2.6. The Bash Shell Constructs

The Korn and Bash shells are very similar, but there are some differences. The Bash constructs are listed in Table 2.4.

Table 2.4. Bash Shell Syntax and Constructs

The shbang line	The "shbang" line is the very first line of the script and lets the kernel know what shell will be interpreting the lines in the script. The shbang line consists of a #! followed by the full pathname to the shell, and can be followed by options to control the behavior of the shell. EXAMPLE #!/bin/bash
Comments	Comments are descriptive material preceded by a # sign. They are in effect until the end of a line and can be started anywhere on the line. EXAMPLE # This is a comment
Wildcards	There are some characters that are evaluated by the shell in a special way. They are called shell metacharacters or "wildcards." These characters are neither numbers or letters. For example, the *, ?, and [ ] are used for filename expansion. The <, >, 2>, >>, and | symbols are used for standard I/O redirection and pipes. To prevent these characters from being interpreted by the shell they must be quoted. EXAMPLE rm *; ls ??; cat file[1-3]; echo "How are you?"
Displaying output	To print output to the screen, the echo command is used. Wildcards must be escaped with either a backslash or matching quotes. EXAMPLE echo "How are you?"
Local variables	Local variables are in scope for the current shell. When a script ends, they are no longer available; i.e., they go out of scope. Local variables can also be defined with the built-in declare function. Local variables are set and assigned values. EXAMPLE variable_name=value declare variable_name=value name="John Doe" x=5
Global variables	Global variables are called environment variables and are created with the export built-in command. They are set for the currently running shell and any process spawned from that shell. They go out of scope when the script ends. The built-in declare function with the -x option also sets an environment variable and marks it for export. EXAMPLE export VARIABLE_NAME=value declare -x VARIABLE_NAME=value export PATH=/bin:/usr/bin:.
Extracting values from variables	To extract the value from variables, a dollar sign is used. EXAMPLE echo $variable_name echo $name echo $PATH
Reading user input	The user will be asked to enter input. The read command is used to accept a line of input. Multiple arguments to read will cause a line to be broken into words, and each word will be assigned to the named variable. EXAMPLE echo "What is your name?" read name read name1 name2 ...
Arguments	Arguments can be passed to a script from the command line. Positional parameters are used to receive their values from within the script. EXAMPLE At the command line: $ scriptname arg1 arg2 arg3 ... In a script:
	echo $1 $2 $3		Positional parameters
	echo $*		All the positional paramters
	echo $#		The number of positional parameters
Arrays	The Bourne shell utilizes positional parameters to create a word list. In addition to positional parameters, the Bash shell supports an array syntax whereby the elements are accessed with a subscript, starting at 0. Bash shell arrays are created with the declare -a command. EXAMPLE set apples pears peaches (positional parameters) echo $1 $2 $3 declare -a array_name=(word1 word2 word3 ...) declare -a fruit=( apples pears plums ) echo ${fruit[0]}
Command substitution	Like the C/TC shells and the Bourne shell, the output of a UNIX/Linux command can be assigned to a variable, or used as the output of a command in a string, by enclosing the command in backquotes. The Bash shell also provides a new syntax. Instead of placing the command between backquotes, it is enclosed in a set of parentheses, preceded by a dollar sign. EXAMPLE variable_name=`command` variable_name=$( command ) echo $variable_name echo "Today is `date`" echo "Today is $(date)"
Arithmetic	The Bash shells support integer arithmetic. The declare -i command will declare an integer type variable.The Korn shell's typeset command can also be use for backward compatibility. Integer arithmetic can be performed on variables declared this way. Otherwise the (( )) (let command ) syntax is used for arithmetic operations.
	EXAMPLE
	declare -i variable_name		used for bash
	typeset -i variable_name		can be used to be compatible with ksh
	(( n=5 + 5 )) echo $n
Operators	The Bash shell uses the built-in test command operators to test numbers and strings, similar to C language operators.
	EXAMPLE
	Equality:			Logical:
	==	equal to		&&	and
	!=	not equal to		||	or
				!	not
	Relational:
	>	greater than
	>=	greater than, equal to
	<	less than
	<=	less than, equal to
Conditional statements	The if construct is followed by an expression enclosed in parentheses. The operators are similar to C operators. The then keyword is placed after the closing paren. An if must end with an endif. The new [[ ]] test command is now used to allow pattern matching in conditional expressions. The old [ ] test command is still available for backward compatibility with the Bourne shell. The case command is an alternative to if/else. EXAMPLE
	The if construct is: if command then block of statements fi if [[ expression ]] then block of statements fi if (( numeric expression )) then block of statements else block of statements fi The if/else construct is: if command then block of statements else block of statements fi if [[ expression ]] then block of statements else block of statements fi if (( numeric expression )) then block of statements else block of statements fi The case construct is: case variable_name in pattern1) statements ;; pattern2) statements ;; pattern3) ;; esac case "$color" in blue) echo $color is blue ;; green) echo $color is green ;; red|orange) echo $color is red or orange ;; *) echo "Not a matach" ;; esac			The if/else/else if construct is: if command then block of statements elif command then block of statements else if command then block of statements else block of statements fi ------------------------- if [[ expression ]] then block of statements elif [[ expression ]] then block of statements else if [[ expression ]] then block of statements else block of statements fi -------------------------- if (( numeric expression )) then block of statements elif (( numeric expression )) then block of statements else if ((numeric expression)) then block of statements else block of statements fi
Loops	There are four types of loops: while, until, for, and select. The while loop is followed by an expression enclosed in square brackets, a do keyword, a block of statements, and terminated with the done keyword. As long as the expression is true, the body of statements between do and done will be executed. The compound test operator [[ ]] is new with Bash, and the old-style test operator [ ] can still be used to evaluate conditional expressions for backward compatibility with the Bourne shell. The until loop is just like the while loop, except the body of the loop will be executed as long as the expression is false. The for loop is used to iterate through a list of words, processing a word and then shifting it off, to process the next word. When all words have been shifted from the list, it ends. The for loop is followed by a variable name, the in keyword, a list of words, then a block of statements, and terminates with the done keyword. The select loop is used to provide a prompt and a menu of numbered items from which the user inputs a selection. The input will be stored in the special built-in REPLY variable. The select loop is normally used with the case command. The loop control commands are break and continue. The break command allows control to exit the loop before reaching the end of it, and the continue command allows control to return to the looping expression before reaching the end.
	EXAMPLE while command until command do do block of statements block of statements done done ------------------------- --------------------------- while [[ string expression ]] until [[ string expression ]] do do block of statements block of statements done done ------------------------- ---------------------------- while (( numeric expression )) until (( numeric expression )) do do block of statements block of statements done done for variable in word_list select variable in word_list do do block of statements block of statements done done -------------------------- ---------------------------- for color in red green b PS3="Select an item from the menu" do do item in blue red green echo $color echo $item done done
						Shows menu: blue red green
Functions	Functions allow you to define a section of shell code and give it a name. There are two formats, one from the Bourne shell, and the Bash version that uses the function keyword. EXAMPLE function_name() { block of code } function function_name { block of code } ------------------------ function lister { echo Your present working directory is `pwd` echo Your files are: ls }

