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

Answer 1

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 }'

Answer 2

1. 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.

2. 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.

3. 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.

4. 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*" ]

5. Dump the shell. Running Python or another 'real' language is usually far cleaner and more maintainable than any shell script over five lines.