expr/expr.1

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  31 .\" $FreeBSD: src/bin/expr/expr.1,v 1.31 2011/07/09 12:05:53 se Exp $
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  33 .Dd September 9, 2010
  34 .Dt EXPR 1
  35 .Os
  36 .Sh NAME
  37 .Nm expr
  38 .Nd evaluate expression
  39 .Sh SYNOPSIS
  40 .Nm
  41 .Ar expression
  42 .Sh DESCRIPTION
  43 The
  44 .Nm
  45 utility evaluates
  46 .Ar expression
  47 and writes the result on standard output.
  48 .Pp
  49 All operators and operands must be passed as separate arguments.
  50 Several of the operators have special meaning to command interpreters
  51 and must therefore be quoted appropriately.
  52 All integer operands are interpreted in base 10 and must consist of only
  53 an optional leading minus sign followed by one or more digits.
  54 .Pp
  55 Arithmetic operations are performed using signed integer math with a
  56 range according to the C
  57 .Vt intmax_t
  58 data type (the largest signed integral type available).
  59 All conversions and operations are checked for overflow.
  60 Overflow results in program termination with an error message on stdout
  61 and with an error status.
  62 .Pp
  63 Operators are listed below in order of increasing precedence; all
  64 are left-associative.
  65 Operators with equal precedence are grouped within symbols
  66 .Ql {
  67 and
  68 .Ql } .
  69 .Bl -tag -width indent
  70 .It Ar expr1 Li | Ar expr2
  71 Return the evaluation of
  72 .Ar expr1
  73 if it is neither an empty string nor zero;
  74 otherwise, returns the evaluation of
  75 .Ar expr2
  76 if it is not an empty string;
  77 otherwise, returns zero.
  78 .It Ar expr1 Li & Ar expr2
  79 Return the evaluation of
  80 .Ar expr1
  81 if neither expression evaluates to an empty string or zero;
  82 otherwise, returns zero.
  83 .It Ar expr1 Li "{=, >, >=, <, <=, !=}" Ar expr2
  84 Return the results of integer comparison if both arguments are integers;
  85 otherwise, returns the results of string comparison using the locale-specific
  86 collation sequence.
  87 The result of each comparison is 1 if the specified relation is true,
  88 or 0 if the relation is false.
  89 .It Ar expr1 Li "{+, -}" Ar expr2
  90 Return the results of addition or subtraction of integer-valued arguments.
  91 .It Ar expr1 Li "{*, /, %}" Ar expr2
  92 Return the results of multiplication, integer division, or remainder of integer-valued arguments.
  93 .It Ar expr1 Li : Ar expr2
  94 The
  95 .Dq Li \&:
  96 operator matches
  97 .Ar expr1
  98 against
  99 .Ar expr2 ,
 100 which must be a basic regular expression.
 101 The regular expression is anchored
 102 to the beginning of the string with an implicit
 103 .Dq Li ^ .
 104 .Pp
 105 If the match succeeds and the pattern contains at least one regular
 106 expression subexpression
 107 .Dq Li "\e(...\e)" ,
 108 the string corresponding to
 109 .Dq Li \e1
 110 is returned;
 111 otherwise the matching operator returns the number of characters matched.
 112 If the match fails and the pattern contains a regular expression subexpression
 113 the null string is returned;
 114 otherwise 0.
 115 .El
 116 .Pp
 117 Parentheses are used for grouping in the usual manner.
 118 .Pp
 119 The
 120 .Nm
 121 utility makes no lexical distinction between arguments which may be
 122 operators and arguments which may be operands.
 123 An operand which is lexically identical to an operator will be considered a
 124 syntax error.
 125 See the examples below for a work-around.
 126 .Pp
 127 The syntax of the
 128 .Nm
 129 command in general is historic and inconvenient.
 130 New applications are advised to use shell arithmetic rather than
 131 .Nm .
 132 .Sh EXIT STATUS
 133 The
 134 .Nm
 135 utility exits with one of the following values:
 136 .Bl -tag -width indent -compact
 137 .It 0
 138 the expression is neither an empty string nor 0.
 139 .It 1
 140 the expression is an empty string or 0.
 141 .It 2
 142 the expression is invalid.
 143 .El
 144 .Sh EXAMPLES
 145 .Bl -bullet
 146 .It
 147 The following example (in
 148 .Xr sh 1
 149 syntax) adds one to the variable
 150 .Va a :
 151 .Dl "a=$(expr $a + 1)"
 152 .It
 153 This will fail if the value of
 154 .Va a
 155 is a negative number.
 156 To protect negative values of
 157 .Va a
 158 from being interpreted as options to the
 159 .Nm
 160 command, one might rearrange the expression:
 161 .Dl "a=$(expr 1 + $a)"
 162 .It
 163 More generally, parenthesize possibly-negative values:
 164 .Dl "a=$(expr \e( $a \e) + 1)"
 165 .It
 166 With shell arithmetic, no escaping is required:
 167 .Dl "a=$((a + 1))"
 168 .It
 169 This example prints the filename portion of a pathname stored
 170 in variable
 171 .Va a .
 172 Since
 173 .Va a
 174 might represent the path
 175 .Pa / ,
 176 it is necessary to prevent it from being interpreted as the division operator.
 177 The
 178 .Li //
 179 characters resolve this ambiguity.
 180 .Dl "expr \*q//$a\*q \&: '.*/\e(.*\e)'"
 181 .It
 182 With modern
 183 .Xr sh 1
 184 syntax,
 185 .Dl "\*q${a##*/}\*q"
 186 expands to the same value.
 187 .El
 188 .Pp
 189 The following examples output the number of characters in variable
 190 .Va a .
 191 Again, if
 192 .Va a
 193 might begin with a hyphen, it is necessary to prevent it from being
 194 interpreted as an option to
 195 .Nm ,
 196 and
 197 .Va a
 198 might be interpreted as an operator.
 199 .Bl -bullet
 200 .It
 201 To deal with all of this, a complicated command
 202 is required:
 203 .Dl "expr \e( \*qX$a\*q \&: \*q.*\*q \e) - 1"
 204 .It
 205 With modern
 206 .Xr sh 1
 207 syntax, this can be done much more easily:
 208 .Dl "${#a}"
 209 expands to the required number.
 210 .El
 211 .Sh SEE ALSO
 212 .Xr sh 1 ,
 213 .Xr test 1
 214 .Sh STANDARDS
 215 The
 216 .Nm
 217 utility conforms to
 218 .St -p1003.1-2008 .
 219 .Pp
 220 The extended arithmetic range and overflow checks do not conflict with
 221 POSIX's requirement that arithmetic be done using signed longs, since
 222 they only make a difference to the result in cases where using signed
 223 longs would give undefined behavior.
 224 .Pp
 225 According to the
 226 .Tn POSIX
 227 standard, the use of string arguments
 228 .Va length ,
 229 .Va substr ,
 230 .Va index ,
 231 or
 232 .Va match
 233 produces undefined results. In this version of
 234 .Nm ,
 235 these arguments are treated just as their respective string values.