using an if statement to compare two strings in shell

Question

This might be a stupid question, but i have never had to anything like this in scripting. So any help would be appreciated.

I am dealing with data from a census and have to generate reports. Each line in the file has a last name, frequency, etc etc. I have to grab the frequency. But if i grab some ones last name who has a frequency of "0.000" then i have to change it to "0.00025". I attempted to do an if statement but am not sure how to do it.

#!/bin/bash
all=/homes/ddailey/public_html/data/dist_all_last

count=`grep -w $1 $all | awk '{ print $2 }'`

if [ $count == "0.000" ]; then
   echo .00025
else
   echo $count
fi

This isn't perfect, but it looks like it should do what you describe. What's the problem with it? — kojiro, Oct 14 '13 at 02:41
Yeah i know its not perfect, just put it together in a few minutes. The problem is that if $count greps a string that is 0.000 it doesnt change it to .00025. It just keeps it at 0.000 — Sampson, Oct 14 '13 at 02:43
As written it shouldn't change `count`'s value, but it should *echo* .00025. Does it do that? — kojiro, Oct 14 '13 at 02:45
How many lines are in the file? If there's more than one, that's the trouble. See also [How to debug a shell script](http://stackoverflow.com/questions/951336/). — Jonathan Leffler, Oct 14 '13 at 02:46
it doesnt echo .00025, it echo's .000... but their is more than one line in the file — Sampson, Oct 14 '13 at 02:49

score 1 · Answer 1 · answered Oct 14 '13 at 04:04

The first statement

count=`grep -w $1 $all | awk '{ print $2 }'`

is returning a list of values separated by newlines. A simple echo won't show the newlines but this will:

echo "$count"

The if statement, however, expects a single value, so if you have two or more results in $count it will not match the single test value. What you need to do is loop through the values in $count. Try the following:

counts=`grep -w $1 $all | awk '{ print $2 }'`
for count in $counts
do
    if [ $count == "0.000" ]; then
       echo .00025
    else
       echo $count 
    fi
done

I prefer the following design which puts all your logic into the awk command. That keeps all your processing logic in one script, not split across awk and bash.

grep -w $1 $all |
awk '{ if ($2 == "0.000") $2 = "0.00025"; print $2 }'

score 0 · Answer 2 · edited May 23 '17 at 11:50

Pick a shell. The Bourne Again Shell (bash) and the basic /bin/sh are both popular. There are a number of other options which you should not use.
Specific advice. Google. Really. See Compare a string in Unix for your answer, or How do I compare strings in Bourne Shell? if using bash.
Watch spaces. Shell programming is sensitive to white space, so having or forgetting a space breaks scripts. For example, make sure you remember the space before and after '[' or ']'. Getting it to run often involves adding and removing spaces.
Watch for empty replacements. if [ $nosuchvar = 3 ] makes an error but if [ "$nosuchvar" = 3 ] does not. Also, glob replacements, so if [[ $foo == "a*" ]] is different than if [ $foo == "a*" ]
Dump the shell. Running Python or another 'real' language is usually far cleaner and more maintainable than any shell script over five lines.

using an if statement to compare two strings in shell

2 Answers2