How to read in variables from multi-line file?

Question

I am trying to read from a file that is multiple lines with multiple variables per line. There are four variables: item, price, max_qty, and min_qty. The file looks like this:

item price
min_qty max_qty

I have tried:

while IFS=' ' read -r first second third fourth; do
    echo "$first $second $third $fourth
done

This does not work. I want the output to be:

item price min_qty max_qty

I have also thought about somehow replacing the new lines with spaces and then reading from that line. I don't want to actually change the file though.

score 3 · Accepted Answer · answered Apr 15 '19 at 15:54

3

read twice:

while read -r first second && read -r third fourth; do 
  echo "$first $second $third $fourth"
done < file

answered Apr 15 '19 at 15:54

oguz ismail

1
16
47
69

It works, thank you. Can you explain why using read twice works? I had tried before using read twice, but on two lines (not &&). It just read from the beginning of the file twice. – rrfeva Apr 17 '19 at 14:43
@rrfeva it works because each `read ` call reads one line from input. – oguz ismail Apr 17 '19 at 14:48

score 0 · Answer 2 · answered Apr 15 '19 at 18:55

If it is true that "the file looks like this" (two lines, two values per line, not a larger file with many pairs of lines like that), then you can do it with a single read by using the -d option to set the line delimiter to empty:

read -r -d '' first second third fourth <file
echo "$first $second $third $fourth"

The code prints 'item price min_qty max_qty' with the example file.
If you have errexit or the ERR trap set, the program will exit immediately, and silently, immediately after the read is executed. See Bash ignoring error for a particular command for ways of avoiding that.
The code will work with the default value of IFS (space+tab+newline). If IFS could be set to something different elsewhere in the program, set it explicitly with IFS=$' \t\n' read -r -d '' ....
The code will continue to work if you change the file format (all values on one line, one value per line, ...), assuming that it still contains only four values.
The code won't work if the file actually contains many line pairs. In that case the "read twice" solution by oguz ismail is good.

How to read in variables from multi-line file?

2 Answers2