chap2.html

unix 下的C开发手册,还用详细的例程。

<pre>
<code>
x=1
echo $(( $(echo 3)+$x ))
</code>
</pre>
<h4><a name = "tag_001_006_001">&nbsp;</a>Tilde Expansion</h4>
<xref type="3" name="tildexp"></xref>
A
<i>tilde-prefix</i>
consists of an unquoted tilde character at the beginning of a word,
followed by
all of the characters preceding the first unquoted slash in the word,
or all the characters in the word if there is no slash.
In an assignment
(see
<b>variable assignment</b>
in the <b>XBD</b> specification, <a href="../xbd/glossary.html"><b>Glossary</b>&nbsp;</a> )
multiple tilde-prefixes can be used:
at the beginning
of the word (that is, following the equal sign of the
assignment), following any unquoted colon or both.
A tilde-prefix in an assignment
is terminated by the first unquoted colon or slash.
If none of the characters in the tilde-prefix are quoted,
the characters in the tilde-prefix
following the tilde are treated as a possible login name
from the user database.
A portable login name cannot contain characters outside
the set given in the description of the
<i>LOGNAME</i>
environment variable in the <b>XSH</b> specification.
If the login name is null
(that is, the tilde-prefix contains only the tilde),
the tilde-prefix will be replaced by the value of the variable
<i>HOME</i>.
If
<i>HOME</i>
is unset, the results are unspecified.
Otherwise, the tilde-prefix will be replaced by a pathname of the
home directory associated with the login name
obtained using the <b>XSH</b> specification
<i><a href="../xsh/getpwnam.html">getpwnam()</a></i>
function.
If the system does not recognise the login name,
the results are undefined.
<p>
Tilde expansion generally occurs only at the beginning
of words, but
an exception based on historical practice has been included:
<pre>
<code>
PATH=/posix/bin:~dgk/bin
</code>
</pre>
is eligible for tilde expansion
because tilde follows a colon
and none of the relevant characters is quoted.
Consideration was given to prohibiting this behaviour
because any of the following are reasonable substitutes:
<pre>
<code>
PATH=$(printf %s: ~rms/bin ~bfox/bin ...)  

PATH=$(printf %s ~karels/bin : ~bostic/bin)

for Dir in ~maart/bin ~srb/bin ...
do
    PATH=${PATH:+$PATH:}$Dir
done
</code>
</pre>
<p>
In the first command, any number of directory names are
concatenated and separated with colons, but it may be
undesirable to end the variable with a colon because this
is an obsolescent means to include dot at the end of the
<i>PATH</i>.
&nbsp;In the second, explicit colons
are used for each directory.
In all cases, the shell performs tilde expansion
on each directory because all are separate words to the shell.
<p>
Note that expressions in operands such as:
<pre>
<code>
make -k mumble LIBDIR=~chet/lib
</code>
</pre>
do not qualify as shell variable assignments
and tilde expansion is not performed (unless the command
does so itself, which
<i><a href="make.html">make</a></i>
does not).
<p>
The special sequence <b>$~</b>
has been designated for future implementations to evaluate
as a means of forcing tilde expansion in any word.
<p>
Because of the requirement that the word is not quoted, the
following are not equivalent; only the last will cause tilde expansion:
<pre>
<code>
\~hlj/   ~h\lj/   ~"hlj"/   ~hlj\/   ~hlj/
</code>
</pre>
<p>
The results of giving tilde with an unknown
login name are undefined because the KornShell
~+
and
~-
constructs make use of this condition, but, in general
it is an error to give an incorrect login name with tilde.
The results of having
<i>HOME</i>
unset are unspecified because some historical shells
treat this as an error.
<h4><a name = "tag_001_006_002">&nbsp;</a>Parameter Expansion</h4>
<xref type="3" name="parmexp"></xref>
The format for parameter expansion is as follows:
<pre>
<code>
${<i>expression</i>}
</code>
</pre>
where
<i>expression</i>
consists of all characters until the matching
"}".
Any
"}"
escaped by a backslash or within a quoted string,
and characters in embedded
arithmetic expansions,
command substitutions
and variable expansions,
are not examined in determining the matching
"}".
<p>
The simplest form for parameter expansion is:
<pre>
<code>
${<i>parameter</i>}
</code>
</pre>
<p>
The value, if any, of
<i>parameter</i>
will be substituted.
<p>
The
parameter
name or symbol can be enclosed in
braces, which are optional except for positional parameters
with more than one digit or when
<i>parameter</i>
is followed by a character that
could be interpreted as part of the name.
The matching closing brace will be determined
by counting brace levels, skipping
over enclosed quoted strings and command substitutions.
<p>
If the parameter name or symbol is not enclosed in braces,
the expansion will use the longest
valid name (see <b>name</b>
in the <b>XBD</b> specification, <a href="../xbd/glossary.html"><b>Glossary</b>&nbsp;</a> ),
whether or not the symbol represented by that name exists.
When the shell is scanning its input to determine the boundaries
of a name, it is not bound by its knowledge of what names
are already defined.
For example, if
<b>F</b>
is a defined shell variable, the command:
<pre>
<code>
echo&nbsp;$Fred
</code>
</pre>
does not echo the value of
<b>$F</b>
followed by
<b>red</b>;
it selects the longest possible valid name,
<b>Fred</b>,
which in this case might be unset.
<p>
If a parameter expansion occurs inside double-quotes:
<ul>
<p>
<li>
Pathname expansion will not be performed
on the results of the expansion.
<p>
<li>
Field splitting will not be performed
on the results of the expansion,
with the exception of "@"; see
<xref href=specpar><a href="#tag_001_005_002">
Special Parameters
</a></xref>.
<p>
</ul>
<p>
In addition, a
parameter expansion
can be modified by using one of the following formats.
In each case
that a value of
<i>word</i>
is needed (based on the state of
<i>parameter</i>,
as described below),
<i>word</i>
will be subjected to
tilde expansion,
parameter expansion, command substitution
and arithmetic expansion.
If
<i>word</i>
is not needed, it will not be expanded.
The
"}"
character that delimits the following parameter expansion
modifications is determined
as described previously in this section and in
<xref href=dquote><a href="#tag_001_002_003">
Double-quotes
</a></xref>.
(For example,
${foo-bar}xyz}
would result in the expansion of
<b>foo</b>
followed by the string
xyz}
if
<b>foo</b>
is set, else the string
barxyz}).
<dl compact>

