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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-09-18 20:03:23 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-09-18 20:03:23 +0000 |
| commit | f316a2ed52195135a35e32d7096e876357c48c69 (patch) | |
| tree | 5f4486b9a5a213a69e66ef574d6bc643a207981c /doc/multisubstitute.html | |
| download | execline-f316a2ed52195135a35e32d7096e876357c48c69.tar.xz | |
initial commit: rc for execline-2.0.0.0
Diffstat (limited to 'doc/multisubstitute.html')
| -rw-r--r-- | doc/multisubstitute.html | 121 |
1 files changed, 121 insertions, 0 deletions
diff --git a/doc/multisubstitute.html b/doc/multisubstitute.html new file mode 100644 index 0000000..6400187 --- /dev/null +++ b/doc/multisubstitute.html @@ -0,0 +1,121 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>execline: the multisubstitute command</title> + <meta name="Description" content="execline: the multisubstitute command" /> + <meta name="Keywords" content="execline command multisubstitute" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">execline</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> The <tt>multisubstitute</tt> program </h1> + +<p> +<tt>multisubstitute</tt> performs several substitutions at once in +its <em>argv</em>, then executes another program. +</p> + +<h2> Interface </h2> + +<p> + In an <a href="execlineb.html">execlineb</a> script: +</p> + +<pre> + multisubstitute + { + [ <a href="define.html">define</a> [ -n ] [ -s ] [ -C | -c ] [ -d <em>delim</em> ] <em>variable</em> <em>value</em> ] + [ <a href="importas.html">importas</a> [ -i | -D <em>default</em> ] [ -n ] [ -s ] [ -C | -c ] [ -d <em>delim</em> ] <em>variable</em> <em>envvar</em> ] + [ <a href="import.html">import</a> [ -i | -D <em>default</em> ] [ -n ] [ -s ] [ -C | -c ] [ -d <em>delim</em> ] <em>envvar</em> ] + [ <a href="elglob.html">elglob</a> [ -v ] [ -w ] [ -s ] [ -m ] [ -e ] [ -0 ] <em>variable</em> <em>pattern</em> ] + [ <a href="elgetpositionals.html">elgetpositionals</a> [ -P <em>sharp</em> ] ] + [ <a href="multidefine.html">multidefine</a> <em>value</em> { <em>variable...</em> } ] + <em>...</em> + } + <em>prog...</em> +</pre> + +<ul> + <li> <tt>multisubstitute</tt> reads a <a href="el_semicolon.html">block</a> +containing a series of substitution commands. It performs all +those <a href="el_substitute.html">substitutions</a> on +<em>prog...</em> in parallel. Check the relevant documentation page +to learn about the syntax of each substitution command. </li> + <li> <tt>multisubstitute</tt> then execs into the modified <em>prog...</em>. </li> +</ul> + +<h2> Options </h2> + +<ul> + <li> If a <tt>backtick</tt> directive was given with the <tt>-i</tt> option, +and <em>command</em> crashes or exits nonzero, <tt>multisubstitute</tt> will +also exit with the same exit code. </li> + <li> If an <tt>import</tt> or <tt>importas</tt> directive was given with the +<tt>-i</tt> option, and the looked up variable is undefined, +<tt>multisubstitute</tt> will exit 100. </li> +</ul> + +<h2> Rationale </h2> + +<h3> Security </h3> + +<p> + <tt>multisubstitute</tt> can be used to avoid unwanted +<em>serial substitutions</em>. Consider the following script: +</p> + +<pre> + #!/command/execlineb + export A wrong + define B ${A} + import A + echo ${B} +</pre> + +<p> + Running it will print <tt>wrong</tt>, because <tt>A</tt> is substituted +<em>after</em> B. On the contrary, the following script: +</p> + +<pre> + #!/command/execlineb + export A wrong + multisubstitute + { + define B ${A} + import A + } + echo ${B} +</pre> + +<p> + will print <tt>${A}</tt>, because A and B are substituted at the same +time. Serial substitution may be what you want - but when in doubt, +always perform parallel substitution. +</p> + +<h3> Efficiency </h3> + +<p> +<a href="el_substitute.html">Substitution</a> is a costly mechanism: +the whole <em>argv</em> is read three times and rewritten twice. +Serial substitution multiplies the cost by the number of +substitutions, whereas parallel substitution pays the price only once. +</p> + +<h2> Credits </h2> + +<p> +<a href="http://code.dogmap.org/">Paul Jarc</a> first originated the +idea of the <tt>multisubstitute</tt> command and a possible syntax. +</p> + +</body> +</html> |