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

Answer 1

Two little tricks

1. Implicit array for script or functions:

In addition to anubhava's correct answer: If basic syntax for loop is:

for var in "${arr[@]}" ;do ...$var... ;done

there is a special case in bash:

When running a script or a function, arguments passed at command lines will be assigned to $@ array variable, you can access by $1, $2, $3, ... and so on.

This can be populated (for test) by

set -- arg1 arg2 arg3 ...

A loop over this array could be written simply:

for item ;do
    echo "This is item: $item."
  done

Note that the reserved work in is not present and no array name too!

Sample:

set -- arg1 arg2 arg3 ...
for item ;do
    echo "This is item: $item."
  done

This is item: arg1.
This is item: arg2.
This is item: arg3.
This is item: ....

Note that this is same than

for item in "$@";do
    echo "This is item: $item."
  done

Then into a script:

#!/bin/bash

for item ;do
    printf "Doing something with '%s'.\n" "$item"
  done

Save this in a script myscript.sh, chmod +x myscript.sh, then

./myscript.sh arg1 arg2 arg3 ...

Doing something with 'arg1'.
Doing something with 'arg2'.
Doing something with 'arg3'.
Doing something with '...'.

Same in a function:

myfunc() { for item;do cat <<<"Working about '$item'."; done ; }

Then

myfunc item1 tiem2 time\ 3

Working about 'item1'.
Working about 'tiem2'.
Working about 'time 3'.

2. Using List of array keys

man bash | sed 'H;/^$/{x;s/^\n//;s/\n$//;/List of array keys/p;};d'

  ${!name[@]}
  ${!name[*]}
         List of array keys.  If name is an array  variable,  expands  to
         the  list  of array indices (keys) assigned in name.  If name is
         not an array, expands to 0 if name is set  and  null  otherwise.
         When  @  is used and the expansion appears within double quotes,
         each key expands to a separate word.

declare -a arr=("element 1" "element 2" "element 3")

for i in "${!arr[@]}";do
    printf 'Field id:%2d is "%s".\n' "$i" "${arr[i]}"
    # Doing something with "${arr[i]}"
done

Field id: 0 is "element 1".
Field id: 1 is "element 2".
Field id: 2 is "element 3".

2.1 Use case sample

Index of array are commonly beginning by 0, but you could assign array element by number directly. For sample, using birthday as index:

#!/bin/bash

exec {people}<<eoPeopleList
    Alice        20020401
    Bob          20011109
    Charles      20050728
    Zoe          20030401
eoPeopleList
printf -v thisYear '%(%Y)T' -1
while read -ru $people name birthdate; do
    birthdays[10#${birthdate:4}]+="$name ($(( thisYear - ${birthdate::4} ))y), "
done
for day in "${!birthdays[@]}";do
    printf 'People who\47s birthday is %(%b)T %2d: %s\n' \
        $((${day::-2}*2462400)) $((10#${day: -2})) "${birthdays[day]%, }"
done

Then running this script (in 2024) will output:

People who's birthday is Apr  1: Alice (22y), Zoe (21y)
People who's birthday is Jul 28: Charles (19y)
People who's birthday is Nov  9: Bob (23y)

2.1b Using more arrays

As we are working aroung arrays, here's a modified version using locale for assigning month's names to an array variable

#!/bin/bash

exec {people}<<eoPeopleList
    Alice        20020401
    Bob          20011109
    Charles      20050728
    Zoe          20030401
eoPeopleList
thisYear=$(date +%Y)
while read -ru $people name birthdate; do
    birthdays[10#${birthdate:4}]+="$name ($(( thisYear - ${birthdate::4} ))y), "
done
exec {people}<&-

mapfile -d\; -t months <<<";$(locale abmon)"

for day in "${!birthdays[@]}";do
    printf 'People who\47s birthday is %s %2d: %s\n' \
        ${months[${day::-2}]} $((10#${day: -2})) "${birthdays[day]%, }"
done

For strictly same output.

People who's birthday is Apr  1: Alice (22y), Zoe (21y)
People who's birthday is Jul 28: Charles (19y)
People who's birthday is Nov  9: Bob (23y)

Answer 2

Below code declares an array of locales and then loop over them to fetch content from sitecore using curl command and write them in relevant file.

declare -a SUPPORTED_LOCALES=("en" "fr" "de" "nl" "it" "es" "pt")

for locale in "${SUPPORTED_LOCALES[@]}";
do
  curl -s "http://sitecoreurl.com?sc_lang=$locale" > assets/sitecore/indexpage_$locale.json
done

Tip As common in many languages, in bash the array declaration dont need comma as a separator. So if you happen to define it like

declare -a SUPPORTED_LOCALES=("en" ,"fr", "de", "nl", "it", "es", "pt")

This will create array of only one element which is "en" ,"fr", "de", "nl", "it", "es", "pt" This was weird for me, but took a while to understand whats happening.

Answer 3

I used this approach for my GitHub updates, and I found it simple.

