blob: aebdde8f60f6d5efc02cef702306deccdc8d789e (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
|
<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 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="//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>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="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 an <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}
importas A 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}
importas A 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="https://code.dogmap.org/">Paul Jarc</a> first originated the
idea of the <tt>multisubstitute</tt> command and a possible syntax.
</p>
</body>
</html>
|
