Bash Script – Assign Command Output Lines to Array Values

arraybashshell-script

When running a command, I get 10 lines of output. I want to take lines 2-4-6-8-10 and put them into an array.

Every time I run my command, the order changes so I need to do this in one go. I had tried running my command and picking out line 2, then running again and picking out line 4 etc., but because the order changes this does not work:

value1=$(my_command |sed '2q;d')
value2=$(my_command |sed '4q;d')
value3=$(my_command |sed '6q;d')
value4=$(my_command |sed '8q;d')
value5=$(my_command |sed '10q;d')

MY_ARRAY=("${value1}" "${value2}" "${value3}" "${value4}" "${value5}")

Best Answer

Using readarray in the bash shell, and GNU sed:

readarray -t my_array < <( my_command | sed '1~2d' )

The built-in readarray reads the lines into an array. The lines are read from a process substitution. The sed command in the process substitution will only output every second line read from my_command (and could also be written sed '1!n;d', or as sed -n 'n;p' with standard sed).

In GNU sed, the address n~m addresses every m:th line starting at line n. This is a GNU extension to standard sed, for convenience.

The my_command command will only ever be called once.

Testing:

$ readarray -t my_array < <( seq 10 | sed '1~2d' )

$ printf '%s\n' "${my_array[@]}"
2
4
6
8
10

Edit

You seem to be using bash version 3. Unfortunately, associative arrays are not available until version 4. I recommend you post a sanitized version of your original deploy.sh script with sensitive personal information removed so you can get ideas from other people about alternative approaches.

Edit 2

Just to summarize a bit of exchange in the chat:

The easiest way to do some action over all the arguments is to just iterate over them with a for loop:

    for arg; do
        scp login1@host1:"$arg" login2@host2:/dest/
    done

Remember to double-quote all instances of "$arg". You do not need to put the arguments in an array yourself, as they already exist in the array $@, which is what for uses by default when you don't give an explicit in list....

Bash – How to reassign new values to array elements

I would be suspicious of your version of Bash to start. I'm on this version and I can set arrays just fine.

$ bash --version
GNU bash, version 4.2.45(1)-release (x86_64-redhat-linux-gnu)

Example

$ array[0]=0
$ array[1]=1
$ array[2]=2

You can then recall them individually:

$ echo ${array[0]}
0

$ echo ${array[1]}
1

$ echo ${array[2]}
2

Or as a list:

$ echo "${array[@]}"
0 1 2

You cannot just call the array as another variable:

$ echo $array
0

I believe this may have been what was misleading you into thinking it wasn't working correctly. Also setting an array element with another variable's value also works.

Example

$ newval=3
$ array[2]=$newval

$ echo "${array[@]}"
0 1 3

You could also use printf here, doing so will show you why you want to quote the results of ${array[@]}. But first let's add a string to the array:

$ array[0]="a string"

Without the quotes:

$ printf "%s\n" ${array[@]}
a
string
1
3

With quotes:

$ printf "%s\n" "${array[@]}"
a string
1
3

Passing things around

Well you have 2 tactics that you can make use of within Bash. You can simply set variables so that they're globally scoped. This is often the technique that most people employ.

The other method involves passing by value, values to functions within Bash. I'll show a basic function where I pass it 2 values.

Example

$ function hello() {
 printf "I got arg1: %d and arg2: %d\n" $1 $2
}

Now when I execute it with different arguments:

$ hello 1 2
I got arg1: 1 and arg2: 2

$ hello 3 4
I got arg1: 3 and arg2: 4

$ hello 3 blah
bash: printf: blah: invalid number
I got arg1: 3 and arg2: 0

You can read more about arrays and functions within Bash here to get a better understanding of the technologies.

Best Answer

Related Solutions

Bash – Passing a bash command-line argument containing a dot

Edit

Edit 2

Bash – How to reassign new values to array elements

Example

Example

Passing things around

Example

References

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