execute bash expression without temporary file in bash native way

Question

I have worked code

id | tr -s ',' '\n' > output1 && tail -n $( expr $(cat output1 | wc -l) - 1 ) output1

but i want it without temporary file. thank you

Answer 1

If the objective is to skip the 1st line of output from the id | tr command pair:

$ id | tr -s ',' '\n' | tail -n +2

# or

$ id | tr -s ',' '\n' | awk 'FNR>1'

Or eliminating the tr call and printing everything after the 1st ,:

$ id | awk -F, '{for (i=2;i<=NF;i++) print $i}'

Answer 2

An arguably more "bash native" way to do this is:

IFS=, read -r -a id_parts  < <(id)
printf '%s\n' "${id_parts[@]:1}"

• See How to split a string into an array in Bash? for an explanation of IFS=, read -r -a id_parts ....
• See ProcessSubstitution - Greg's Wiki for an explanation of < <(id).
• See How to slice an array in Bash and Is it normal for printf '%s\n' "${array[@]}" to print one line per array element? for an explanation of printf '%s\n' "${id_parts[@]:1}".

Answer 3

If you want get all user-groups and for a better output

awk '
   NR==1{split($0,ids); next} 
   {split($0,names)}
   END{
    #for(i in ids) printf "%d(%s)\n", ids[i], names[i]
    for(i in ids) printf "%5d ==> %s\n", ids[i], names[i]
}' <(id -G) <(id -nG)

1000 ==> ufopilot
   4 ==> adm
  27 ==> sudo
 139 ==> docker