HOWTO zum Kommando "expr"

(C) 2006-2025 T.Birnthaler/H.Gottschalk <howtos(at)ostc.de>
              OSTC Open Source Training and Consulting GmbH
              www.ostc.de

$Id: shell-expr-HOWTO.txt,v 1.2 2025/02/23 20:14:55 tsbirn Exp $

Inhaltsverzeichnis

0) Einführung   (Toc)

"expr" (expression) is a command which calculates the value of an expression
(numerical, string, comparison, logical, ...) and prints it out on the terminal:

  Numerical expression --> Result of calculation
  String expression    --> Result of string operation
  Comparison           --> Result "1" if true, "0" if false (opposite of exit status)
  Logical expression   --> Result "1" if true, "0" if false (opposite of exit status)

"expr" supports only INTEGER numbers. For FLOATING POINT numbers tools like
"bc" (binary calculator), "dc" (decimal calculator) or "awk" have to be used.

Furthermore "expr" produces an EXIT STATUS:

  0 = Expression syntactically correct and result is NOT 0 and NOT empty
  1 = Expression syntactically correct and result is     0 or      empty
  2 = Expression syntactically wrong
  3 = An error occurred

The elements of an expression have to be separated by SPACES.
Shell special characters (e.g. * & | ( ) < >) have to be QUOTED.

A lot of replacements for "expr" have been added to the shell over time
(faster, more features, don't need/allow spaces inside expression, ...):

  declare -i VAR   # Variable of type integer --> Assignment evaluated as expression
  let VAR=...      # Numeric expression
  VAR=$[ ... ]     # Numeric expression
  (( VAR = ... ))  # Numeric expression
  VAR=$(( ... ))   # Numeric expression

1) "expr"-Operatoren   (Toc)

1.1) Arithmetik   (Toc)

  expr 10 + 3      # --> 13  Addition
  expr 10 - 3      # --> 7   Subtraktion
  expr 10 \* 3     # --> 30  Multiplikation
  expr 10 / 3      # --> 3   Division
  expr 10 % 3      # --> 1   Modulo (Divisionsrest)

1.2) Numerischer Vergleich   (Toc)

  expr 10 = 3      # --> 1  Zahl gleich             (Exit status 0)
  expr 10 != 3     # --> 0  Zahl ungleich           (Exit status 1)
  expr 10 \< 3     # --> 1  Zahl kleiner            (Exit status 0)
  expr 10 \<= 3    # --> 1  Zahl kleiner gleich     (Exit status 0)
  expr 10 \> 3     # --> 0  Zahl größer             (Exit status 1)
  expr 10 \>= 3    # --> 0  Zahl größer gleich      (Exit status 1)

1.3) String-Vergleich   (Toc)

  expr abc =   def   # --> 0  String gleich         (Exit status 1)
  expr abc !=  def   # --> 1  String ungleich       (Exit status 0)
  expr abc \<  def   # --> 1  String kleiner        (Exit status 0)
  expr abc \<= def   # --> 1  String kleiner gleich (Exit status 0)
  expr abc \>  def   # --> 0  String größer         (Exit status 1)
  expr abc \>= def   # --> 0  String größer gleich  (Exit status 1)

1.4) Inkrementieren / Dekrementieren   (Toc)

  COUNT=`expr $COUNT + 1`    # COUNT um 1 erhöhen (Inkrement)
  COUNT=$(expr $COUNT - 1)   # COUNT um 1 verringern (Dekrement)

1.5) Logische Verknüpfung (es gibt KEINE logische Negation!)   (Toc)

  expr 10 \& 2      # --> 10 AND  (Exit status 0)
  expr 10 \& 0      # -->  0 AND  (Exit status 1)
  expr  0 \& 2      # -->  0 AND  (Exit status 1)
  expr  0 \& 0      # -->  0 AND  (Exit status 1)
  expr 10 \| 2      # --> 10 OR   (Exit status 0)
  expr 10 \| 0      # --> 10 OR   (Exit status 0)
  expr  0 \| 2      # -->  2 OR   (Exit status 0)
  expr  0 \| 0      # -->  0 OR   (Exit status 1)

