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Loop through an array of strings in Bash?


Question

I want to write a script that loops through 15 strings (array possibly?) Is that possible?

Something like:

for databaseName in listOfNames
then
  # Do something
end
2019/04/22
1
1556
4/22/2019 9:44:39 PM

Accepted Answer

You can use it like this:

## declare an array variable
declare -a arr=("element1" "element2" "element3")

## now loop through the above array
for i in "${arr[@]}"
do
   echo "$i"
   # or do whatever with individual element of the array
done

# You can access them using echo "${arr[0]}", "${arr[1]}" also

Also works for multi-line array declaration

declare -a arr=("element1" 
                "element2" "element3"
                "element4"
                )
2016/11/14
2475
11/14/2016 7:53:22 AM


None of those answers include a counter...

#!/bin/bash
## declare an array variable
declare -a array=("one" "two" "three")

# get length of an array
arraylength=${#array[@]}

# use for loop to read all values and indexes
for (( i=1; i<${arraylength}+1; i++ ));
do
  echo $i " / " ${arraylength} " : " ${array[$i-1]}
done

Output:

1  /  3  :  one
2  /  3  :  two
3  /  3  :  three
2015/07/09

Yes

for Item in Item1 Item2 Item3 Item4 ;
  do
    echo $Item
  done

Output:

Item1
Item2
Item3
Item4

To preserve spaces; single or double quote list entries and double quote list expansions.

for Item in 'Item 1' 'Item 2' 'Item 3' 'Item 4' ;
  do
    echo "$Item"
  done

Output:

Item 1
Item 2
Item 3
Item 4

To make list over multiple lines

for Item in Item1 \
            Item2 \
            Item3 \
            Item4
  do
    echo $Item
  done

Output:

Item1
Item2
Item3
Item4


Simple list variable

List=( Item1 Item2 Item3 )

or

List=(
      Item1 
      Item2 
      Item3
     )

Display the list variable:

echo ${List[*]}

Output:

Item1 Item2 Item3

Loop through the list:

for Item in ${List[*]} 
  do
    echo $Item 
  done

Output:

Item1
Item2
Item3

Create a function to go through a list:

Loop(){
  for item in ${*} ; 
    do 
      echo ${item} 
    done
}
Loop ${List[*]}

Using the declare keyword (command) to create the list, which is technically called an array:

declare -a List=(
                 "element 1" 
                 "element 2" 
                 "element 3"
                )
for entry in "${List[@]}"
   do
     echo "$entry"
   done

Output:

element 1
element 2
element 3

Creating an associative array. A dictionary:

declare -A continent

continent[Vietnam]=Asia
continent[France]=Europe
continent[Argentina]=America

for item in "${!continent[@]}"; 
  do
    printf "$item is in ${continent[$item]} \n"
  done

Output:

 Argentina is in America
 Vietnam is in Asia
 France is in Europe

CVS variables or files in to a list.
Changing the internal field separator from a space, to what ever you want.
In the example below it is changed to a comma

List="Item 1,Item 2,Item 3"
Backup_of_internal_field_separator=$IFS
IFS=,
for item in $List; 
  do
    echo $item
  done
IFS=$Backup_of_internal_field_separator

Output:

Item 1
Item 2
Item 3

If need to number them:

` 

this is called a back tick. Put the command inside back ticks.

`commend` 

It is next to the number one on your keyboard and or above the tab key. On a standard american english language keyboard.

List=()
Start_count=0
Step_count=0.1
Stop_count=1
for Item in `seq $Start_count $Step_count $Stop_count`
    do 
       List+=(Item_$Item)
    done
for Item in ${List[*]}
    do 
        echo $Item
    done

Output is:

Item_0.0
Item_0.1
Item_0.2
Item_0.3
Item_0.4
Item_0.5
Item_0.6
Item_0.7
Item_0.8
Item_0.9
Item_1.0

Becoming more familiar with bashes behavior:

Create a list in a file

cat <<EOF> List_entries.txt
Item1
Item 2 
'Item 3'
"Item 4"
Item 7 : *
"Item 6 : * "
"Item 6 : *"
Item 8 : $PWD
'Item 8 : $PWD'
"Item 9 : $PWD"
EOF

Read the list file in to a list and display

List=$(cat List_entries.txt)
echo $List
echo '$List'
echo "$List"
echo ${List[*]}
echo '${List[*]}'
echo "${List[*]}"
echo ${List[@]}
echo '${List[@]}'
echo "${List[@]}"

BASH commandline reference manual: Special meaning of certain characters or words to the shell.

2019/12/08

In the same spirit as 4ndrew's answer:

listOfNames="RA
RB
R C
RD"

# To allow for other whitespace in the string:
# 1. add double quotes around the list variable, or
# 2. see the IFS note (under 'Side Notes')

for databaseName in "$listOfNames"   #  <-- Note: Added "" quotes.
do
  echo "$databaseName"  # (i.e. do action / processing of $databaseName here...)
done

# Outputs
# RA
# RB
# R C
# RD

B. No whitespace in the names:

listOfNames="RA
RB
R C
RD"

for databaseName in $listOfNames  # Note: No quotes
do
  echo "$databaseName"  # (i.e. do action / processing of $databaseName here...)
done

# Outputs
# RA
# RB
# R
# C
# RD

Notes

  1. In the second example, using listOfNames="RA RB R C RD" has the same output.

Other ways to bring in data include:

Read from stdin

# line delimited (each databaseName is stored on a line)
while read databaseName
do
  echo "$databaseName"  # i.e. do action / processing of $databaseName here...
done # <<< or_another_input_method_here
  1. the bash IFS "field separator to line" [1] delimiter can be specified in the script to allow other whitespace (i.e. IFS='\n', or for MacOS IFS='\r')
  2. I like the accepted answer also :) -- I've include these snippets as other helpful ways that also answer the question.
  3. Including #!/bin/bash at the top of the script file indicates the execution environment.
  4. It took me months to figure out how to code this simply :)

Other Sources (while read loop)

2017/05/23

You can use the syntax of ${arrayName[@]}

#!/bin/bash
# declare an array called files, that contains 3 values
files=( "/etc/passwd" "/etc/group" "/etc/hosts" )
for i in "${files[@]}"
do
    echo "$i"
done
2017/10/25

Surprised that nobody's posted this yet -- if you need the indices of the elements while you're looping through the array, you can do this:

arr=(foo bar baz)

for i in ${!arr[@]}
do
    echo $i "${arr[i]}"
done

Output:

0 foo
1 bar
2 baz

I find this a lot more elegant than the "traditional" for-loop style (for (( i=0; i<${#arr[@]}; i++ ))).

(${!arr[@]} and $i don't need to be quoted because they're just numbers; some would suggest quoting them anyway, but that's just personal preference.)

2018/08/10

Source: https://stackoverflow.com/questions/8880603
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