How to initialize a bash array with output piped from another command?

Question

Is there any way to pipe the output of a command which lists a bunch of numbers (each number in a separate line) and initialize a bash array with those numbers?

Details: This lists 3 changelist numbers which have been submitted in the following date range. The output is then piped to cut to filter it further to get just the changelist numbers.

p4 changes -m 3 -u edk -s submitted @2009/05/01,@now | cut -d ' ' -f 2

E.g. :

422311
543210
444000

How is it possible to store this list in a bash array?

nik · Accepted Answer · 2009-06-11T13:33:29.710

72

You can execute the command under ticks and set the Array like,

ARRAY=(`command`)

Alternatively, you can save the output of the command to a file and cat it similarly,

command > file.txt
ARRAY=(`cat file.txt`)

Or, simply one of the following forms suggested in the comments below,

ARRAY=(`< file.txt`)
ARRAY=($(<file.txt))

edited Jun 11 '09 at 13:33

answered Jun 09 '09 at 16:41

nik

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Useless use of cat: ` – ephemient Jun 09 '09 at 17:46
fixed the answer for future reference. – nik Jun 09 '09 at 17:54
10

Whenever possible, avoid the use of back quotes. This is more readable and can work with nesting without awkward quoting: ARRAY=($(command)) or ARRAY=($(< file.txt)) – Dennis Williamson Jun 09 '09 at 22:15
2

Be warned that this will ALWAYS use whitespace to separate elements. http://stackoverflow.com/questions/9449417/how-do-i-assign-the-output-of-a-command-into-an-array reports possibility to handle whitespace by setting `IFS` to `'\n'` but I couldn't make it accept `'\0'`. – ivan_pozdeev May 14 '16 at 19:49
Why does `folders=(\`ls -1 ${FOLDER} | xargs\`)` give me `Syntax error: "(" unexpected` ? – TheEagle Aug 16 '21 at 14:45
Self-answer: because I had `#!/bin/sh` instead of `#!/bin/bash` … :roll: – TheEagle Aug 16 '21 at 15:43
This splits the input by spaces, not by newlines. – Jindrich Vavruska Oct 02 '21 at 09:38

score 16 · Answer 2 · answered Dec 11 '15 at 13:21

16

If you use bash 4+, it has special command for this: mapfile also known as readarray, so you can fill your array like this:

declare -a a
readarray -t a < <(command)

for more portable version you can use

declare -a a
while read i; do
  a=( "${a[@]}" "$i" )
done < <(command)

answered Dec 11 '15 at 13:21

Andrey Starodubtsev

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This is my first time seeing `< <(command)`. Can you please help me find more detail on this syntax. I'm interested in knowing why it needs 2 `<` to work. Thanks – bazz Dec 18 '15 at 01:54
3

First `<` is redirection of input stream, see https://www.gnu.org/software/bash/manual/html_node/Redirections.html#Redirecting-Input for detains. Construction `<(command)` means that output of `command` is piped to named fifo (usually `/dev/fdn`), you can find details on it at https://www.gnu.org/software/bash/manual/html_node/Process-Substitution.html. – Andrey Starodubtsev Dec 18 '15 at 07:09
Be warned that `readarray` doesn't support an alternative separator, it always uses newline. – ivan_pozdeev May 14 '16 at 18:48
3

`a[${#a[*]}]=value` or `a+=(value)` is a better syntax to append to an array. – ivan_pozdeev May 14 '16 at 18:52
1

Re @ivan_pozdeev `readarray`/`mapfile` apparently does support defining alternative delimiters. The [docs](https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html#index-mapfile) specify `-d delim`: "The first character of delim is used to terminate each input line, rather than newline." – Allan Lewis Dec 08 '17 at 12:24
@AllanLewis `-d` for `readarray` [is new in `bash 4.4`](http://git.savannah.gnu.org/cgit/bash.git/diff/NEWS?id=a0c0a00fc419b7bc08202a79134fcd5bc0427071). – ivan_pozdeev Dec 08 '17 at 19:40

score 3 · Answer 3 · answered Aug 10 '18 at 10:57

3

Quite similar to #4 but looks a bit better for me. :)

declare -a a
readarray -t a <<< $(command)

answered Aug 10 '18 at 10:57

Maciej Wawrzyńczuk

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Interesting alternative. However the methods differ in case of an empty output from the command, the here-string method gives an array having one empty item, whereas the names pipe method gives an empty array. – luciole75w Nov 19 '19 at 02:47

How to initialize a bash array with output piped from another command?

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