[arrays] Creating an array from a text file in Bash

A script takes a URL, parses it for the required fields, and redirects its output to be saved in a file, file.txt. The output is saved on a new line each time a field has been found.

file.txt

A Cat
A Dog
A Mouse 
etc... 

I want to take file.txt and create an array from it in a new script, where every line gets to be its own string variable in the array. So far I have tried:

#!/bin/bash

filename=file.txt
declare -a myArray
myArray=(`cat "$filename"`)

for (( i = 0 ; i < 9 ; i++))
do
  echo "Element [$i]: ${myArray[$i]}"
done

When I run this script, whitespace results in words getting split and instead of getting

Desired output

Element [0]: A Cat 
Element [1]: A Dog 
etc... 

I end up getting this:

Actual output

Element [0]: A 
Element [1]: Cat 
Element [2]: A
Element [3]: Dog 
etc... 

How can I adjust the loop below such that the entire string on each line will correspond one-to-one with each variable in the array?

You can do this too:

oldIFS="$IFS"
IFS=$'\n' arr=($(<file))
IFS="$oldIFS"
echo "${arr[1]}" # It will print `A Dog`.

Note:

Filename expansion still occurs. For example, if there's a line with a literal * it will expand to all the files in current folder. So use it only if your file is free of this kind of scenario.

mapfile and readarray (which are synonymous) are available in Bash version 4 and above. If you have an older version of Bash, you can use a loop to read the file into an array:

arr=()
while IFS= read -r line; do
  arr+=("$line")
done < file

In case the file has an incomplete (missing newline) last line, you could use this alternative:

arr=()
while IFS= read -r line || [[ "$line" ]]; do
  arr+=("$line")
done < file

Related:

• Need alternative to readarray/mapfile for script on older version of Bash

You can simply read each line from the file and assign it to an array.

#!/bin/bash
i=0
while read line 
do
        arr[$i]="$line"
        i=$((i+1))
done < file.txt

This answer says to use

mapfile -t myArray < file.txt

I made a shim for mapfile if you want to use mapfile on bash < 4.x for whatever reason. It uses the existing mapfile command if you are on bash >= 4.x

Currently, only options -d and -t work. But that should be enough for that command above. I've only tested on macOS. On macOS Sierra 10.12.6, the system bash is 3.2.57(1)-release. So the shim can come in handy. You can also just update your bash with homebrew, build bash yourself, etc.

It uses this technique to set variables up one call stack.

Use mapfile or read -a

Always check your code using shellcheck. It will often give you the correct answer. In this case SC2207 covers reading a file that either has space separated or newline separated values into an array.

Don't do this

array=( $(mycommand) )

Files with values separated by newlines

mapfile -t array < <(mycommand)

Files with values separated by spaces

IFS=" " read -r -a array <<< "$(mycommand)"

The shellcheck page will give you the rationale why this is considered best practice.