How to print certain columns by name?

Question

I have the following file:

id  name  age
1   ed    50
2   joe   70   

I want to print just the id and age columns. Right now I just use awk:

cat file.tsv | awk '{ print $1, $3 }'

However, this requires knowing the column numbers. Is there a way to do it where I can use the name of the column (specified on the first row), instead of the column number?

Answer 1

Maybe something like this:

$ cat t.awk
NR==1 {
    for (i=1; i<=NF; i++) {
        ix[$i] = i
    }
}
NR>1 {
    print $ix[c1], $ix[c2]
}
$ awk -f t.awk c1=id c2=name input 
1 ed
2 joe
$ awk -f t.awk c1=age c2=name input 
50 ed
70 joe

If you want to specify the columns to print on the command line, you could do something like this:

$ cat t.awk 
BEGIN {
    split(cols,out,",")
}
NR==1 {
    for (i=1; i<=NF; i++)
        ix[$i] = i
}
NR>1 {
    for (i in out)
        printf "%s%s", $ix[out[i]], OFS
    print ""
}
$ awk -f t.awk -v cols=name,age,id,name,id input 
ed 1 ed 50 1 
joe 2 joe 70 2 

(Note the -v switch to get the variable defined in the BEGIN block.)

Answer 2

If you just want to refer to those fields by their names instead of numbers, you can use read:

while read id name age
do
  echo "$id $age"
done < file.tsv 

EDIT

I saw your meaning at last! Here's a bash function that will print out only the columns you specify on the command line (by name).

printColumns () 
{ 
read names
while read $names; do
    for col in $*
    do
        eval "printf '%s ' \$$col"
    done
    echo
done
}

Here's how you can use it with the file you've presented:

$ < file.tsv printColumns id name
1 ed 
2 joe 

(The function reads stdin. < file.tsv printColumns ... is equivalent of printColumns ... < file.tsv and cat file.tsv | printColumns ...)

$ < file.tsv printColumns name age
ed 50 
joe 70 

$ < file.tsv printColumns name age id name name name
ed 50 1 ed ed ed 
joe 70 2 joe joe joe

Note: Pay attention to the names of the columns you request! This version lacks sanity checks, so nasty things can happen if one of the arguments is something like "anything; rm /my/precious/file"

Answer 3

For what it's worth. This can handle any number of columns in the source, and any number of columns to print, in whatever output sequence you choose; just re-arrange the args...

eg. call: script-name id age

outseq=($@)
colnum=($( 
  for ((i; i<${#outseq[@]}; i++)) ;do 
    head -n 1 file |
     sed -r 's/ +/\n/g' |
      sed -nr "/^${outseq[$i]}$/="
  done ))
tr ' ' '\t' <<<"${outseq[@]}"
sed -nr '1!{s/ +/\t/gp}' file |
  cut -f $(tr ' ' ','<<<"${colnum[@]}") 

output

id      age
1       50
2       70

Answer 4

Just trowing a Perl solution into the lot:

#!/usr/bin/perl -wnla

BEGIN {
    @f = ('id', 'age');   # field names to print
    print "@f";           # print field names
}

if ($. == 1) {            # if line number 1
    @n = @F;              #   get all field names
} else {                  # or else
    @v{@n} = @F;          #   map field names to values
    print "@v{@f}";       #   print values based on names
}