><a name = "tag_001_014_1780">&nbsp;</a>EXAMPLES</h4><blockquote>
To alert the user and then print and read a series of prompts:
<pre>
<code>
printf "\aPlease fill in the following: \nName: "
read name
printf "Phone number: "
read phone
</code>
</pre>
<p>
To read out a list of right and wrong answers from a file, calculate
the percentage correctly, and print them out.
The numbers are right-justified and separated by a single
tab character.
The percentage is written to one decimal place of accuracy:
<pre>
<code>
while read right wrong ; do
   percent=$(echo "scale=1;($right*100)/($right+$wrong)" | bc)
   printf "%2d right\t%2d wrong\t(%s%%)\n" \
       $right $wrong $percent
done &lt; database_file
</code>
</pre>
The command:
<pre>
<code>
printf "%5d%4d\n" 1 21 321 4321 54321
</code>
</pre>
produces:
<pre>
<code>
    1  21
  3214321
54321   0
</code>
</pre>
<p>
Note that the
<i>format</i>
operand is used three times to print all of the
given strings and that a
0
was supplied by
<i>printf</i>
to satisfy the last
<b>%4d</b>
conversion specification.
<p>
The
<i>printf</i>
utility is required to notify the user when conversion
errors are detected while producing numeric output; thus, the
following results would be expected on an implementation with
32-bit twos-complement integers when
<b>%d</b>
is specified as the
<i>format</i>
operand:
<pre>
<table  bordercolor=#000000 border=1 align=center>
<tr valign=top><th align=center><b>Argument</b>
<th align=center><b>Standard<br>Output</b>
<th align=center><b>Diagnostic Output</b>
<tr valign=top><td align=left>5a
<td align=left>5
<td align=left>printf: "5a" not completely converted
<tr valign=top><td align=left>9999999999
<td align=left>2147483647
<td align=left>printf: "9999999999" arithmetic overflow
<tr valign=top><td align=left>-9999999999
<td align=left>-2147483648
<td align=left>printf: "-9999999999" arithmetic overflow
<tr valign=top><td align=left>ABC
<td align=left>0
<td align=left>printf: "ABC" expected numeric value
</table>
</pre>
<p>
The diagnostic message format is not specified, but these
examples convey the type of information that should be reported.
Note that the value shown on standard output is what would be
expected as the return value from the <b>XSH</b> specification
function
<i><a href="../xsh/strtol.html">strtol()</a></i>.
A similar correspondence exists between
<b>%u</b>
and
<i><a href="../xsh/strtoul.html">strtoul()</a></i>
and
<b>%e</b>,
<b>%f</b>
and
<b>%g</b>
(if the implementation supports floating-point conversions) and
<i><a href="../xsh/strtod.html">strtod()</a></i>.
<p>
In a locale using the ISO/IEC&nbsp;646:1991 standard
as the underlying codeset, the command:
<pre>
<code>
printf "%d\n" 3 +3 -3 \'3 \"+3 "'-3"
</code>
</pre>
produces:
<pre>
<table  bordercolor=#000000 border=1 align=center><tr valign=top><td align=left>3
<td align=left>Numeric value of constant 3
<tr valign=top><td align=left>3
<td align=left>Numeric value of constant 3
<tr valign=top><td align=left>-3
<td align=left>Numeric value of constant -3
<tr valign=top><td align=left>51
<td align=left>Numeric value of the character `3' in the ISO/IEC&nbsp;646:1991 standard codeset
<tr valign=top><td align=left>43
<td align=left>Numeric value of the character `+' in the ISO/IEC&nbsp;646:1991 standard codeset
<tr valign=top><td align=left>45
<td align=left>Numeric value of the character `-' in the ISO/IEC&nbsp;646:1991 standard codeset
</table>
</pre>
<p>
Note that in a locale with multi-byte characters, the value of a
character is intended to be the value of the equivalent of the
<b>wchar_t</b>
representation of the character as described in the <b>XSH</b> specification.
</blockquote><h4><a name = "tag_001_014_1781">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_001_014_1782">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="awk.html">awk</a></i>,
<i><a href="bc.html">bc</a></i>,
<i><a href="echo.html">echo</a></i>,
the <b>XSH</b> specification description of
<i><a href="../xsh/printf.html">printf()</a></i>.
</blockquote><hr size=2 noshade>
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