Print lines using line-numbers stored in shell variable, using sed

Question

I have a tab file from, which I am extracting the column containing the row numbers I need to extract lines from another file. I got the line numbers with cut -f and I stored them into a variable list. I tried to use sed with the following:

$ list="2 5 7 10"
$ echo $list
2 5 7 10
$ sed -n "$list p" longText.txt 
sed: -e expression #1, char 3: unknown command: `5'
$ sed -n "${list}p" longText.txt 
sed: -e expression #1, char 3: unknown command: `5'

What is the error? What is the correct syntax?

Answer 1

The issue with your code is that you try to run a sed command that ooks like

2 5 7 10p

This obviously makes no sense to sed, so it complains.

Part of your issue is using a single string to hold many different items (the line numbers). This is better done with an array. Each array element (line number) will then need to have a p inserted after it to make it a separate sed command.

For the numbers 2, 5, 7, and 10, you want a sed script that looks something like

2p; 5p; 7p; 10p;

or

2p;5p;7p;10p;

or

2p
5p
7p
10p

You can create a script like that rather easily from an array in bash:

lines=( 2 5 7 10 )
printf '%sp\n' "${lines[@]:-"q;"}" | sed -n -f /dev/stdin longText.txt

This uses printf to create a sed script from the list of numbers, and feeds it to sed which reads it from standard input with -f /dev/stdin, and applies it to you file.

If the list is empty, the printf will emit a sed expression that does not do anything and then terminates sed.

---

If you need to use the numbers in a string, then you could convert that string into a valid sed script and apply it to your file like so:

sed 's/[[:digit:]]\{1,\}/&p;/g' <<<"$list" | sed -n -f /dev/stdin longText.txt

Given the string 2 5 7 10, this would generate the sed script 2p; 5p; 7p; 10p; and then apply that to your file in a similar manner as above.

An empty list would not cause sed to output anything.

Answer 2

Just place an echo before your sed command to see where n why you are going wrong.

One way to go, using the sed editor is:

# assuming default IFS
## based on Kusalananda's observations
set -f ;# disable globbing
list="2 5 7 10"
for i in $list;do
  echo "${i}p"
done | sed -nf - longText.txt

## or per Kusalanda method has a bug in it
## for empty list it generates wrong output
# printf '%sp\n' $list | sed -nf - lonxt.txt