abc:123:myname:1231
 def:423324:arbitrary:value:string
 StackExchange:Unix:Linux

#!/bin/bash
key_word_I_am_looking_for=$1
my_variable=`cat myfile.csv | awk -F: -v keyword="$key_word_I_am_looking_for" '( $1 == keyword )' END{print "$@" }'`
echo "$my_variable"

#!/bin/bash
awk -F: -v keyword="$1" '$1 == keyword {$1=$1; print}' myfile.csv

grep "^$1:" myfile.csv | tr ":" " "

sed -n '/^def:/s/:/ /gp' myfile.csv

sed -n "/^$1:/s/:/ /gp" myfile.csv

$ python -c "import sys;
with open(sys.argv[2]) as f:
    for line in f:
        if sys.argv[1] == line.split(':')[0]:
            print ' '.join(line.strip().split(':'))" def file
def 423324 arbitrary value string