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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/execlineb.html | |
| download | execline-f316a2ed52195135a35e32d7096e876357c48c69.tar.xz | |
initial commit: rc for execline-2.0.0.0
Diffstat (limited to 'doc/execlineb.html')
| -rw-r--r-- | doc/execlineb.html | 246 |
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diff --git a/doc/execlineb.html b/doc/execlineb.html new file mode 100644 index 0000000..64f29bd --- /dev/null +++ b/doc/execlineb.html @@ -0,0 +1,246 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-6" /> + <meta http-equiv="Content-Language" content="en" /> + <title>execline: the execlineb command</title> + <meta name="Description" content="execline: the execlineb command" /> + <meta name="Keywords" content="execline command execlineb launcher" /> + <!-- <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>execlineb</tt> program </h1> + +<p> +<tt>execlineb</tt> reads and executes a script. +</p> + +<h2> Interface </h2> + +<pre> + execlineb [ -q | -w | -W ] [ -p | -P | -S <em>nmin</em> ] -c <em>script</em> [ <em>args...</em> ] +</pre> + +<p> +or +</p> + +<pre> + execlineb [ -q | -w | -W ] [ -p | -P | -S <em>nmin</em> ] <em>scriptfile</em> [ <em>args...</em> ] +</pre> + +<p> +or in an executable file: +</p> + +<pre> +#!/command/execlineb [ -qwWpPS<em>nmin</em> ] +<em>script</em> +</pre> + +<p> + <em>Parsing phase</em>. +</p> + +<ul> + <li> <tt>execlineb</tt> reads and parses the script it is given. +It exits 100 on a syntax error and 111 on a temporary error. +It makes an <em>argv</em>, i.e. a system command line, with the +parsed script. If the <em>argv</em> is empty, <tt>execlineb</tt> +exits 0. </li> +</ul> + +<p> + <em>Environment management phase</em>. +</p> + +<ul> + <li> <em>Pushing the current stack frame.</em> If neither the +<tt>-p</tt> nor the <tt>-P</tt> nor the <tt>-S</tt> option is set: +<tt>execlineb</tt> <a href="el_pushenv.html">pushes</a> +the current positional parameters, i.e. environment variables that +start with <tt>#</tt>, <tt>0</tt>, <tt>1</tt>, ..., <tt>9</tt>. +To get the previous values back, use +<a href="emptyenv.html"><tt>emptyenv -P</tt></a>. </li> + <li> <em>Setting the new stack frame.</em> If neither the <tt>-P</tt> +nor the <tt>-S</tt> option is set: + <ul> + <li> <tt>execlineb</tt> sets the <tt>#</tt> environment variable to +the number <em>n</em> of <em>args</em> it is given. </li> + <li> It sets the <tt>0</tt> environment variable to the name +of the script - or to the <tt>execlineb</tt> invocation name +if the <tt>-c</tt> option is used. </li> + <li> It sets the <tt>1</tt>, <tt>2</tt>, ... <tt><em>n</em></tt> +environment variables to the different <em>args</em>. </li> + </ul> </li> +</ul> + +<p> + <em>Execution phase</em>. +</p> + +<ul> + <li> <tt>execlineb</tt> executes into the <em>argv</em> it +has built from the script. +There is only one command line for the +whole script: the <tt>execlineb</tt> binary is a <em>launcher</em>, +whose sole purpose is to execute into that command line. It does +not stay in memory like a traditional <em>interpreter</em> would. </li> +</ul> + +<h2> Options </h2> + +<ul> + <li> <tt>-c <em>script</em></tt> : execute <em>script</em>, do not +look for a file. </li> +</ul> + +<p> + See below for the other options. +</p> + +<h2> Syntax of scripts </h2> + +<p> + An execlineb script is a string that must not contain the null character. +<tt>execlineb</tt> parses it and divides it into <em>words</em>. + + The parser recognizes the following components: +</p> + +<ul> + <li> <em>whitespace</em> is defined as spaces, tabs, newlines and +carriage returns. Words are always separated by whitespace.</li> + <li> A <em>quoted string</em> begins with a doublequote (<tt>"</tt>) +and ends with another doublequote. Quoted doublequotes must be prefixed +by a backslash (<tt>\</tt>). Quoted strings always evaluate to exactly +one word. For instance, <tt>""</tt> evaluates to the empty word. </li> + <li> The <tt>\a</tt>, <tt>\b</tt>, <tt>\t</tt>, <tt>\n</tt>, <tt>\v</tt>, +<tt>\f</tt>, and <tt>\r</tt> sequences are recognized in quoted +strings, and are converted to the ASCII numbers 7, 8, 9, 10, 11, 12 and +13 respectively. </li> + <li> Inside a quoted string, backslashed +newlines disappear completely. </li> + <li> <tt>\0x<em>ab</em></tt> sequences are recognized in quoted strings +and evaluate to ASCII hexadecimal number <em>ab</em>. </li> + <li> <tt>\0<em>abc</em></tt> sequences are recognized in quoted strings +and evaluate to ASCII octal number <em>abc</em>. </li> + <li> <tt>\<em>abc</em></tt> sequences are recognized in quoted strings +and evaluate to ASCII decimal number <em>abc</em>. <em>a</em> must not +be zero. </li> + <li> A comment starts