How do I convert tabs to exact number of spaces in bash script? -

i need convert string tabs string spaces.

but need number of spaces match tabbing format identical.

i need because gnuplot labels don't tabs.

so want:

a            fat      cat 

which a\tvery\tfat\tcat, converted to:

a            fat      cat 

with spaces, , not to

a fat cat 

edit 1

i think misunderstood problem:

$ cat -t aggregate/summary.txt | head -n1       date        pnl     annpnl  days  avtrds  avevol  avedur tdays      pnl/$  avpnl     stddev       maxd  shrpe 

but when assign variable spaces lost:

$ ff=`cat aggregate/summary.txt | head -n1`; echo $ff date pnl annpnl days avtrds avevol avedur tdays pnl/$ avpnl stddev maxd shrpe 

you can use expand command:

~$ cat >c       fast    cat     nrstaui e ~$ expand c>d 

when using cat -a, can see spaces used (tabs represented ^i):

~$ cat -a c a^ifast^icat^inrstaui^ie$ ~$ cat -a d       fast    cat     nrstaui e$ 

edit: if assign line variable, have use double quotes see differences:

~$ f=$(cat c) ~$ echo "$f" |cat -a a^ifast^icat^inrstaui^ie$ ~$ echo $f |cat -a fast cat nrstaui e$ 

same problem spaces:

~$ f=$(cat d) ~$ echo "$f" |cat -a       fast    cat     nrstaui e$ ~$ echo $f |cat -a fast cat nrstaui e$