How do I correctly retrieve, using bash' cut, the first field from a line with only 1 field in a text file? - cut

Question

In a text file (accounts.txt) with (financial) accounts the sub-accounts are, and need to be, separated by an underscore, looking like this:
assets
assets_hh
assets_hh_reimbursements
assets_hh_reimbursements_ff
... etc.
Now I want to get specific sub-accounts from specific line numbers, e.g.:
field 3 from line 4:
$ lnr=4; fnr=3
$ cut -d $'\n' -f "$lnr" < accounts.txt | cut -d _ -f "$fnr"
reimbursements
$
But both fnr=1 and fnr=2 give for the first line, which has only 1 field:
$ cut -d $'\n' -f 1 < accounts.txt | cut -d _ -f "fnr"
assets
$
which is undesired behaviour.
Now I can get around this by prefixing an underscore to each account and add 1 to each required field number, but this is not an elegant solution.
Am I doing something wrong and/or can this be changed by issuing a different retrieval command?

Answer 1

Using the cut -d $'\n' -f "$lnr" for getting the lnr-th line from the file is somewhat strange. More common approach is using sed, like:
sed -n "${lnr}p" file | cmd ...
However, for this the awk is better - in one invocation could handle the lnr and fnr too.
file=accounts.txt
lnr=1
fnr=2
awk -F_ -v l=$lnr -v f=$fnr 'NR==l{print $f}' "$file"
The above for the all combinations lnr/fnr produces:
line field1 field2 field3 field4
------------------------------------------------------------------------
assets assets
assets_hh assets hh
assets_hh_reimbursements assets hh reimbursements
assets_hh_reimbursements_ff assets hh reimbursements ff

Answer 2

Check below solution -
cat f
assets
assets_hh
assets_hh_reimbursements
assets_hh_reimbursements_ff
Based on your comment try below commands -
$ lnr=1; fnr=2
$ echo $lnr $fnr
1 2
$ awk -v lnr=$lnr -v fnr=$fnr -F'_' 'NR==lnr {print $fnr}' f
###Output is nothing as line 1 column 2 is blank when FS="_"
$ lnr=4;fnr=1
$ echo $lnr $fnr
4 1
$ awk -v lnr=$lnr -v fnr=$fnr -F'_' 'NR==lnr {print $fnr}' f
assets
$ lnr=4;fnr=3
$ echo $lnr $fnr
4 3
$ awk -v lnr=$lnr -v fnr=$fnr -F'_' 'NR==lnr {print $fnr}' f
reimbursements

Answer 3

One solution is to head|tail and read into an array so it's easier to work with the items:
lnr=4
fnr=2
IFS=_ read -r -a arr < <(head -n "$lnr" accounts.txt | tail -n 1)
#note that the array is 0-indexed, so the fieldnumber has to fit that
echo "${arr[$fnr]}"
Then you could expand the idea into a more usable function:
get_field_from_file() {
local fname="$1"
local lnr="$2"
local fnr="$3"
IFS=_ read -r -a arr < <(head -n "$lnr" "$fname" | tail -n 1)
if (( $fnr > ${#arr[#]} )); then
return 1
else
echo "${arr[$fnr]}"
fi
}
field=$(get_field_from_file "accounts.txt" "4" "2") || echo "no such line or field"
[[ -n $field ]] && echo "field: $field"