Bash and IFS: string split to an array with specific separator add an extra empty element

Question

About split a string into an array we have two scenarios:

if the string has empty spaces how a separator, according with the following post:

Reading a space-delimited string into an array in Bash

So If I use:

string="Hello Unix World"
array1=($string)
echo ${array1[@]}
echo "size: '${#array1[@]}'"

read -a array2 <<< $string
echo ${array2[@]}
echo "size: '${#array2[@]}'"

The output is:

Hello Unix World
size: '3'
Hello Unix World
size: '3'

Both approaches works as expected.

Now, if the string has something different than an empty space how a separator, according with the following post:

How to split a string into an array in Bash?

So If I use:

path="/home/human/scripts"
IFS='/' read -r -a array <<< "$path"

echo "Approach 1"
echo ${array[@]}
echo "size: '${#array[@]}'"

echo "Approach 2"
for i in "${array[@]}"; do
   echo "$i"
done

echo "Approach 3"
for (( i=0; i < ${#array[@]}; ++i )); do
    echo "$i: ${array[$i]}"
done

It prints:

Approach 1
home human scripts    <--- apparently all is ok, but see the line just below!
size: '4'
Approach 2
                      <--- an empty element
home
human
scripts
Approach 3
0:                    <--- confirmed, the empty element
1: home
2: human
3: scripts

Why appears that empty element? How fix the command to avoid this situation?

a delimiter has a 'before' field and an 'after' field; when / is the delimiter this - /home/human/scripts - has 4x fields ... the empty string before the first /, and the other 3 fields that you know about; if the input was //home/human/scripts/ you'd have 6 fields ... 2 empty strings + home + human + scripts + 1 empty string — markp-fuso
– markp-fuso, Commented Jan 7, 2022 at 23:20
I should say, too, that if you had proper quoted the array in approach 1, echo "${array[@]}" would have output ` home human scripts, with the space separating the empty string from home` visible. Without quotes, echo really does only get three arguments; the unquoted empty string "vanishes" during word-splitting. — chepner
– chepner, Commented Jan 7, 2022 at 23:25
fwiw, you'll see the same behavior with other tools/commands that use delimiters (eg, cut and awk) or that use IFS to split the input (eg, read - see konsolebox's example) — markp-fuso
– markp-fuso, Commented Jan 7, 2022 at 23:42
The empty string is not the same as a space. The leading space in the output of echo "${array[@]}" comes from echo itself, not the array. — chepner
– chepner, Commented Jan 8, 2022 at 1:14

konsolebox · Accepted Answer · 2022-01-07 23:19:42Z

1

Your string split into 4 parts: an empty one, and the three words.

path="/home/human/scripts"
IFS='/' read -r -a array <<< "$path"
declare -p array

Output:

declare -a array=([0]="" [1]="home" [2]="human" [3]="scripts")

There are many ways to fix it. One is to delete the empty values. Another is to exclude the beginning slash before splitting.

for i in "${!array[@]}"; do
    [[ ${array[i]} ]] || unset 'array[i]'
done

Or

IFS='/' read -r -a array <<< "${path#/}"

The first one is adaptable to path forms were slashes are repeated not only in the beginning.

answered Jan 7, 2022 at 23:19

konsolebox

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5 Comments

Manuel Jordan Over a year ago

I used your second approach. Thanks. Can you expand the idea about why appeared the empty element?

chepner Over a year ago

Because it's a component of the string you split. Just because the strings starts with / doesn't mean the initial / is treated differently.

Manuel Jordan Over a year ago

Can you explain how works "${path#/}"? specially the "#/ part.

Manuel Jordan Over a year ago

@chepner now I see ... sorry very tired to realise about that. Thanks

konsolebox Over a year ago

It's one of those parameter expansion methods.

glenn jackman · Accepted Answer · 2022-01-08 02:55:31Z

1

Just to pile on with what is really a formatted comment:

The manual (in 3.5.7 Word Splitting) describes IFS as a "field terminator":

The shell treats each character of $IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators.

For IFS=/ read -a fields <<< "/home/user", the first field is the empty string terminated by the first slash.

answered Jan 8, 2022 at 2:55

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