with a <tt>#</tt> and ends with the line. Comments +are not recognized inside quoted strings. </li> + <li> Anything else is an unquoted string, that can evaluate to +zero or more words. </li> + <li> Any character can be escaped in unquoted strings by prepending +it with a backslash. It works the same way in quoted strings, except +for the special sequences described above. </li> +</ul> + +<p> + You can see an example of distinct <tt>execlineb</tt> components +<a href="componentsb.txt">here</a>. +</p> + +<p> + In addition to that simple lexing, +<tt>execlineb</tt> performs the following higher-level parsing: +</p> + +<ul> + <li> A word consisting of a single <em>opening brace</em> (<tt>{</tt>) +increments an internal level counter, <em>blevel</em>, and disappears from the +<em>argv</em>. Quoted open braces do not have that behaviour. </li> + <li> A word consisting of a single <em>closing brace</em> (<tt>}</tt>) +decrements <em>blevel</em>, and is replaced with the empty word. +Quoted closing braces do not have that behaviour. </li> + <li> If <tt>execlineb</tt> finds that braces are unmatched (i.e. +<em>blevel</em> goes below 0 during the parsing, or is not 0 at the end +of the script), it exits 100 with an error message. </li> + <li> <tt>execlineb</tt> automatically quotes +<a href="el_semicolon.html">blocks</a>. Which means that everytime it +finds a word, it prepends it with <em>blevel</em> spaces. </li> +</ul> + +<p> +For proper execution, the sequence of words must follow +the <a href="grammar.html">execline grammar</a>. +</p> + +<h2> Options for block syntax checking </h2> + +<p> + External execline commands that read blocks, like +<a href="foreground.html">foreground</a>, use the <tt>EXECLINE_STRICT</tt> +environment variable: if it is set to 1, they will print a warning message +on stderr if they find their blocks not to be properly quoted. If it is set +to 2, they will also die. If it is set to 0, or unset, they won't complain +at all. +</p> + +<p> + Normally the <tt>EXECLINE_STRICT</tt> environment variable is +inherited from the caller. You can +force it unset, set to 1, or set to 2 by giving respectively the +<tt>-q</tt>, <tt>-w</tt> or <tt>-W</tt> option to <tt>execlineb</tt>. +</p> + +<h2> Options for environment management </h2> + +<p> + Normally, execline scripts are <em>reentrant</em>: environment variables +potentially overwritten by <tt>execlineb</tt>, such as <tt>#</tt> or +<tt>0</tt>, are +<a href="el_pushenv.html">pushed</a>. This is the standard, safe +behaviour. Nevertheless, it is rather costly, and may be unneeded for +small scripts: for those cases, execline comes with two options +that bypass the environment management. Be warned that the purpose +of these options is <strong>optimization</strong>, and you should not +use them if you're not familiar with the way execlineb uses the +environment to store positional parameters. Alternatively, there's also +an integrated substitution mechanism that doesn't make use +of the environment at all. +</p> + +<ul> + <li> The <tt>-p</tt> option will bypass the +<a href="el_pushenv.html">push</a> phase: the current frame of positional +parameters will be <em>overwritten</em>. The script will <em>not</em> be +reentrant. </li> + <li> The <tt>-P</tt> option will bypass positional parameter handling +<em>completely</em>: the environment will not be pushed, and positional +parameters will be ignored. <tt>execlineb -P -c "<em>script</em>"</tt> is +equivalent to, but more efficient than, <tt>execlineb -c +"emptyenv -P <em>script</em>"</tt>. You should use the <tt>-P</tt> option +only in standalone scripts that take no arguments, such as +<a href="http://skarnet.org/software/s6/">s6</a>'s or +<a href="http://smarden.org/runit/">runit</a>'s <em>run scripts</em>. </li> + <li> The <tt>-S <em>nmin</em></tt> option <em>will</em> substitute the +positional parameters - up to at least <em>nmin</em> - but <em>will not</em> +push nor set environment +variables. <tt>execlineb -S3 -c "<em>script</em>"</tt> is equivalent to, +but more efficient than, <tt>execlineb -c "elgetpositionals -P3 emptyenv +-P <em>script</em>"</tt>. See +<a href="el_pushenv.html#integrated">the details</a>. </li> +</ul> + +<h2> Current limitations </h2> + +<p> + <tt>execlineb</tt> builds and executes a unique +<em>argv</em> with the script: hence scripts are subject to OS-dependent +limitations such as the kernel buffer size for <em>argv</em> and <em>envp</em> + - at least 64 kB on most systems. This means that <tt>execlineb</tt> cannot +execute arbitrarily large scripts. Be careful with deeply nested scripts too: +without the <tt>-p</tt>/<tt>-P</tt>/<tt>-S</tt> option, each execlineb +invocation uses up some space in the environment. +</p> + +</body> +</html> |