read and if statement in bash

casperdaghost · 10-12-2011, 02:59 PM

When i hit the y, it still goes to the else statement, and I can't figure out why. I even took out the "sudo /data/bouce light05" statement becasue I thought that was messing it up, but it is not.

Code:

casperh@init01 ~ $ more bounce_the_engine
#!/bin/bash

echo "Do you want to bounce the session"
read $BOUNCEREPLY
if [ "$BOUNCEREPLY" == 'y' ]; then
        echo "Bouncing the CASH engine"
        sudo /data/bounce light05
else
        echo "Not bouncing the CASH engine"
fi
woljoh@ashqoptsinit01 ~ $ ./bounce_the_engine
Do you want to bounce the session
y
Not bouncing the CASH engine
casper@init01 ~ $

jthill · 10-12-2011, 03:05 PM

The comparison operator for test aka [ is '=', not '=='

sycamorex · 10-12-2011, 03:08 PM

You need to get rid of $ in "read $BOUNCEREPLY"

casperdaghost · 10-12-2011, 03:49 PM

yes both of you are right - if you only use one equal symbol in bash comparison, if that is the syntax, that is fine, I will study it.
however i don't understand why I have to take the $ sign off the variable for the read function.

is it that you only use the $ when you are expanding variables in bash?

sycamorex · 10-12-2011, 03:57 PM

Quote:

Originally Posted by casperdaghost

yes both of you are right - if you only use one equal symbol in bash comparison, if that is the syntax, that is fine, I will study it.
however i don't understand why I have to take the $ sign off the variable for the read function.

is it that you only use the $ when you are expanding variables in bash?

$ is used to RETRIEVE the value of a variable. You don't use it when you assign the value to a variable.

colucix · 10-12-2011, 04:07 PM

Actually = and == are synonyms. Ref: http://tldp.org/LDP/abs/html/comparison-ops.html

GazL · 10-12-2011, 04:18 PM

A single '=' is compatible with traditional bourne shells which may choke on '=='. Good habit to get into using the single form for the sake of portability.

wpeckham · 10-12-2011, 05:14 PM

BOUNCEREPLY=5
Sets variable BOUNCEREPLY to "5"
${BOUNCEREPLY}=6
Now evaluates
5=6

$ can be read as "the value of" in many cases without changing meaning.

read BOUNCEREPLY
reads an input and puts that input into the variable BOUNCEREPLY

read $BOUNCEREPLY should not work at all, but if it did would put the input string into a variable whose name was held by BOUCEREPLY.

Alas, that amount of indirection is not managed well by any current shell.
---------------------------------------------
As for '=' and '==', '=' is generally correct for numeric comparison, while '==' is more correct for strings. Shell programmers have been dealing with users who cannot 'get' that for so long that they have about given up and made the if evaluator handle them through the same code.

It makes that code do the work the programmer should have done, but what else are computers FOR?

David the H. · 10-13-2011, 07:26 AM

If you're using bash then you're really better off using the newer [[ test for strings, and ((..)) for numerical tests. They were designed to avoid a lot of the weaknesses of the old single-bracket test command.

http://mywiki.wooledge.org/BashFAQ/031
http://mywiki.wooledge.org/ArithmeticExpression

MTK358 · 10-13-2011, 07:39 AM

Note that unlike "[", which is a synonym for the "test" command, "[[ ]]" and "(( ))" actually are shell syntax.

linuxwin2 · 10-13-2011, 08:19 AM

Code:

!/bin/bash

echo "Do you want to bounce the session"
read BOUNCEREPLY

if [ $BOUNCEREPLY = "y" ]; then
        echo "Bouncing the CASH engine"
        sudo /data/bounce light05

else
        echo "Not bouncing the CASH engine"
fi

MTK358 · 10-13-2011, 08:30 AM

Quote:

Originally Posted by linuxwin2

Code:

!/bin/bash

echo "Do you want to bounce the session"
read BOUNCEREPLY

if [ $BOUNCEREPLY = "y" ]; then
        echo "Bouncing the CASH engine"
        sudo /data/bounce light05

else
        echo "Not bouncing the CASH engine"
fi

It's a bad idea to not enclose a variable in quotes in a test command. If the variables contains whitespace, it can cause bad things to happen. Try this:

Code:

echo "Do you want to bounce the session"
read BOUNCEREPLY

if [[ $BOUNCEREPLY = y ]]; then
    echo "Bouncing the CASH engine"
    sudo /data/bounce light05
else
    echo "Not bouncing the CASH engine"
fi

One of the reasons [[]] is better is becuase it properly handles whitespace in variables, even if you don't use quotes.