BASH: read line and split it to array

cristalp · 11-24-2011, 11:14 AM

Dear all,

I have a file with several fields for each line:

Code:

aaa   bbb   ccc
ddd   eee   fff
ggg   hhh   iii
...

I want to read this file line by line and when it read each line, this line is sent to an array with each filed as its element. I use a while loop to do this:

Code:

while read LINE
do
a=${LINE[0]}
b=${LINE[1]}
c=${LINE[2]}
echo $b
done < $1

I hope variable $a to be the first filed of each line in each cycle of while loop, that is aaa, ddd and ggg ..
same idea for $b and $c.

However, what I got is $b and $c are empty strings and $a is a string includes the whole line which has three fields together. I test it by echo them separately.

I have tried many tests and still do not find where is the problem. Could any one help me to figure it out? Thanks a lot!!

colucix · 11-24-2011, 11:24 AM

It's not clear from your example which variable should be an array. Here are three different cases:

1. Let read create an array with the content of each line: you need the -a option:

Code:

while read -a LINE
do
  echo ${LINE[0]}
  echo ${LINE[1]}
  echo ${LINE[2]}
done < file

2. Explicitly assign the content of each line to an array:

Code:

while read line
do
  array=($line)
  echo ${array[0]}
  echo ${array[1]}
  echo ${array[2]}
done < file

3. Assign the content of each line to three variables individually:

Code:

while read a b c
do
  echo $a
  echo $b
  echo $c
done < file

What is that one matching your requirement?

David the H. · 11-24-2011, 03:12 PM

To give line of text its own array element, you can use the new (bash v4+) mapfile command. a.k.a. readarray.

Code:

mapfile -t LINE < filename	#use -t to remove the trailing newlines.

mapfile also has a convenient -n option, allowing you to explictly set the index number to start on. Using -n 1 means that line one = index one, etc.

In older systems you can do something like this:

Code:

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

I cleared the IFS setting for read, because otherwise it removes all leading and trailing whitespace from each line. You can of course remove it if this behavior is desirable.

There's also an issue with loops using read when a file doesn't contain a final newline. The contents get stored in the variable, but the subsequent commands aren't executed. You may need to explicitly save it after the loop with an additional command:

Code:

[[ -n $line ]] && LINE+=( "$line" )

http://mywiki.wooledge.org/BashFAQ/001

And finally a caution: colucix's second example likely does not do what you want. It breaks each line up into individual words and saves them to the array, but then overwrites them again when the next line executes. So only the final line remains contained in the array at the end of the loop.