Bash if statement with prepended character

Question

I'm trying to understand a script that will stop when executed as root:

#!/usr/bin/env bash

if [ x"$(whoami)" = x"root" ]; then
    echo "Error: don't run this script as root"
    exit 1
fi

I have tested this and it works as intended even if I remove the x in the if statement. My question is why is the x in x"$(whoami)" and x"root" needed?

The `x` in this construct was an old idiom to work around bugs in some shells that failed to properly handle empty strings. It is not generally necessary in modern shells. — William Pursell, Nov 25 '18 at 18:47
@bluerojo bash version 4.3.48(1). I realized that the script also works without the `x`. I have edited the question. — builder-7000, Nov 25 '18 at 18:54
@Sergio if it works without the x, then I am confused by the question? — IT_User, Nov 25 '18 at 19:09

score 1 · Answer 1 · answered Nov 25 '18 at 21:20

basically the [ is a softlink to an external program called test, therefore the condition is passed to it as program arguments, and doing so if you don't surround a $variable with "$quotes" , and the variable happens to be empty it won't be considered as an empty argument, it will be considered as no argument (nothing)

#!/bin/bash -eu

var=bla

if [[ $var == bla ]];then
  echo first test ok
fi

var=""

if [[ $var == "" ]];then
  echo second test ok
fi

if [ "$var" == "" ];then
  echo third test ok
fi

if [ x$var == "x" ];then
  echo fourth test ok
fi

echo this will fail:

if [ $var == "" ];then
  echo fifth test ok
fi

echo because it is the same as writing:

if [ == "" ];then
  echo sixth test is obviously eroneous
fi

echo but also you should quote your variables because this will work:

var="a b"

if [ "$var" == "a b" ];then
  echo seventh test ok
fi

echo ... but this one won\'t as test now has four arguments:
if [ $var == "a b" ];then
  echo eighth test ok
fi

agree about `[`, but otherwise a excellent early post. Your showing all to possibilities is an excellent example of "code explained by illustrations" (IMHO), Keep posting! Good luck to all. — shellter, Nov 26 '18 at 04:51

Bash if statement with prepended character

1 Answers1