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if
Using the shell, we can code the logic above as follows:
x=5
if [ $x -eq 5 ]; then echo "x equals 5."
else
echo "x does not equal 5."
fi
x=5
if [ $x -eq 5 ]; then echo "x equals 5."
else
echo "x does not equal 5."
fi
or we can enter it directly at the command line (slightly shortened):
[me@linuxbox ~]$ x=5
[me@linuxbox ~]$ if [ $x -eq 5 ]; then echo "equals 5"; else echo "does not equal 5"; fi
equals 5 [me@linuxbox ~]$ x=0
[me@linuxbox ~]$ if [ $x -eq 5 ]; then echo "equals 5"; else echo "does not equal 5"; fi
does not equal 5
[me@linuxbox ~]$ x=5
[me@linuxbox ~]$ if [ $x -eq 5 ]; then echo "equals 5"; else echo "does not equal 5"; fi
equals 5 [me@linuxbox ~]$ x=0
[me@linuxbox ~]$ if [ $x -eq 5 ]; then echo "equals 5"; else echo "does not equal 5"; fi
does not equal 5
In this example, we execute the command twice. Once, with the value of x set to 5, which results in the string “equals 5” being output, and the second time with the value of x set to 0, which results in the string “does not equal 5” being output.
The if statement has the following syntax:
if commands; then
commands
[elif commands; then
commands...] [else
commands]
fi
where commands is a list of commands. This is a little confusing at first glance. But be- fore we can clear this up, we have to look at how the shell evaluates the success or failure of a command.