Commit | Line | Data |
---|---|---|
b378a021 BJ |
1 | .nr H1 3 |
2 | .NH | |
3 | Other, less commonly used, shell features | |
4 | .NH 2 | |
5 | Loops at the terminal; variables as vectors | |
6 | .PP | |
7 | It is occasionally useful to use the | |
8 | .I foreach | |
9 | control structure at the terminal to aid in performing a number | |
10 | of similar commands. | |
11 | For instance, there were at one point three shells in use on the Cory \s-2UNIX\s0 | |
12 | system at Cory Hall, | |
13 | `/bin/sh', | |
14 | `/bin/nsh', | |
15 | and | |
16 | `/bin/csh'. | |
17 | To count the number of persons using each shell one could have issued | |
18 | the commands | |
19 | .DS | |
20 | % grep \-c csh$ /etc/passwd | |
21 | 27 | |
22 | % grep \-c nsh$ /etc/passwd | |
23 | 128 | |
24 | % grep \-c \-v sh$ /etc/passwd | |
25 | 430 | |
26 | % | |
27 | .DE | |
28 | Since these commands are very similar we can use | |
29 | .I foreach | |
30 | to do this more easily. | |
31 | .DS | |
32 | % foreach i (\'sh$\' \'csh$\' \'\-v sh$\') | |
33 | ? grep \-c $i /etc/passwd | |
34 | ? end | |
35 | 27 | |
36 | 128 | |
37 | 430 | |
38 | % | |
39 | .DE | |
40 | Note here that the shell prompts for | |
41 | input with `? ' when reading the body of the loop. | |
42 | .PP | |
43 | Very useful with loops are variables which contain lists of filenames | |
44 | or other words. | |
45 | You can, for example, do | |
46 | .DS | |
47 | % set a=(\`ls\`) | |
48 | % echo $a | |
49 | csh.n csh.rm | |
50 | % ls | |
51 | csh.n | |
52 | csh.rm | |
53 | % echo $#a | |
54 | 2 | |
55 | % | |
56 | .DE | |
57 | The | |
58 | .I set | |
59 | command here gave the variable | |
60 | .I a | |
61 | a list of all the filenames in the current directory as value. | |
62 | We can then iterate over these names to perform any chosen function. | |
63 | .PP | |
64 | The output of a command within `\`' characters is converted by | |
65 | the shell to a list of words. | |
66 | You can also place the `\`' quoted string within `"' characters | |
67 | to take each (non-empty) line as a component of the variable; | |
68 | preventing the lines from being split into words at blanks and tabs. | |
69 | A modifier `:x' exists which can be used later to expand each component | |
70 | of the variable into another variable splitting it into separate words | |
71 | at embedded blanks and tabs. | |
72 | .NH 2 | |
73 | Braces { ... } in argument expansion | |
74 | .PP | |
75 | Another form of filename expansion, alluded | |
76 | to before involves the characters `{' and `}'. | |
77 | These characters specify that the contained strings, separated by `,' | |
78 | are to be consecutively substituted into the containing characters | |
79 | and the results expanded left to right. | |
80 | Thus | |
81 | .DS | |
82 | A{str1,str2,...strn}B | |
83 | .DE | |
84 | expands to | |
85 | .DS | |
86 | Astr1B Astr2B ... AstrnB | |
87 | .DE | |
88 | This expansion occurs before the other filename expansions, and may | |
89 | be applied recursively (i.e. nested). | |
90 | The results of each expanded string are sorted separately, left | |
91 | to right order being preserved. | |
92 | The resulting filenames are not required to exist if no other expansion | |
93 | mechanisms are used. | |
94 | This means that this mechanism can be used to generate arguments which are | |
95 | not filenames, but which have common parts. | |
96 | .PP | |
97 | A typical use of this would be | |
98 | .DS | |
99 | mkdir ~/{hdrs,retrofit,csh} | |
100 | .DE | |
101 | to make subdirectories `hdrs', `retrofit' and `csh' | |
102 | in your home directory. | |
103 | This mechanism is most useful when the common prefix is longer | |
104 | than in this example, i.e. | |
105 | .DS | |
106 | chown bin /usr/{bin/{ex,edit},lib/{ex1.1strings,how_ex}} | |
107 | .DE | |
108 | .NH 2 | |
109 | Command substitution | |
110 | .PP | |
111 | A command enclosed in `\`' characters is replaced, just before | |
112 | filenames are expanded, by the output from that command. | |
113 | Thus it is possible to do | |
114 | .DS | |
115 | set pwd=\`pwd\` | |
116 | .DE | |
117 | to save the current directory in the variable | |
118 | .I pwd | |
119 | or to do | |
120 | .DS | |
121 | ex \`grep \-l TRACE *.c\` | |
122 | .DE | |
123 | to run the editor | |
124 | .I ex | |
125 | suppling as arguments those files whose names end in `.c' | |
126 | which have the string `TRACE' in them.* | |
127 | .FS | |
128 | *Command expansion also occurs in input redirected with `<<' | |
129 | and within `"' quotations. | |
130 | Refer to the shell manual section for full details. | |
131 | .FE | |
132 | .NH 2 | |
133 | Other details not covered here | |
134 | .PP | |
135 | In particular circumstances it may be necessary to know the exact | |
136 | nature and order of different substitutions performed by the shell. | |
137 | The exact meaning of certain combinations of quotations is also | |
138 | occasionally important. | |
139 | These are detailed fully in its manual section. | |
140 | .PP | |
141 | The shell has a number of command line option flags mostly of use | |
142 | in writing \s-2UNIX\s0 programs, | |
143 | and debugging shell scripts. | |
144 | See the shells manual section for a list of these options. | |
145 | .bp |