## declare an array variable
arr_variable=("kofi" "kwame" "Ama")

## now loop through the above array
for i in "${arr_variable[@]}"
do
   echo "$i"


done
   

You can iterate through bash array values using a counter with three-expression (C style) to read all values and indexes for loops syntax:

declare -a kofi=("kofi" "kwame" "Ama")
 
# get the length of the array
length=${#kofi[@]}

for (( j=0; j<${length}; j++ ));
do
  print (f "Current index %d with value %s\n" $j "${kofi[$j]}")
done

Answer 4

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=0; i<${arraylength}; i++ ));
do
  echo "index: $i, value: ${array[$i]}"
done

Output:

index: 0, value: one
index: 1, value: two
index: 2, value: three

Answer 5

How you loop through an array, depends on the presence of new line characters. With new line characters separating the array elements, the array can be referred to as "$array", otherwise it should be referred to as "${array[@]}". The following script will make it clear:

#!/bin/bash

mkdir temp
mkdir temp/aaa
mkdir temp/bbb
mkdir temp/ccc
array=$(ls temp)
array1=(aaa bbb ccc)
array2=$(echo -e "aaa\nbbb\nccc")

echo '$array'
echo "$array"
echo
for dirname in "$array"; do
    echo "$dirname"
done
echo
for dirname in "${array[@]}"; do
    echo "$dirname"
done
echo
echo '$array1'
echo "$array1"
echo
for dirname in "$array1"; do
    echo "$dirname"
done
echo
for dirname in "${array1[@]}"; do
    echo "$dirname"
done
echo
echo '$array2'
echo "$array2"
echo
for dirname in "$array2"; do
    echo "$dirname"
done
echo
for dirname in "${array2[@]}"; do
    echo "$dirname"
done
rmdir temp/aaa
rmdir temp/bbb
rmdir temp/ccc
rmdir temp

Answer 6

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

CSV 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.

`command` 

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.

Answer 7

What I really needed for this was something like this:

for i in $(the_array); do something; done

For instance:

for i in $(ps -aux | grep vlc  | awk '{ print $2 }'); do kill -9 $i; done

(Would kill all processes with vlc in their name)

Answer 8

Simple way :

arr=("sharlock"  "bomkesh"  "feluda" )  ##declare array

len=${#arr[*]}  # it returns the array length

#iterate with while loop
i=0
while [ $i -lt $len ]
do
    echo ${arr[$i]}
    i=$((i+1))
done


#iterate with for loop
for i in $arr
do
  echo $i
done

#iterate with splice
 echo ${arr[@]:0:3}

Answer 9

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.)

Answer 10

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

Answer 11

Single line looping,

 declare -a listOfNames=('db_a' 'db_b' 'db_c')
 for databaseName in ${listOfNames[@]}; do echo $databaseName; done;

you will get an output like this,

db_a
db_b
db_c

Answer 12

I loop through an array of my projects for a git pull update:

#!/bin/sh
projects="
web
ios
android
"
for project in $projects do
    cd  $HOME/develop/$project && git pull
end

Answer 13

Possible first line of every Bash script/session:

say() { for line in "${@}" ; do printf "%s\n" "${line}" ; done ; }

Use e.g.:

$ aa=( 7 -4 -e ) ; say "${aa[@]}"
7
-4
-e

May consider: echo interprets -e as option here

Answer 14

Try this. It is working and tested.

for k in "${array[@]}"
do
    echo $k
done

# For accessing with the echo command: echo ${array[0]}, ${array[1]}

Answer 15

The declare array doesn't work for Korn shell. Use the below example for the Korn shell:

promote_sla_chk_lst="cdi xlob"

set -A promote_arry $promote_sla_chk_lst

for i in ${promote_arry[*]};
    do
            echo $i
    done

Answer 16

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:

• stdin (listed below),
• variables,
• an array (the accepted answer),
• a file...

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

2. 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')
3. I like the accepted answer also :) -- I've include these snippets as other helpful ways that also answer the question.
4. Including #!/bin/bash at the top of the script file indicates the execution environment.
5. It took me months to figure out how to code this simply :)

Other Sources (while read loop)

Answer 17

That is possible, of course.

for databaseName in a b c d e f; do
  # do something like: echo $databaseName
done 

See Bash Loops for, while and until for details.

Answer 18

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"
                )

Answer 19

This is similar to user2533809's answer, but each file will be executed as a separate command.

#!/bin/bash
names="RA
RB
R C
RD"

while read -r line; do
    echo line: "$line"
done <<< "$names"

Answer 20

listOfNames="db_one db_two db_three"
for databaseName in $listOfNames
do
  echo $databaseName
done

or just

for databaseName in db_one db_two db_three
do
  echo $databaseName
done

Answer 21

This is also easy to read:

FilePath=(
    "/tmp/path1/"    #FilePath[0]
    "/tmp/path2/"    #FilePath[1]
)

#Loop
for Path in "${FilePath[@]}"
do
    echo "$Path"
done

Answer 22

If you are using Korn shell, there is "set -A databaseName ", else there is "declare -a databaseName"

To write a script working on all shells,

 set -A databaseName=("db1" "db2" ....) ||
        declare -a databaseName=("db1" "db2" ....)
# now loop 
for dbname in "${arr[@]}"
do
   echo "$dbname"  # or whatever

done

It should be work on all shells.