Question

I tried to create a shell script, which sum the given numbers. If there is no given parameter, then it tries to read the pipe output, but I get an error.

#!/bin/sh

sum=0

if [ $# -eq 0 ]
then
  while read data
  do
    sum=`expr $sum + $data`
  done
else
  for ((  i = 1 ;  i <= $#;  i++  ))
  do
    sum=`expr $sum + ${!i}`
  done
fi

echo $sum

This works: sum 10 12 13
But this one doesn’t: echo 10 12 13| sum

Thanks in advance,

Answer 1

Here you go (assuming bash, not sh):

#!/bin/bash

sum=0

if (( $# == 0 )); then
  # Read line by line
  # But each line might consist of separate numbers to be added
  # So read each line as an array!
  while read -a data; do
    # Now data is an array... but if empty, continue
    (( ${#data[@]} )) || continue
    # Convert this array into a string s, with elements separated by a +
    printf -v s "%s+" ${data[@]}
    # Append 0 to s (observe that s ended with a +)
    s="${s}0"
    # Add these numbers to sum
    (( sum += s ))
  done
else
    # If elements come from argument line, do the same!
    printf -v s "%s+" $@
    # Append 0 to s (observe that s ended with a +)
    s="${s}0"
    # Add these numbers to obtain sum
    (( sum = s ))
fi

echo $sum

You can invoke it thus:

$ echo 10 12 13 | ./sum
35
$ ./sum 10 12 13
35
$ # With several lines and possibly empty lines:
$ { echo 10 12 13; echo; echo 42 22; } | ./sum
99

Hope this helps!

Edit. You might also be interested in learning cool stuff about IFS. I’ve noticed that people tend to confuse @ and * in bash. If you don’t know what I’m talking about, then you should use @ instead of *, also for array subscripts! In the bash manual, you’ll find that when double quoted, $* (or ${array[*]}) expands to all the elements of the array separated by the value of the IFS. This can be useful in our case:

#!/bin/bash

sum=0

if (( $# == 0 )); then
  # Read line by line
  # But each line might consist of separate numbers to be added
  # So read each line as an array!
  while read -a data; do
    # Now data is an array... but if empty, continue
    (( ${#data[@]} )) || continue
    # Setting IFS=+ (just for the sum) will yield exactly what I want!
    IFS=+ sum=$(( sum + ${data[*]} ))
  done
else
    # If elements come from argument line, do the same!
    # Setting IFS=+ (just for the sum) will yield exactly what I want!
    IFS=+ sum=$(( $* ))
fi

echo $sum

Gniourf now exits from teacher mode. :-)