<dt>${<i>parameter</i>:-<i>word</i>}<dd><b>Use Default Values</b>. If
<i>parameter</i>
is unset or null, the expansion of
<i>word</i>
will be substituted; otherwise, the value of
<i>parameter</i>
will be substituted.

<dt>${<i>parameter</i>:=<i>word</i>}<dd><b>Assign Default Values</b>. If
<i>parameter</i>
is unset or null, the expansion of
<i>word</i>
will be assigned to
<i>parameter</i>.
In all cases, the final value of
<i>parameter</i>
will be substituted.
Only variables, not positional parameters or special parameters,
can be assigned in this way.

<dt>${<i>parameter</i>:?<b>[</b><i>word</i><b>]</b>}<dd><b>Indicate Error if Null or Unset</b>. If
<i>parameter</i>
is unset or null, the expansion of
<i>word</i>
(or a message indicating it is unset if
<i>word</i>
is omitted)
will be written to
standard error and the shell will exit
with a non-zero exit status.
Otherwise, the value of
<i>parameter</i>
will be substituted.
An interactive shell
need not exit.

<dt>${<i>parameter</i>:+<i>word</i>}<dd><b>Use Alternative Value</b>. If
<i>parameter</i>
is unset or null,
null will be substituted;
otherwise, the expansion of
<i>word</i>
will be substituted.

</dl>
<p>
In the parameter expansions shown previously,
use of the colon in the format results in a test
for a parameter that is unset or null;
omission of the colon results in a test
for a parameter that is only unset.
The following table summarises the effect of the colon:
<pre>
<table  bordercolor=#000000 border=1 align=center><tr valign=top><th align=center><b>&nbsp;</b>
<th align=center><b>parameter<br>set and not null</b>
<th align=center><b>parameter<br>set but null</b>
<th align=center><b>parameter<br>unset</b>
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>:-</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>substitute <i>word</i>
<td align=left>substitute <i>word</i>
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>-</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>substitute null
<td align=left>substitute <i>word</i>
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>:=</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>assign <i>word</i>
<td align=left>assign <i>word</i>
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>=</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>substitute <i>parameter</i>
<td align=left>assign null
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>:?</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>error, exit
<td align=left>error, exit
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>?</b><i>word</i><b>}</b>
<td align=left>substitute <i>parameter</i>
<td align=left>substitute null
<td align=left>error, exit
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>:+</b><i>word</i><b>}</b>
<td align=left>substitute <i>word</i>
<td align=left>substitute null
<td align=left>substitute null
<tr valign=top><td align=left><b>${</b><i>parameter</i><b>+</b><i>word</i><b>}</b>
<td align=left>substitute <i>word</i>
<td align=left>substitute <i>word</i>
<td align=left>substitute null
</table>
</pre>
<p>
In all cases shown with &quot;substitute&quot;,
the expression is replaced with the value shown.
In all cases shown with &quot;assign&quot;,
<i>parameter</i>
is assigned that value, which also replaces the expression.
<dl compact>

