Change variables in bash

Question

How do I change this var ?

max=0;
min=20000000;
cat |while read
do
    read a
    if [[ $a -gt $max ]]
    then
        max=a`
    fi
    `if [[ $a -lt $min ]]
    then
        min=a
    fi
done
echo $max 
echo $min

My min and max are still the same, 0 and 2000000. Can anybody help me with this ? I have no idea.

also max=a should be max=$a, same with min=a should be min=$a — Andrew Brock
– Andrew Brock, Commented May 1, 2012 at 18:49
What should be min if every values are greater than 20000000 ? I guess that min and max should be your first value... — Luc M
– Luc M, Commented May 1, 2012 at 20:25
Possible duplicate of A variable modified inside a while loop is not remembered — melpomene
– melpomene, Commented Jan 18, 2018 at 2:33

Tomáš Zato · Accepted Answer · 2015-09-30 12:55:04Z

48

The (main) problem with your script is that setting min and max happens in a subshell, not your main shell. So the changes aren't visible after the pipeline is done.

Another one is that you're calling read twice - this might be intended if you want to skip every other line, but that's a bit unusual.

The last one is that min=a sets min to a, literally. You want to set it to $a.

Using process substitution to get rid of the first problem, removing the (possibly) un-necessary second read, and fixing the assignments, your code should look like:

max=0
min=20000000
while read a
do
    if [[ $a -gt $max ]]
    then
        max=$a
    fi
    if [[ $a -lt $min ]]
    then
        min=$a
    fi
done < <(cat)    # careful with the syntax
echo $max 
echo $min

edited Sep 30, 2015 at 12:55

Tomáš Zato

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answered May 1, 2012 at 18:56

Mat

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2 Comments

user unknown Over a year ago

4th problem: unmotivated backticks in the middle of the script, not explainable by beginner mistake. :)

Dennis Williamson Over a year ago

In Bash (and Ksh and Zsh), I recommend this form: if (( a > max )). Also, if cat is not a placeholder for something else, then the whole redirection and process substitution can be eliminated since the script will read from stdin by default anyway.

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