2w ago

Check two conditions in bash ?

Hi,

I trying to test two condition together (AND) under bash but it's not working...

The goal is ti have True when two variables are either not set or empty (empty string)

I've tried

 bash

    
if [[ -n VARIABLE1 && -n VARIABLE2 ]]; then
    echo "OK"
fi

Here I get the "OK" no matter what .

Thanks.

16 comments

To check for an empty string, use -z. -n checks to see if a string is not empty.
- in [[, empty strings are falsy, so this also works:
  bash
  
  [[ ! $VARIABLE1 && ! $VARIABLE2 ]] && echo "OK"
If you want true for empty strings, you want -z not -n
bash

if [[ -z "VARIABLE1" && -z "VARIABLE2" ]]; then echo "OK" fi
- in [[ there is generally no need for quoting, except the right hand side of an = comparison
[ condition1 ] && [ condition2 ] && echo "Good" || echo "Bad"
- Explanation
  [ is an alias for the program test, so you can call man test for more info.
  && is bash syntax for conjunction. In A && B, B will only be called if A returned a exit code >0 (error). You can call man bash for more info.
  || is bash syntax for disjunction. In A || B, B will only be called if A returned exit code =0 (success). true and false are programs that just return exit codes 0 respectively 1.
- Never use a && b || c. It is not the same as if a; then b; else c; fi: when a succeeds but b fails, it will run both b and c.
  I would not bother with [ unless you absolutely need compatibility with non-bash shells.
You're probably wanting [ -z "${VAR1}" -a -z "${VAR2}" ]. Note in bash that there are minor differences in how [ ] and [[ ]] tests are handled. You can pull up a handy cheat sheet of the operands on most distros by running man test, though you'll need to read through the CONDITIONAL EXPRESSIONS section of man bash if you want to see the minor differences of the single vs double square bracket commands (mostly whether locale applies to string order, as well as whether operands are evaluated in numeric comparisons).
You need to reference the value of the variable, ie:
bash

if [[ -n "$VARIABLE1" && -n "$VARIABLE2" ]]; then echo "OK" fi
- not working, both variables do not exist and the echo "OK" do not trigger.
  
  Then it is working. That is what that code was checking for.
  Specifically, -n checks if the variable exists and also does not have a null value.
  If you want to reverse it, ie, check that those conditions are not true, put an exclamation mark in front of the whole thing.

Try this:

 bash

    
#!/usr/bin/env bash

a=""
if [[ -z "${a}" && -z "${b}" ]]; then
    echo "OK"
else
    echo "Not OK"
fi

a="OK"
if [[ -n "${a}" && -z "${b}" ]]; then
    echo "More ${a}"
else
    echo "More Unokay"
fi

Thank you all for yours input
What finally did work
bash

if [[ -z VARIABLE1 && -z VARIABLE2 ]]; then echo "OK" fi
If only Linux was using Python syntax that would be so much more intuitive...
- Linux does use Python syntax... in Python.
  In Bash though, it uses Bash syntax.
- The variables need a dollar sign: $VARIABLE1
  help test shows what -n and -z do.
Could try:
bash

if [ condition1 ] && [ condition2 ]; then echo "OK" fi

16 comments