Chapter 8. Operations and Related Topics
8.1. Operators
- variable assignment
Initializing or changing the value of a variable
- =
All-purpose assignment operator, which works for both arithmetic and string assignments.
1 var=27 2 category=minerals # No spaces allowed after the "=".
Do not confuse the "=" assignment operator with the = test operator.
1 # = as a test operator 2 3 if [ "$string1" = "$string2" ] 4 then 5 command 6 fi 7 8 # if [ "X$string1" = "X$string2" ] is safer, 9 #+ to prevent an error message should one of the variables be empty. 10 # (The prepended "X" characters cancel out.)
- +
plus
- -
minus
- *
multiplication
- /
division
- **
exponentiation
1 # Bash, version 2.02, introduced the "**" exponentiation operator. 2 3 let "z=5**3" # 5 * 5 * 5 4 echo "z = $z" # z = 125
- %
modulo, or mod (returns the remainder of an integer division operation)
5/3 = 1, with remainder 2bash$ expr 5 % 3 2
This operator finds use in, among other things, generating numbers within a specific range (see Example 9-11 and Example 9-15) and formatting program output (see Example 27-16 and Example A-6). It can even be used to generate prime numbers, (see Example A-15). Modulo turns up surprisingly often in numerical recipes.
Example 8-1. Greatest common divisor
1 #!/bin/bash 2 # gcd.sh: greatest common divisor 3 # Uses Euclid's algorithm 4 5 # The "greatest common divisor" (gcd) of two integers 6 #+ is the largest integer that will divide both, leaving no remainder. 7 8 # Euclid's algorithm uses successive division. 9 # In each pass, 10 #+ dividend <--- divisor 11 #+ divisor <--- remainder 12 #+ until remainder = 0. 13 # The gcd = dividend, on the final pass. 14 # 15 # For an excellent discussion of Euclid's algorithm, see 16 #+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm. 17 18 19 # ------------------------------------------------------ 20 # Argument check 21 ARGS=2 22 E_BADARGS=85 23 24 if [ $# -ne "$ARGS" ] 25 then 26 echo "Usage: `basename $0` first-number second-number" 27 exit $E_BADARGS 28 fi 29 # ------------------------------------------------------ 30 31 32 gcd () 33 { 34 35 dividend=$1 # Arbitrary assignment. 36 divisor=$2 #! It doesn't matter which of the two is larger. 37 # Why not? 38 39 remainder=1 # If an uninitialized variable is used inside 40 #+ test brackets, an error message results. 41 42 until [ "$remainder" -eq 0 ] 43 do # ^^^^^^^^^^ Must be previously initialized! 44 let "remainder = $dividend % $divisor" 45 dividend=$divisor # Now repeat with 2 smallest numbers. 46 divisor=$remainder 47 done # Euclid's algorithm 48 49 } # Last $dividend is the gcd. 50 51 52 gcd $1 $2 53 54 echo; echo "GCD of $1 and $2 = $dividend"; echo 55 56 57 # Exercises : 58 # --------- 59 # 1) Check command-line arguments to make sure they are integers, 60 #+ and exit the script with an appropriate error message if not. 61 # 2) Rewrite the gcd () function to use local variables. 62 63 exit 0- +=
plus-equal (increment variable by a constant)nbsp;COMMAND.
As a simple example, consider this alternative to the echo-grep construction.
1 # Instead of: 2 if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] 3 # etc. 4 5 # Try: 6 if grep -q "txt" <<< "$VAR" 7 then # ^^^ 8 echo "$VAR contains the substring sequence \"txt\"" 9 fi 10 # Thank you, Sebastian Kaminski, for the suggestion.
