[SOLVED] bash programming question

KatrinAlec · 08-09-2018, 03:35 AM

I'm trying things out with if and spaces
why is
if [ " " = "String" ];
different from
if [ " "="String" ];
?

jlinkels · 08-09-2018, 04:38 AM

There must be spaces around the operator.

I assume you have a mistake in the comparison as you compare two constants. One of the should be a variable.

jlinkels

KatrinAlec · 08-09-2018, 04:42 AM

Of course in real life it would be variables.
Just for the sake of example I used values, same problem if I use variables.

michaelk · 08-09-2018, 06:01 AM

As stated bash conditional statements have a required syntax. There must be a space between the brackets and between the operand. No spaces will usually produce an error.

Code:

if [ "foo" = "bar" ]
then
   ...
fi

if [ "foo" == "bar" ]
then
   ...
fi

https://www.gnu.org/software/bash/ma...pressions.html

KatrinAlec · 08-09-2018, 06:33 AM

It seems there are different opinions on what is right on the internet.
I've found:
if [ "foo" = "foo" ] - so with spaces.
if [ "$a" = "$b" ] - with spaces and with quotes, probably if a variable contains a$="Hello World" it needs the quotes.
But I've also found
if [ a$=b$ ] - no space, no quotes (might of course always be a mistake)

So you say the correct method is with spaces, and I guess quotes around variables will also be a good idea.
Is there any reason ever not to use
if [ "a$" = "b$" ]
?

michaelk · 08-09-2018, 07:03 AM

There is a difference between double and single quotes. Single quotes will preserve the literal value of each character within the quotes. Double quotes will expand variables i.e. $foo to its actual value and \ which is used as an escape character.

Quote:

[ "$a" = "$b" ]

For basic if statements using quotes is a good practice.

pan64 · 08-09-2018, 07:40 AM

a$ is probably $a, $ at the end is quite unusual. And probably will not do what you want.
Learning variables, quoting is not trivial in bash, but you can have help to understand it.
At first there is a man page and other documentation too (just look for bash quoting).
Also you can use set -xv in your script to check how it is really working and also you can check your script using shellcheck (or similar)