Unexpected Output of Shell Script

Question

I was going through old tutorials and I found one shell script:

a=10
b=20
if  $a -eq $b 
then
   echo "a is equal to b"
else
   echo "a is not equal to b"
fi

When I stored this shell script as test.sh and executed it. It gives me below output

test.sh: 3: test.sh: 10: not found
a is not equal to b

I don't understand the first line of the output. I would appreciate if anyone can throw light on what's going on.

The Internet is full of incompetent shell scripting tutorials. Find a different one. The Stack Overflow bash tag wiki has a reading list, though it is understandably geared towards Bash, not sh. Many things work the same, anyway, and a good tutorial should explain the differences, and teach you both portable shell programming and Bash specifics. — tripleee
– tripleee, Commented Mar 4, 2016 at 20:50

Filkolev · Accepted Answer · 2016-03-04 20:42:27Z

2

The if expects a test, like this: [ $a -eq $b ] or [[ $a -eq $b ]].

Your sample expands to: if 10 -eq 20 at which point bash looks for a program called 10 which obviously doesn't exist, hence the error message.

A missing program apparently evaluates as false which leads to the execution of the else statement.

answered Mar 4, 2016 at 20:42

Filkolev

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2 Comments

Anthony Geoghegan Over a year ago

When Bash tries to execute a program that doesn't exist, it returns an exit status of 127. Like all non-zero exit codes, this is considered by the shell to be false.

chepner Over a year ago

if does not expect a test; it just expects a valid command. test (and its unfortunate alias [) is just one such valid command, albeit the one the OP wants.

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