Bash while loop with done < $1

Question

I'm a bit confused by the done < $1 notation.

I'm trying to write a program "sumnums" that reads in a file called "nums" that has a couple rows of numbers. Then it should print out the rows of the numbers followed by a sum of all the numbers.

Currently I have:

#!/bin/bash
sum=0;
while read myline
do  
  echo "Before for; Current line: \"$myline\""
done
for i in $myline; do  
  sum=$(expr $sum + $i)
done < $1    
echo "Total sum is: $sum"

and it outputs the list of the numbers from nums correctly then says ./sumnums: line 10: $1: ambiguous redirect, then outputs Total sum is: 0.

So somehow it isn't adding. How do I rearrange these lines to fix the program and get rid of the "ambiguous redirect"?

How are you invoking your script? Either call it as ./sumnum nums or change done < $1 to done < nums. — codeforester
– codeforester, Commented Apr 25, 2017 at 2:15
Uhhh. You'll want to redirect stdin for the while read loop, not the for loop. — Charles Duffy
– Charles Duffy, Commented Apr 25, 2017 at 2:57
BTW, consider running scripts through shellcheck.net as a matter of course before asking about them here. — Charles Duffy
– Charles Duffy, Commented Apr 25, 2017 at 3:04

Charles Duffy · Accepted Answer · 2017-04-25 03:04:12Z

Assuming your filename is in $1 (that is, that your script was called with ./yourscript nums):

#!/bin/bash

[[ $1 ]] || set -- nums  ## use $1 if already set; otherwise, override with "nums"

sum=0
while read -r i; do      ## read from stdin (which is redirected by < for this loop)
  sum=$(( sum + i ))     ## ...treat what we read as a number, and add it to our sum
done <"$1"               ## with stdin reading from $1 for this loop
echo "Total sum is: $sum"

If $1 doesn't contain your filename, then use something that does contain your filename in its place, or just hardcode the actual filename itself.

Notes:

<"$1" is applied to a while read loop. This is essential, because read is (in this context) the command that actually consumes content from the file. It can make sense to redirect stdin to a for loop, but only if something inside that loop is reading from stdin.
$(( )) is modern POSIX sh arithmetic syntax. expr is legacy syntax; don't use it.

karakfa · Accepted Answer · 2017-04-25 02:54:56Z

0

awk to the rescue!

awk '{for(i=1;i<=NF;i++) sum+=$i} END{print "Total sum is: " sum}' file

bash is not the right tool for this task.

answered Apr 25, 2017 at 2:54

karakfa

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2 Comments

Jacob Wegelin Over a year ago

Where is the "done <" documented? What is the meaning of the "<" in bash? Does it have a name such as "redirect," under which its documentation can be found?

Connor Over a year ago

@JacobWegelin If I knew a simple explanation I would provide it, but I'm figuring it out too. Here's a link to some documentation. Find this line: done < <(route -n) # ^ ^ First < is redirection, second is process substitution.

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