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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2022-12-20 09:46:56 +0000 |
|---|---|---|
| committer | Laurent Bercot <ska@appnovation.com> | 2022-12-20 09:46:56 +0000 |
| commit | 43a1ac4f3514406da9ce2a8eb41cc178c7531eca (patch) | |
| tree | 14a9c92aa1fd54ddb618e3671a1f5b26e0586630 /doc/eltest.html | |
| parent | a041d89c0543d5917eaef9181add440025b7ebf0 (diff) | |
| download | execline-43a1ac4f3514406da9ce2a8eb41cc178c7531eca.tar.xz | |
Prepare for 2.9.1.0; add eltest
Signed-off-by: Laurent Bercot <ska@appnovation.com>
Diffstat (limited to 'doc/eltest.html')
| -rw-r--r-- | doc/eltest.html | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/doc/eltest.html b/doc/eltest.html new file mode 100644 index 0000000..068cd4c --- /dev/null +++ b/doc/eltest.html @@ -0,0 +1,172 @@ +<html> + <head> + <meta name="viewport" content="width=device-width, initial-scale=1.0" /> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>execline: the eltest program</title> + <meta name="Description" content="execline: the eltest program" /> + <meta name="Keywords" content="execline command eltest test" /> + <!-- <link rel="stylesheet" type="text/css" href="//skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">execline</a><br /> +<a href="//skarnet.org/software/">Software</a><br /> +<a href="//skarnet.org/">skarnet.org</a> +</p> + +<h1> The <tt>eltest</tt> program </h1> + +<p> + eltest evaluates an expression and indicates the result via its +exit status. +</p> + +<h2> Interface </h2> + +<pre> + eltest <em>expression...</em> +</pre> + +<p> + <tt>eltest</tt> acts as the generic POSIX +<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">test</a> utility, +but it diverges from the specification on how it parses ambiguous arguments, see below. +</p> + +<p> + <tt>eltest</tt> supports all the standard +<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">test</a> +operands, plus all the extensions from +<a href="https://man7.org/linux/man-pages/man1/test.1.html">GNU test</a>, plus a few +extensions from the <tt>test</tt> builtin from +<a href="https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bash-Conditional-Expressions">bash</a>. +The extensions to POSIX <tt>test</tt> are listed below. +</p> + +<p> + <tt>eltest</tt> accepts an arbitrary number of arguments and, if the expression is +valid, always returns the result of the expression no matter how complex it +is. +</p> + +<h2> Exit codes </h2> + +<ul> + <li> 0: the test is true </li> + <li> 1: the test is false </li> + <li> 100: wrong usage </li> + <li> 101: internal error (should never happen, warrants a bug-report) </li> + <li> 111: system call failure </li> +</ul> + +<h2> Posixness </h2> + +<p> + <tt>eltest</tt> <strong>is not</strong> suitable as a Single Unix +<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">test</a> +program, due to the way it disambiguates between arguments and operators, see below. +However, if you never use arguments that start with a backslash, or that have the +same name as an existing operator, then +<tt>eltest</tt> exhibits the same behaviour as <tt>test</tt>. +</p> + +<h2> Extensions to POSIX </h2> + +<ul> + <li> <tt><em>expr1</em> -a <em>expr2</em></tt> : +tests whether <em>expr1</em> <strong>and</strong> <em>expr2</em> are true. +If <em>expr1</em> is false, then <em>expr2</em> is not evaluated. </li> + <li> <tt><em>expr1</em> -o <em>expr2</em></tt> : +tests whether <em>expr1</em> <strong>or</strong> <em>expr2</em> is true. +If <em>expr1</em> is true, then <em>expr2</em> is not evaluated. </li> + <li> <tt>-k <em>file</em></tt> : tests whether <em>file</em> +has the sticky bit. </li> + <li> <tt>-O <em>file</em></tt> : tests whether <em>file</em> +is owned by the effective uid of the current process. </li> + <li> <tt>-U <em>file</em></tt> : same. </li> + <li> <tt>-G <em>file</em></tt> : tests whether <em>file</em>'s gid +is the effective gid of the current process. </li> + <li> <tt>-N <em>file</em></tt> : tests whether <em>file</em> exists +and has been modified