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| <h4 class="subsection">10.4.6 Details of Variable Substitution</h4> |
| |
| <p>Part of ordinary shell syntax is the use of ‘<samp><span class="samp">$</span><var>variable</var></samp>’ to |
| substitute the value of a shell variable into a command. This is called |
| <dfn>variable substitution</dfn>, and it is one part of doing word expansion. |
| |
| <p>There are two basic ways you can write a variable reference for |
| substitution: |
| |
| <dl> |
| <dt><code>${</code><var>variable</var><code>}</code><dd>If you write braces around the variable name, then it is completely |
| unambiguous where the variable name ends. You can concatenate |
| additional letters onto the end of the variable value by writing them |
| immediately after the close brace. For example, ‘<samp><span class="samp">${foo}s</span></samp>’ |
| expands into ‘<samp><span class="samp">tractors</span></samp>’. |
| |
| <br><dt><code>$</code><var>variable</var><dd>If you do not put braces around the variable name, then the variable |
| name consists of all the alphanumeric characters and underscores that |
| follow the ‘<samp><span class="samp">$</span></samp>’. The next punctuation character ends the variable |
| name. Thus, ‘<samp><span class="samp">$foo-bar</span></samp>’ refers to the variable <code>foo</code> and expands |
| into ‘<samp><span class="samp">tractor-bar</span></samp>’. |
| </dl> |
| |
| <p>When you use braces, you can also use various constructs to modify the |
| value that is substituted, or test it in various ways. |
| |
| <dl> |
| <dt><code>${</code><var>variable</var><code>:-</code><var>default</var><code>}</code><dd>Substitute the value of <var>variable</var>, but if that is empty or |
| undefined, use <var>default</var> instead. |
| |
| <br><dt><code>${</code><var>variable</var><code>:=</code><var>default</var><code>}</code><dd>Substitute the value of <var>variable</var>, but if that is empty or |
| undefined, use <var>default</var> instead and set the variable to |
| <var>default</var>. |
| |
| <br><dt><code>${</code><var>variable</var><code>:?</code><var>message</var><code>}</code><dd>If <var>variable</var> is defined and not empty, substitute its value. |
| |
| <p>Otherwise, print <var>message</var> as an error message on the standard error |
| stream, and consider word expansion a failure. |
| |
| <!-- ??? How does wordexp report such an error? --> |
| <!-- WRDE_BADVAL is returned. --> |
| <br><dt><code>${</code><var>variable</var><code>:+</code><var>replacement</var><code>}</code><dd>Substitute <var>replacement</var>, but only if <var>variable</var> is defined and |
| nonempty. Otherwise, substitute nothing for this construct. |
| </dl> |
| |
| <dl> |
| <dt><code>${#</code><var>variable</var><code>}</code><dd>Substitute a numeral which expresses in base ten the number of |
| characters in the value of <var>variable</var>. ‘<samp><span class="samp">${#foo}</span></samp>’ stands for |
| ‘<samp><span class="samp">7</span></samp>’, because ‘<samp><span class="samp">tractor</span></samp>’ is seven characters. |
| </dl> |
| |
| <p>These variants of variable substitution let you remove part of the |
| variable's value before substituting it. The <var>prefix</var> and |
| <var>suffix</var> are not mere strings; they are wildcard patterns, just |
| like the patterns that you use to match multiple file names. But |
| in this context, they match against parts of the variable value |
| rather than against file names. |
| |
| <dl> |
| <dt><code>${</code><var>variable</var><code>%%</code><var>suffix</var><code>}</code><dd>Substitute the value of <var>variable</var>, but first discard from that |
| variable any portion at the end that matches the pattern <var>suffix</var>. |
| |
| <p>If there is more than one alternative for how to match against |
| <var>suffix</var>, this construct uses the longest possible match. |
| |
| <p>Thus, ‘<samp><span class="samp">${foo%%r*}</span></samp>’ substitutes ‘<samp><span class="samp">t</span></samp>’, because the largest |
| match for ‘<samp><span class="samp">r*</span></samp>’ at the end of ‘<samp><span class="samp">tractor</span></samp>’ is ‘<samp><span class="samp">ractor</span></samp>’. |
| |
| <br><dt><code>${</code><var>variable</var><code>%</code><var>suffix</var><code>}</code><dd>Substitute the value of <var>variable</var>, but first discard from that |
| variable any portion at the end that matches the pattern <var>suffix</var>. |
| |
| <p>If there is more than one alternative for how to match against |
| <var>suffix</var>, this construct uses the shortest possible alternative. |
| |
| <p>Thus, ‘<samp><span class="samp">${foo%r*}</span></samp>’ substitutes ‘<samp><span class="samp">tracto</span></samp>’, because the shortest |
| match for ‘<samp><span class="samp">r*</span></samp>’ at the end of ‘<samp><span class="samp">tractor</span></samp>’ is just ‘<samp><span class="samp">r</span></samp>’. |
| |
| <br><dt><code>${</code><var>variable</var><code>##</code><var>prefix</var><code>}</code><dd>Substitute the value of <var>variable</var>, but first discard from that |
| variable any portion at the beginning that matches the pattern <var>prefix</var>. |
| |
| <p>If there is more than one alternative for how to match against |
| <var>prefix</var>, this construct uses the longest possible match. |
| |
| <p>Thus, ‘<samp><span class="samp">${foo##*t}</span></samp>’ substitutes ‘<samp><span class="samp">or</span></samp>’, because the largest |
| match for ‘<samp><span class="samp">*t</span></samp>’ at the beginning of ‘<samp><span class="samp">tractor</span></samp>’ is ‘<samp><span class="samp">tract</span></samp>’. |
| |
| <br><dt><code>${</code><var>variable</var><code>#</code><var>prefix</var><code>}</code><dd>Substitute the value of <var>variable</var>, but first discard from that |
| variable any portion at the beginning that matches the pattern <var>prefix</var>. |
| |
| <p>If there is more than one alternative for how to match against |
| <var>prefix</var>, this construct uses the shortest possible alternative. |
| |
| <p>Thus, ‘<samp><span class="samp">${foo#*t}</span></samp>’ substitutes ‘<samp><span class="samp">ractor</span></samp>’, because the shortest |
| match for ‘<samp><span class="samp">*t</span></samp>’ at the beginning of ‘<samp><span class="samp">tractor</span></samp>’ is just ‘<samp><span class="samp">t</span></samp>’. |
| |
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