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Comments on How to read lines into an array in Bash

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How to read lines into an array in Bash

+6
−0

I wish to fill an array with strings, using Bash scripting. The strings are in a file, one per line. Here is what I've tried:

declare -a my_array=()

while read line; do
    my_array+=( "$line" )
done < my_file.txt

But it seems to only add the first line as tested with echo "$my_array".

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+8
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1st issue

As others have said, echo "$array" only prints the first element of the array; I suggest

printf '%s\n' "${array[@]}"

to print each element on a line.

2nd issue

More importantly, the loop is partially incorrect. Consider this file:

pear yellow
   apple \\ red   
orange brown    

Look closely: Lines 2 and 3 contain trailing spaces.

That's what the array gets (using underscores as delimiters to better see exactly what each element contains):

$ printf '_%s_\n' "${array[@]}"
_pear yellow_
_apple \ red_
_orange brown_
  1. All the leading and trailing spaces were mangled. Address that with an empty IFS.
  2. \\ became \. The read command interprets backslash sequences, add the -r flag to disable that.

Solution

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

or, better, as suggested by r~~, use mapfile -t array < file (mapfile and readarray are synonyms). It is a single command and thus more efficient.

Caveat

Finally, mind that shell loops are very slow. A test on a 10 MB file:

$ wc < /var/log/kern.log.0
  117555  1389245 10000045

~$ time { mapfile -t a < /var/log/kern.log.0; }
real    0m0.060s
user    0m0.052s
sys     0m0.008s

$ time { while IFS= read -r line; do b+=("$line"); done < /var/log/kern.log.0; }
real    0m0.720s
user    0m0.652s
sys     0m0.068s

And that is just reading the file into an array and doing nothing with it.

If attempting to process text, use actual text-processing tools (Awk, Sed, Jq, Csvkit, etc. depending on the task).

Further reading

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1 comment thread

Another caveat with the loop is trailing lines without newline. read will exit because it hit EOF, so... (2 comments)
Another caveat with the loop is trailing lines without newline. read will exit because it hit EOF, so...
Someone‭ wrote about 3 years ago

Another caveat with the loop is trailing lines without newline. read will exit because it hit EOF, so you have to check if $line is empty after the loop. A simple demonstrator: echo -n $'a\nb' | (while read -r line; do echo $line; done; echo "After: $line")

Quasímodo‭ wrote about 3 years ago

Someone‭ Well noted. Though, also to note, is that text-files are required to always have end in a newline character in Unix systems.