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AgileSoul
Asked: 2022-05-23 11:36:08 +0800 CST 2022-05-23 11:36:08 +0800 CST

What does "// /" mean inside an if that negates a variable?

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I'm trying to understand a function created by > airmon-ng but when I get to this conditional line:

if [ ! "${InterfacePhysical// /}" ]; then 
      InterfacePhysical="$(ls -l "$interface_physical_path" | sed 's/^.*\/\([a-zA-Z0-9_-]*\)$/\1/')"
fi

Taking into account that InterfacePhysical="phy0"
What I don't understand is:
Why is the variable enclosed in braces? And what is the functionality of those "// /" that are between the braces?
If someone can make me understand what validates this condition, it would be very helpful.

bash

1 Answers

  1. Best Answer

    TL;DR

    It is evaluating if the variable, removing the spaces, is empty or not.

    extended explanation

    There is a Bash mechanism called parameter expansion .

    We can invoke this mechanism using curly braces ${variable}. Now that we have this, we can call other "sub-mechanisms" (personal term) like substitution:

    "${variable/patrón/reemplazo}"

    Which implies that the variable variable is going to be searched for a pattern and replaced with the (obvious) replacement. But normally only the first pattern found will be replaced :

    $ var1="aEEaaEEaaEEa"; echo "${var1/a/0}"
0EEaaEEaaEEa

    And only the first letter "a" was changed to a zero.

    Instead, if we start the pattern with the character /, all matches are changed :

    $ var1="aEEaaEEaaEEa"; echo "${var1//a/0}"
0EE00EE00EE0

    Now we know that in your case:

    ${InterfacePhysical// /}

    In the variable InterfacePhysical, the mechanism searches for all matches of the pattern of spaces, then removes them. Namely:

    $ var1=" a    a    a "; echo "${var1// /}"
aaa

    And if we translate it into Spanish:

    "${<De la variable var>//<busco por TODOS los espacios>/<se eliminan>}"

    Using just a forward slash /var/pattern/stringwould only delete the first space it finds.

    After we have a variable without spaces, we proceed to do a simple evaluation:

    $ var1="a a"; [ "${var1// /}" ] && echo true || echo false
true

$ var1="  "; [ "${var1// /}" ] && echo true || echo false
false

    That is, we are evaluating if the variable, removing the spaces, is empty.

    And negating the condition:

    $ var1="a a"; [ ! "${var1// /}" ] && echo true || echo false
false

    That is, it will only enter the block ifif the variable is empty.

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