If-statement in Bash script

[Ralph_Donnchadh]

Hello,

I am new to Ubuntu, and I am trying to write my first shell script.
I know in C++ I can use the following if-statment


// for some integers a and b
if (!( (a == 1) &&(b == 2)) )
{
// Do something
}

I also found out that in shell script I can use the following to achieve the same effect.


# for some integers a and b
if [ ! a -eq 1 ] || [! b -eq 2 ] # Difficult to read
then
# Do something
fi

However, in the shell script example, the condition statement is difficult to read. Is there any way in shell script to write the condition as:

(! (condition1 && condition2) )

to improve readability?

Thanks

[KIAaze]

#!/bin/bash
# for some integers a and b
a=1
b=2
echo "a=$a and b=$b"
if ! [ $a -eq 1 ] || ! [ $b -eq 2 ] # Difficult to read
then
# Do something
echo "not(a=1) or not(b=2)"
fi

if ! ( [ $a -eq 1 ] && [ $b -eq 2 ] ) # Difficult to read
then
# Do something
echo "not(a=1 and b=2)"
fi

a=3
b=3
echo "a=$a and b=$b"
if ! [ $a -eq 1 ] || ! [ $b -eq 2 ] # Difficult to read
then
# Do something
echo "not(a=1) or not(b=2)"
fi

if ! ( [ $a -eq 1 ] && [ $b -eq 2 ] ) # Difficult to read
then
# Do something
echo "not(a=1 and b=2)"
fi


You might also want to look at the man page for test:

man test

[Ralph_Donnchadh]

Thanks KIAaze! Your examples help a lot!

[ibuclaw]

tbh, to a small degree, bash can be as readable as you want it to be...


#!/bin/bash

function eq
{
[ $1 -eq $2 ]
return $?
}

a=3;
b=3;

if (eq $a 3) && (eq $b 3)
then
printf "\$a and \$b are both equal to 3\n";
fi

But that doesn't mean that it is readable for others.

IMO ... Perl is a better language for Swiss Army Knife jobs in Linux than what Bash is.


#!/usr/bin/perl

use strict;
use warnings;

my ($a, $b) = (3, 3);

if($a == 3 or $b == 3)
{
print "\$a and \$b are both equal to 3\n";
}

# Alternate Syntax
print "\$a and \$b are both equal to 3\n" if($a eq 3 and $b eq 3);


Regards
Iain