2.6.1 The Bash Shell Script

Example 2.7.

1   #!/bin/bash

    # GNU bash versions 2.x

2   # The Party Program––Invitations to friends from the "guest" file

3   guestfile=~/shell/guests

4   if [[ ! –e "$guestfile" ]]

    then

5       printf "${guestfile##*/} non–existent"

        exit 1

    fi

6   export PLACE="Sarotini's"

7   (( Time=$(date +%H) + 1 ))

8   declare -a foods=(cheese crackers shrimp drinks `"hot dogs"` sandwiches)

9   declare -i  n=0

10  for person in $(cat $guestfile)

    do

11      if  [[ $person == root ]]

        then

              continue

        else

              # Start of here document

12            mail –v –s "Party" $person <<- FINIS

              Hi $person! Please join me at $PLACE for a party!

              Meet me at $Time o'clock.

              I'll bring the ice cream. Would you please bring

              ${foods[$n] and anything else you would like to eat?

              Let me know if you can make it.

                     Hope to see you soon.

                          Your pal,

                          ellie@$(hostname)

              FINIS

13            n=n+1

14            if (( ${#foods[*]} ==  $n ))

              then

15               declare -a foods=(cheese crackers shrimp drinks `"hot dogs"`

                                   sandwiches)

16            n=0

              fi

        fi

17  done

    printf "Bye..."

EXPLANATION