since it was last read. </li> + <li> <tt><em>file1</em> -nt <em>file2</em></tt> : +tests whether <em>file1</em> has a (strictly) newer modification date than <em>file2</em>. </li> + <li> <tt><em>file1</em> -ot <em>file2</em></tt> : +tests whether <em>file1</em> has a (strictly) older modification date than <em>file2</em>. </li> + <li> <tt><em>file1</em> -ef <em>file2</em></tt> : +tests whether <em>file1</em> and <em>file2</em> are physically the same +file (same device and inode numbers). </li> + <li> <tt>-v <em>var</em></tt> : tests whether the +<em>var</em> variable is defined in the current environment. </li> + <li> <tt><em>string</em> =~ <em>pattern</em></tt> : +tries to match <em>string</em> against extended regular expression +<em>pattern</em>. True if any part of <em>string</em> matches <em>pattern</em>; +in order to match whole strings, you must anchor <em>pattern</em> with +<tt>^</tt> and <tt>$</tt> markers. </li> +</ul> + +<h2> Argument disambiguation </h2> + +<p> + Unlike <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">test</a>, +which has different fixed syntax trees depending on the number of arguments it receives and +has undefined behaviour when called with more than 5 arguments, <tt>eltest</tt> accepts any +number of arguments and builds its syntax trees on the fly. This means that expressions such +as <tt>-n = -n</tt> cannot be automatically disambiguated: <tt>eltest</tt> does not know that +there are 3 arguments, so when it reads the first <tt>-n</tt> it assumes that it is an unary +operator, then when it reads <tt>=</tt> it assumes it is the argument to <tt>-n</tt>, then +when it reads the second <tt>-n</tt> it exits with a syntax error. +</p> + +<p> + Doing otherwise would result in a combinatory explosion of possible syntax trees, making +it easy for users to trigger unbounded RAM consumption, and turning a simple utility into +a programming nightmare. This is why POSIX +<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html">test</a> +is so restricted. But we don't want the same restrictions. +</p> + +<p> + So, instead, <tt>eltest</tt> provides the user with a mechanism to make sure that +operands are never mistaken for operators: +</p> + +<ul> + <li> An word that looks like an operator will always be interpreted like an operator. +So, expressions like <tt>-n = -n</tt> will result in a syntax error, because the +first <tt>-n</tt> will never be understood as data for the <tt>=</tt> operator. </li> + <li> A word that starts with a <tt>\</tt> (backslash) will always be interpreted +like data, never like an operator, and the backslash will be removed. This +means: <tt>\-n = \-n</tt> is a valid expression testing the equality between +the strings <tt>-n</tt> and </tt>-n</tt>. + <ul> + <li> Be aware that execline as well as the shell use one backlash for their own +unquoting mechanism, so when using backslashes in an execline or shell script, they +must be doubled. You would probably need to type something like <tt>\\-n = \\-n</tt>. + </ul> </li> + <li> So, if your script tests equality between <tt>$a</tt> and <tt>$b</tt>, and there's +a possiblity that the contents of these variables look like <tt>eltest</tt> operators, +the proper syntax would be: <tt>eltest \\${a} = \\${b}</tt>. </li> +</ul> + +<p> + Note that these details are irrelevant to a huge majority of <tt>eltest</tt> use +cases, because most of the time users only need a simple test +such as <tt>eltest -r ${file}</tt> to check that <tt>$file</tt> is readable, and +there's no possible ambiguity. So don't panic over this. +</p> + +<h2> Notes </h2> + +<ul> + <li> <tt>eltest</tt> is a replacement for the ill-named, and now deprecated, +<a href="https://skarnet.org/software/s6-portable-utils/s6-test.html">s6-test</a> +program, part of the (just as ill-named) +<a href="https://skarnet.org/software/s6-portable-utils/">s6-portable-utils</a> +package. It is too valuable a utility to be part of a marginal package, and +has nothing to do with <a href="https://skarnet.org/software/s6/">s6</a>. </li> +</ul> + +</body> +</html> |