<dt><b>${#</b><i>parameter</i><b>}</b><dd><b>String Length</b>. The length in characters of the value of
<i>parameter</i>.
If
<i>parameter</i>
is "*" or "@",
the result of the expansion is unspecified.

</dl>
<p>
The following four varieties of parameter expansion
provide for substring processing.
In each case, pattern matching notation (see
<xref href=patmat><a href="#tag_001_013">
Pattern Matching Notation
</a></xref>),
rather than regular expression notation, will be
used to evaluate the patterns.
If
<i>parameter</i>
is "*" or "@", the result of the expansion is unspecified.
Enclosing the full parameter expansion string in
double-quotes will not cause the following four
varieties of pattern characters
to be quoted, whereas
quoting characters within the braces will have this effect.
<dl compact>

<dt>${<i>parameter</i>%<i>word</i>}<dd><b>Remove Smallest Suffix Pattern</b>.
The
<i>word</i>
will be expanded to produce a pattern.
The parameter expansion then will result in
<i>parameter</i>,
with the smallest portion of the suffix matched by the
<i>pattern</i>
deleted.

<dt>${<i>parameter</i>%%<i>word</i>}<dd><b>Remove Largest Suffix Pattern</b>.
The
<i>word</i>
will be expanded to produce a pattern.
The parameter expansion then will result in
<i>parameter</i>,
with the largest portion of the suffix matched by the
<i>pattern</i>
deleted.

<dt>${<i>parameter</i>#<i>word</i>}<dd><b>Remove Smallest Prefix Pattern</b>. The
<i>word</i>
will be expanded to produce a pattern.
The parameter expansion then will result in
<i>parameter</i>,
with the smallest portion of the prefix matched by the
<i>pattern</i>
deleted.

<dt>${<i>parameter</i>##<i>word</i>}<dd><b>Remove Largest Prefix Pattern</b>. The
<i>word</i>
will be expanded to produce a pattern.
The parameter expansion then will result in
<i>parameter</i>,
with the largest portion of the prefix matched by the
<i>pattern</i>
deleted.

</dl>
<p>
<h5><a name = "tag_001_006_002_001">&nbsp;</a>Examples</h5>
<dl compact>

<dt>${<i>parameter</i>:-<i>word</i>}<dd>
In this example,
<i><a href="ls.html">ls</a></i>
is executed only if
x
is null or unset.
(The
$(ls)
command substitution notation is explained in
<xref href=cmdsub><a href="#tag_001_006_003">
Command Substitution
</a></xref>.)
<pre>
<code>
${x:-$(ls)}
</code>
</pre>

<dt>${<i>parameter</i>:=<i>word</i>}<dd><code>
<pre>
unset X
echo ${X:=abc}
<b>abc
</b></code>
</pre>

<dt>${<i>parameter</i>:?<i>word</i>}<dd><code>
<pre>
unset posix
echo ${posix:?}
<b>sh: posix: parameter null or not set
</b></code>
</pre>

<dt>${<i>parameter</i>:<i>word</i>}<dd><code>
<pre>
set a b c
echo ${3:+posix}
<b>posix
</b></code>
</pre>

<dt><b>${#</b><i>parameter</i><b>}<dd></b><code>
<pre>
HOME=/usr/posix
echo ${#HOME}
<b>10
</b></code>
</pre>

<dt>${<i>parameter</i>%<i>word</i>}<dd><code>
<pre>
x=file.c
echo ${x%.c}.o
<b>file.o
</b></code>
</pre>

<dt>${<i>parameter</i>%%<i>word</i>}<dd><code>
<pre>
x=posix/src/std
echo ${x%%/*}
<b>posix
</b></code>
</pre>

<dt>${<i>parameter</i>#<i>word</i>}<dd><code>
<pre>
x=$HOME/src/cmd
echo ${x#$HOME}
<b>/src/cmd
</b></code>
</pre>