06. Arrays and Strings (Part 1)

Creating / Initializing Array

• The bash shell supports one dimensional arrays

• An array can contain more than one type of data (both numeric values and strings can be stored in one array)

• Index values for an array starts at 0 and increments by 1

• Index values can be a constant value, or can be the result of an arithmetic expression

• Data in an array do not have to be at consecutive indexes

  • It’s possible to create an array with data at indexes 0, 3, 5, 8. Then if you access the array at index 1 or 2, you will get a null value

• There are 2 ways to create an array and assign values to the array

  • First way:

    • arrayName=(value1  value2  value3  . . . )

      • value1 will be at index 0, value2 will be at index 1, and so on

      • values are listed with space as delimiter

  • Second way to create the same array:

    • arrayName[0]=value1

    • arrayName[1]=value2

    • arrayName[2]=value3   . . .

    • This way is most often accomplished with a loop

• For both ways: If the values to be stored in the array contain spaces or other shell metacharacters, quote them to preserve these characters

Accessing Arrays

• To access one element of an array: 

${arrayName[index]}

• Example

classes=(cis18a  cis18b  cis18c)

echo ${classes[2]} print cis18c

• To access all elements of an array:

${arrayName[@]}   or     ${arrayName[*]}

• Example

echo ${classes[@]}   print cis18a  cis18b  cis18c

echo ${classes[*]}   print cis18a  cis18b  cis18c

• Just as with command line arguments, the @ and * are only different when used within double quotes, where “[@]” is the list of individual elements, and “[*]” is a string containing all elements

• If an array name is used without an index value, it refers to the first element of the array

  • Example:  echo $classes print cis18a

Length of an Array

• To find the number of non-null elements of an array: 

${#arrayName[@]}

• Example

classes=(cis18a  cis18b  cis18c)

echo ${#classes[@]} print  3

• To find the number of characters of one element of an array:

${#arrayName[index]}

• Example

echo ${#classes[2]}   print  6

Extract Elements from an Array

• Just like with filters, when the shell extracts data from an array or an element, it makes a copy and does not modify the array or element

• To extract some particular elements from an array: 

${arrayName[@]:startIndex:num}

where startIndex is the starting index, and num is the number of elements to extract

• Example

classes=(cis18a  cis18b  cis18c  cis22a  cis22b  cis22c)

echo ${classes[@]:2:3} print  cis18c  cis22a  cis22b

• To extract some particular characters of one element of an array:

${arrayName[index]:startChar:num}

where startChar is the starting character, num is the number of characters to extract, and character count starts at 0 for the start character

• Example

echo ${classes[2]:2:4}  print   s18c

Adding Elements to an Array

• To add an element to an array:      arrayName[index]=value

  • If index is larger than the last index of the array, then the new value is appended to the array

  • If index is less than or equal to the last index of the array, then the new value replaces an existing value in the array

• Since the indexes don’t have to be continuous, it’s possible to add an element at an index which is much larger than the last index of the array

• To append multiple elements to an array

arrayName=( ${arrayName[@]}  value1  value2  value3 )

• Example

classes=(cis18a  cis18b)

classes=( ${classes[@]}  cis33a   cis33b )

echo ${classes[@]}       print  cis18a  cis18b  cis33a  cis33b

classes[8] = cis18c     cis18c is at index 8, values at indexes 4-7 are null

• To join one array to another array: 

array1=( ${array1[@]}    ${array2[@]}   $array3[@]} )

# array1is now made up of all elements of array1, followed by all elements of array2, then all elements of array3

• To create a new array from other arrays: 

newArray=( ${array1[@]}   ${array2[@]}   ${array3[@]} )

# newArray is made up of all elements of array1, followed by all elements of array2, then all elements of array3

• Example

classes=(cis18a   cis18b   cis18c)

nums=(1  2  3)

classes=( ${classes[@]}  ${nums[@]} )

echo ${classes[@]}       print  cis18a  cis18b  cis18c  1  2  3

Removing Elements from an Array

• To null an element of the array: 

unset   arrayName[index]

• The value at index is now null, and ${#arrayName[@]} is smaller by 1

• Example: 

classes=(cis18a  cis18b  cis18c)

unset   classes [1]

echo  ${classes[1]} print nothing

echo ${#classes[@]}   print  2 (size is smaller by 1)

• To remove an entire array

unset   arrayName

Search and Replace Elements in an Array

• To search and replace an element in the array: 

arrayName=( ${arrayName[@]/searchString/replacementString} )

• Note the pattern is similar to the vi or sed substitute pattern

  • searchString is the what the shell will look for in the array. It can be a regular expression

  • replacementString is what the shell uses to replace the searchString if it’s found in an element of an array

• Example: 

classes=(cis18a  cis18b  cis18c  cis26a )

classes=( ${classes[@]/18/25} )

echo ${classes[@]}       print  cis25a   cis25b   cis25c   cis26a  

Access Each Element in an Array

• To access each element in the array, use a for in loop

• Example:

classes=(cis18a   cis18b   cis18c  cis26a  cis26b)

for  class  in  ${classes[@]}

do

    echo $class print each class in the array

done

Store Multiple Input Data in an Array

• Use an array to store multiple input data values

• Example:

files=( `ls` )

for  file  in  ${files[@]}

do

    echo $file print each file in the current directory

done

• Use an array to read multiple data values from the user

• Use the –a option of read to store input data into an array

• Example:

read  -p “Enter numbers:”  -a  nums

for  num   in  ${nums[@]}

do

    echo $num print each number that user enters

done

• Use an array to store data from a file

• Example:

data=( `cat fileA` )

for  val in  ${data[@]}

do

    echo $val print each value in fileA

done

• Each value in the above example is a whitespace delimited word. So if each line in fileA has 8 words, and there are 5 lines total, then there will be 40 elements in the data array

• To read a whole line into one element of an array, set the IFS shell variable to \n:   IFS=$’\n’   

  • IFS is for Input Field Separator – just like with awk

Example:

IFS=$’\n’

lines=( ‘cat fileA’ ) lines is an array with 5 elements

• If each value of the file can be read in and processed one at a time, without having to keep track of all the values, then it is more efficient not to use an array

• Alternate way to read data from a file, without using an array:

for  val  in  `cat fileA` 

do

    echo $val print each value in fileA

done

• The downside of not using an array is that val is overwritten each time a new value is read in from fileA