Question

My problem can be simplified down to making the following script work (which takes one command line argument):

#!/bin/bash
if ["$1" == "0"]; then
    echo "good"
else
    echo "bad"
fi

This should print good when I run script 0, but I can’t get it to. I’ve tried various combinations of quotes around the numbers, and I’ve tried =, ==, and -eq. So… bash, how does it work?

Answer 1

The [ is actually a command. Do a ls /bin/[ or an ls /usr/bin/[. You’ll see it’s actually an executable file.

The [...] is from the old Bourne shell days. The if command executes the statement, and if the exit code of that statement is a zero, the statement is considered true and the if clause is executed. If the exit code is not zero, the else clause is executed (if present).

Try these:

$ date
Fri May 18 00:04:03 EDT 2012
echo $?   #Prints the exit code of the date command
0

$ date -Q  #Shouldn't work, I hope...
date: illegal option -- q
usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ... 
    [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]
$ echo $?    #Exit code for the date command
1

You can see that date is a valid command and returns an exit code of 0 (the value of $?), but date -Q isn’t valid, and returns an exit code of 1.

Now let’s try them in the if statement:

if date
then
   echo "I've successfully printed out the date!"
else
   echo "I made a mistake in the command"
fi

Now try this:

if date -q
then
   echo "I've successfully printed out the date!"
else
   echo "I made a mistake in the command"
fi

Originally, the [...] was an alias for the test command. The following are equivalent:

if test -f /bin/ls    #Does a file called /bin/ls exist?
then
   echo "There's a /bin/ls file"
fi

and

if [ -f /bin/ls ]
then
   echo "There's a /bin/ls file"
fi

This is why it’s very important to put spaces around the [ and ]. Because these are actually commands. In BASH, there’s built into the shell, but they are commands. That’s also why all the test parameters (things like -f, -z, and -eq) all are prefixed with dashes. They were originally parameters for the test command.