Or, in combination with read:
1 String="This is a string of words." 2 3 read -r -a Words <<< "$String" 4 # The -a option to "read" 5 #+ assigns the resulting values to successive members of an array. 6 7 echo "First word in String is: ${Words[0]}" # This 8 echo "Second word in String is: ${Words[1]}" # is 9 echo "Third word in String is: ${Words[2]}" # a 10 echo "Fourth word in String is: ${Words[3]}" # string 11 echo "Fifth word in String is: ${Words[4]}" # of 12 echo "Sixth word in String is: ${Words[5]}" # words. 13 echo "Seventh word in String is: ${Words[6]}" # (null) 14 # Past end of $String. 15 16 # Thank you, Francisco Lobo, for the suggestion.Example 19-13. Prepending a line to a file
1 #!/bin/bash 2 # prepend.sh: Add text at beginning of file. 3 # 4 # Example contributed by Kenny Stauffer, 5 #+ and slightly modified by document author. 6 7 8 E_NOSUCHFILE=85 9 10 read -p "File: " file # -p arg to 'read' displays prompt. 11 if [ ! -e "$file" ] 12 then # Bail out if no such file. 13 echo "File $file not found." 14 exit $E_NOSUCHFILE 15 fi 16 17 read -p "Title: " title 18 cat - $file <<<$title > $file.new 19 20 echo "Modified file is $file.new" 21 22 exit # Ends script execution. 23 24 from 'man bash': 25 Here Strings 26 A variant of here documents, the format is: 27 28 <<<word 29 30 The word is expanded and supplied to the command on its standard input. 31 32 33 Of course, the following also works: 34 sed -e '1i\ 35 Title: ' $file
Example 19-14. Parsing a mailbox
1 #!/bin/bash 2 # Script by Francisco Lobo, 3 #+ and slightly modified and commented by ABS Guide author. 4 # Used in ABS Guide with permission. (Thank you!) 5 6 # This script will not run under Bash versions < 3.0. 7 8 9 E_MISSING_ARG=67 10 if [ -z "$1" ] 11 then 12 echo "Usage: $0 mailbox-file" 13 exit $E_MISSING_ARG 14 fi 15 16 mbox_grep() # Parse mailbox file. 17 { 18 declare -i body=0 match=0 19 declare -a date sender 20 declare mail header value 21 22 23 while IFS= read -r mail 24 # ^^^^ Reset $IFS. 25 # Otherwise "read" will strip leading & trailing space from its input. 26 27 do 28 if [[ $mail =~ "^From " ]] # Match "From" field in message. 29 then 30 (( body = 0 )) # "Zero out" variables. 31 (( match = 0 )) 32 unset date 33 34 elif (( body )) 35 then 36 (( match )) 37 # echo "$mail" 38 # Uncomment above line if you want entire body of message to display. 39 40 elif [[ $mail ]]; then 41 IFS=: read -r header value <<< "$mail" 42 # ^^^ "here string" 43 44 case "$header" in 45 [Ff][Rr][Oo][Mm] ) [[ $value =~ "$2" ]] && (( match++ )) ;; 46 # Match "From" line. 47 [Dd][Aa][Tt][Ee] ) read -r -a date <<< "$value" ;; 48 # ^^^ 49 # Match "Date" line. 50 [Rr][Ee][Cc][Ee][Ii][Vv][Ee][Dd] ) read -r -a sender <<< "$value" ;; 51 # ^^^ 52 # Match IP Address (may be spoofed). 53 esac 54 55 else 56 (( body++ )) 57 (( match )) && 58 echo "MESSAGE ${date:+of: ${date[*]} }" 59 # Entire $date array ^ 60 echo "IP address of sender: ${sender[1]}" 61 # Second field of "Received" line ^ 62 63 fi 64 65 66 done < "$1" # Redirect stdout of file into loop. 67 } 68 69 70 mbox_grep "$1" # Send mailbox file to function. 71 72 exit $? 73 74 # Exercises: 75 # --------- 76 # 1) Break the single function, above, into multiple functions, 77 #+ for the sake of readability. 78 # 2) Add additional parsing to the script, checking for various keywords. 79 80 81 82 $ mailbox_grep.sh scam_mail 83 MESSAGE of Thu, 5 Jan 2006 08:00:56 -0500 (EST) 84 IP address of sender: 196.3.62.4Exercise: Find other uses for here strings, such as, for example, feeding input to dc.