1.6) Shortcut/Short Circuit-Evaluation von logischen Ausdrücken   (Toc)

  expr 5 \> 7 \| 3 + 2    # --> 5  (Exit status 0)
  expr 5 \> 7 \& 3 + 2    # --> 0  (Exit status 1)

1.7) Klammerung   (Toc)

  expr \( 1 + 2 \) \* 3    # --> 9
  expr    1 + 2    \* 3    # --> 7

1.8.1) String-Operationen   (Toc)

  expr "abcde" : ".*"              # --> 5    Match Länge
  expr "abcde" : "abc"             # --> 3    Match Länge
  expr "abcde" : '\(.*c\)'         # --> abc  Match Muster

1.8.2) GNU-String-Operationen   (Toc)

  expr length "abcde"              # --> 5    String Länge
  expr index "abcde" "cd"          # --> 3    Match start Index
  expr substr "abcde" 2 3          # --> bcd  Substring ab Index 2 mit Länge 3
  expr match "abcde" "abc"         # --> 3    Match Länge
  expr match "abcde" "\(.*c\)"     # --> abc  Match Muster

1.9) Alternativen zum Rechnen ohne "expr" (nur in bash + ksh möglich)   (Toc)

  declare -i SUM                  #          1a) Variable hat Typ Integer
  SUM=111+111                     # --> 222  1b) KEINE Leerzeichen erlaubt!
  unset SUM                       #          2+3+4) Variable hat keinen Typ
  let SUM=111+222                 # --> 333  2a) KEINE Leerzeichen erlaubt!
  let SUM=" 111 + 333 "           # --> 444  2b) Leerzeichen erlaubt!
  SUM=$(( 111 + 444))             # --> 555  3a) Leerzeichen erlaubt!
  ((SUM = 111 + 555))             # --> 666  3b) Leerzeichen erlaubt!
  SUM=$[ 111 + 666 ]              # --> 777  4) Leerzeichen erlaubt, veraltet!

1.9.1) Inkrement/Dekrement ohne "expr"   (Toc)

  N=5                             # --> 5
  ((++N))                         # --> 6   Inkrement (prefix)
  ((N++))                         # --> 7   Inkrement (postfix)
  ((--N))                         # --> 6   Dekrement (prefix)
  ((N--))                         # --> 5   Dekrement (postfix)
  ((N += 5))                      # --> 10  Zusammengesetzte Addition
  ((N -= 5))                      # --> 10  Zusammengesetzte Subtraktion
  ((N *= 5))                      # --> 25  Zusammengesetzte Multiplikation
  ((N /= 5))                      # --> 5   Zusammengesetzte Division
  ((N %= 5))                      # --> 0   Zusammengesetzte Modulo/Divisionsrest

1.10) Fließkomma-Berechnungen (bc/dc = binary/decimal calculator)   (Toc)

  RES=$(echo "1.2 + 3.4 - 5.6 * 7.8" | bc -S6)   # --> -39.08 (-S6 = 6 digits)
  RES=$(bc -S6 -e "1.2 + 3.4 - 5.6 * 7.8")       # --> -39.08 (-e = expression)
  RES=$(echo "1.2 3.4 + 5.6 7.8 * - p" | dc)     # --> -39.08
  RES=$(dc -S6 -e "1.2 3.4 + 5.6 7.8 * - p")     # --> -39.08 (p = print)
  RES=$(awk "BEGIN { print 1.2 + 3.4 - 5.6 * 7.8 }")              # --> -39.08
  RES=$(echo "BEGIN { print 1.2 + 3.4 - 5.6 * 7.8 }" | awk -f -)  # --> -39.08