Check if a condition is false

Question

It is seems to be an easy question, I wonder why googling didn't give anything helpful -- nor in StackOverflow, nor in tutorials. I just need to check using bash that a condition is false.

Of what I found I tried

if ! [ 0==2 ]; then echo Hello; fi

and

if [ ! 0==2 ]; then echo Hello; fi 

none of them print Hello.

I found only two similar questions, but the end answer in both cases was restructured code to not use the "false" condition.

Solution 1

Do you mean:

if ! [ 0 == 2 ]; then
  echo Hello;
fi

You lacked space around the equality operator.

This might be the time to read http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html - especially the sections about if then else and operators. I usually have this open when I am writing scripts..

Solution 2

For mathematical evaluations use (( )) in bash. For text use [[ ]].

if (( $i == 0 )); then
  echo "i is 0"
else
  echo "i is unequal 0"

Read more here about comparison operators in bash.

And more on double parenthesis

Solution 3

If you are using the test command ([..]) you can use the comparison option for integer: -eq, equal, and -ne, not equal.

if [ 0 -eq 2 ]; then echo true ; else echo false ; fi # false
if [ 0 -eq 0 ]; then echo true ; else echo false ; fi # true

if [ 0 -ne 2 ]; then echo true ; else echo false ; fi # true
if [ 0 -ne 0 ]; then echo true ; else echo false ; fi # false

---

In bash the operator [...] is the equivalent of test, a command that checks file types and compare values; test is an internal command: if you ask to your shell with type [ it will answer [ is a built in shell command. You can find the binary too usually in /usr/bin/[.

The SYNOPSIS is test EXPRESSION, as you can read from man test or from info coreutils test invocation.

An omitted EXPRESSION defaults to false. Otherwise, EXPRESSION is true or false and sets exit status.

This is an excerpt from man that cam help to understand a little better

• ( EXPRESSION ) EXPRESSION is true. So it's easy to incur in the error to consider as an operation 0==1. (The operation is 0 == 1 with spaces, 0==1 is an expression).

• ! EXPRESSION EXPRESSION is false.

• ...
• INTEGER1 -eq INTEGER2 INTEGER1 is equal to INTEGER2
• INTEGER1 -ne INTEGER2 INTEGER1 is NOT equal to INTEGER2

From info coreutils test invocation you can read about the exit status of test.

Exit status:

 0 if the expression is true,
 1 if the expression is false,
 2 if an error occurred.

Solution 4

In addition to bash's mathematical evaluations, you can use boolean expressions instead of if:

[max@localhost:~] $ (( 0 == 0 )) && echo True || echo False
True
[max@localhost:~] $ (( 0 != 0 )) && echo True || echo False
False

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