How to add multiple line of output one by one to a variable in Bash?

Question

This might be a very basic question but I was not able to find solution. I have a script:

If I run w | awk '{print $1}' in command line in my server I get:

f931
smk591
sc271
bx972
gaw844
mbihk988
laid640
smk59
ycc951

Now I need to use this list in my bash script one by one and manipulate some operation on them. I need to check their group and print those are in specific group. The command to check their group is id username. How can I save them or iterate through them one by one in a loop.

what I have so far is

tmp=$(w | awk '{print $1})

But it only return first record! Appreciate any help.

Ed Morton · Accepted Answer · 2014-05-01 12:56:33Z

Populate an array with the output of the command:

$ tmp=( $(printf "a\nb\nc\n") )
$ echo "${tmp[0]}"
a
$ echo "${tmp[1]}"
b
$ echo "${tmp[2]}"
c

Replace the printf with your command (i.e. tmp=( $(w | awk '{print $1}') )) and man bash for how to work with bash arrays.

For a lengthier, more robust and complete example:

$ cat ./tstarrays.sh
# saving multi-line awk output in a bash array, one element per line
# See http://www.thegeekstuff.com/2010/06/bash-array-tutorial/ for
# more operations you can perform on an array and its elements.

oSET="$-"; set -f       # save original set flags and turn off globbing
oIFS="$IFS"; IFS=$'\n'  # save original IFS and make IFS a newline

array=( $(
    awk 'BEGIN{
        print "the      quick   brown"
        print "    fox jumped\tover\tthe"
        print "lazy    dogs back    "
    }'
) )

IFS="$oIFS"             # restore original IFS value
set +f -$oSET           # restore original set flags

for (( i=0; i < ${#array[@]}; i++ ));
do
    printf "array[%d] of length=%d: \"%s\"\n" "$i" "${#array[$i]}" "${array[$i]}"
done

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

printf -- "----------\n"
printf -- "array[*]=\n\"%s\"\n" "${array[*]}"

.

$ ./tstarrays.sh
array[0] of length=22: "the      quick   brown"
array[1] of length=23: "    fox jumped  over    the"
array[2] of length=21: "lazy    dogs back    "
----------
array[@]=
"the      quick   brown"
array[@]=
"    fox jumped over    the"
array[@]=
"lazy    dogs back    "
----------
array[*]=
"the      quick   brown     fox jumped  over    the lazy    dogs back    "

A couple of non-obvious key points to make sure your array gets populated with exactly what your command outputs:

If your command output can contain globbing characters than you should disable globbing before the command (oSET="$-"; set -f) and re-enable it afterwards (set +f -$oSET).
If your command output can contain spaces then set IFS to a newline before the command (oIFS="$IFS"; IFS=$'\n') and set it back to it's old value after the command (IFS="$oIFS").

saving multi-line awk output in a bash array I was looking for that. Learned much.

a5hk · Accepted Answer · 2014-05-01 11:55:36Z

2

tmp=$(w | awk '{print $1}')

while read i
do
    echo "$i"
done <<< "$tmp"

answered May 1, 2014 at 11:55

a5hk

7,9043 gold badges29 silver badges41 bronze badges

2 Comments

William Pursell Over a year ago

Note that the variable is not necessary. w | awk ... | while ... works just as well.

Ed Morton Over a year ago

and you should always use while IFS= read -r i unless you have a specific modification of the input data you're trying to accomplish by not setting IFS (e.g. skipping leading blanks) and/or not using -r (e.g. interpolating backslashes).

chepner · Accepted Answer · 2014-05-01 12:07:11Z

1

You can use a for loop, i.e.

for user in $(w | awk '{print $1}'); do echo $user; done

which in a script would look nicer as:

for user in $(w | awk '{print $1}')
do
    echo $user
done

edited May 1, 2014 at 12:07

chepner

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answered May 1, 2014 at 11:49

Bernhard

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Comments

Max · Accepted Answer · 2014-05-01 12:30:25Z

0

You can use the xargs command to do this:

w | awk '{print $1}' | xargs -I '{}' id '{}'

With the -I switch, xargs will take each line of its standard input separately, then construct and execute a command line by replacing the specified string '{}' in the command line template with the input line

answered May 1, 2014 at 12:30

Max

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Comments

yejinxin · Accepted Answer · 2014-05-01 15:03:02Z

0

I guess you should use who instead of w. Try this out,

who | awk '{print $1}' | xargs -n 1 id

answered May 1, 2014 at 15:03

yejinxin

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