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1.1 root 1: .TI CSHELL/EXPRESSIONS
2: Expressions: Arithmetic, Logic Operations, and Decisions [DRAFT]
3:
4:
5: An expression is a group of words (numbers, strings, and
6: operators separated by spaces) which the C shell
7: replaces by a single number, called its value.
8: The act of replacing an expression by its value
9: is a kind of substitution called evaluation, and forms the
10: basis for all arithmetic, logic operations, and decisions.
11: Loosely speaking, the term "expression" applies to any construct
12: whatever in the C shell, but here it has a specific sense.
13:
14: Expressions are only evaluated, hence useful,
15: in conjunction with four built-in commands:
16: .IP
17: .nf
18: \fBif (\fIexpression\fP) then
19: \fBwhile (\fIexpression\fP)
20: \fB@ \fIname\fP = (\fIexpression\fP)
21: exit (\fIexpression\fP)\fR
22: .fi
23: .LP
24: The if and while statements use the value of \fIexpression\fP
25: to determine whether to execute the next command or group of commands.
26: In particular, when the C shell produces a value of zero (false),
27: the next group of commands is not done; otherwise it is done.
28: The \fB@\fP command (yes, it is actually a command),
29: not to be confused with the "at" command or the old default
30: line-kill character, stores the value of \fIexpression\fP
31: in the variable \fIname\fP.
32: Except for providing the only way to store the results of an
33: arithmetic or logic computation, it is identical to the "set" command.
34: The exit command quits the current shell (e.g., in a script)
35: and sets the command completion status (which other programs
36: sometimes want to know) with the value of \fIexpression\fP.
37:
38: The command forms above do not all require parentheses,
39: but including them always is probably a good practice.
40: Moreover, the words inside an expression generally require spaces
41: between each other.
42: These words ultimately consist of strings (such as filenames),
43: numbers (such as 3.1415), and operators (such as +, \-, <, and >).
44: Usually, however, you have entered some of the words as
45: references to variable values, and so the shell will provide
46: the actual values during variable substitution just prior
47: to evaluating the expression.
48:
49: The C shell has two main kinds of expressions: arithmetic and logic.
50: Of logic expressions it has three kinds: boolean, file enquiry,
51: and command status enquiry.
52: Each category is described below.
53:
54: Arithmetic Expressions
55:
56: Here evaluation simply means doing the arithmetic indicated,
57: and the value that replaces the expression is just the result.
58: You may use any of the operators and precedences valid in the
59: C programming language (without tremendously reliable results),
60: and octal numbers must begin with a 0.
61: The only numbers recognized by the shell are integers.
62:
63: In the following examples, suppose
64: we have issued "% set y = 8 ; set num = (4 168 10 8808)":
65:
66: .nf
67: EXPRESSION VALUE EXPRESSION VALUE
68: (2 + 2) 4 (5 - 27 + 7) -15
69: (3 + $y + $num[3]) 21 ($num[4] - 5) 8803
70: (5 * 5) 25 (24 / $y - 6) -3
71:
72: % @ x = (354 - 128 + 52 * 5 / 3)
73: % echo Result is $x
74: Result is 174
75: % set y = (354 - 128 + 52 / 3)
76: % echo Result is $y
77: Result is 354 - 128 + 52 / 3
78: .fi
79:
80:
81: Logic Expressions
82:
83: Evaluation means asking the question "Is it true that \fIexpression\fP?",
84: where \fIexpression\fP is a statement or assertion phrased in C Shell terms.
85: The value replacing it is 1 if the statement is true, 0 if false.
86: There are three kinds of logic expressions: boolean, file enquiry,
87: and command status enquiry.
88: Any nonzero valued expression is taken as true, whatever you
89: may have intended (e.g., (3 + 2) is both 5 and true).
90:
91: Boolean Expressions
92:
93: These expressions, named after the English mathematician George Boole,
94: make assertions about the relative sizes of two numbers. There are
95: 8 kinds of assertions,
96:
97: .nf
98: == literal equals != literal not-equals
99: =~ regular expr. equals !~ regular expr. not equals
100: > greater than < less than
101: >= > or = <= < or =,
102:
103: 2 ways to combine two assertions,
104:
105: && and || or
106:
107: and one way to negate an assertion
108:
109: ! not
110:
111: Here are some examples:
112:
113: EXPRESSION VALUE EXPRESSION VALUE
114: (3 > 2) 1 (-5 < $num[3]) 1
115: (234 <= 234) 1 (234 <= 233) 0
116: (2 == $y || 3 > 2) 1 (1 > -8 && 3 > 1) 1
117: (1) 1 (2 + (5 == 5)) 3
118: (prog.c == *.c) 0 (prog.c =~ *.c) 1
119:
120: % set x = 340
121: % if ($x > 200) echo You cannot afford it.
122: You cannot afford it.
123: .fi
124:
125: File Enquiry Expressions
126:
127: These have the form (-L \fIfilename\fP), where the `L' specifies one of
128: 8 different assertions about \fIfilename\fP.
129: You may employ these to have the shell determine whether or not
130: permission is granted for a certain file, whether it is a directory, etc.
131: The possible choices for `L' are:
132:
133: .nf
134: r read access o ownership
135: w write access z zero size
136: x execute access f plain file
137: e existence d directory
138:
139: Here are some examples.
140:
141: % if (! -z /usr/spool/mail/jak) echo You have mail.
142: You have mail.
143: % if (! -z netcopy.out) echo ready
144: % if (! -e stuff) echo Not executable
145: .fi
146:
147: Command Status Expressions
148:
149: These have the form ( { \fIcommand\fP } ), where the shell
150: executes \fIcommand\fP during evaluation, and uses the
151: completion status as the value of the variable.
152: Note the spaces between \fIcommand\fP and the {}'s.
153: Unlike command substitution, nothing special happens to
154: the output of \fIcommand\fP.
155: In fact, frequently you do not need output anyway since
156: the expression relies upon the return status.
157: In the examples below the \-s option
158: suppresses any printed output in both cases.
159:
160: For instance, to take a certain action in case two files differ,
161:
162: if ( { cmp -s file1 file2 } ) then ...
163:
164: or to leave a script and let the calling process know
165: of a match for a string in a file,
166:
167: exit ( { grep -s string file } )
168:
169:
170: jak
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