bash - I just assigned a variable, but echo $variable shows something else

Here are a series of cases where echo $var can show a different value than what was just assigned. This happens regardless of whether the assigned value was "double quoted", 'single quoted' or unquoted.

How do I get the shell to set my variable correctly?

Asterisks

The expected output is /* Foobar is free software */, but instead I get a list of filenames:

$ var="/* Foobar is free software */"

$ echo $var 
/bin /boot /dev /etc /home /initrd.img /lib /lib64 /media /mnt /opt /proc ...

Square brackets

The expected value is [a-z], but sometimes I get a single letter instead!

$ var=[a-z]
$ echo $var

c

Line feeds (newlines)

The expected value is a a list of separate lines, but instead all the values are on one line!

$ cat file
foo
bar

baz

$ var=$(cat file)
$ echo $var
foo bar baz

Multiple spaces

I expected a carefully aligned table header, but instead multiple spaces either disappear or are collapsed into one!

$ var="       title     |    count"
$ echo $var
title | count

Tabs

I expected two tab separated values, but instead I get two space separated values!

$ var=$'key\tvalue'
$ echo $var
key value

Answer

In all of the cases above, the variable is correctly set, but not correctly read! The right way is to use double quotes when referencing:

echo "$var"

This gives the expected value in all the examples given. Always quote variable references!

---

Why?

When a variable is unquoted, it will:

1. Undergo field splitting where the value is split into multiple words on whitespace (by default):

   
   
   
   

   Before: /* Foobar is free software */

   
   
   

   After: /*, Foobar, is, free, software, */




2. Each of these words will undergo pathname expansion, where patterns are expanded into matching files:

   
   
   

   Before: /*

   
   
   

   After: /bin, /boot, /dev, /etc, /home, ...




3. Finally, all the arguments are passed to echo, which writes them out separated by single spaces, giving

   
   
   
   

   /bin /boot /dev /etc /home Foobar is free software Desktop/ Downloads/
   

   
   
   

   instead of the variable's value.

When the variable is quoted it will:

1. Be substituted for its value.


2. There is no step 2.

This is why you should always quote all variable references, unless you specifically require word splitting and pathname expansion. Tools like shellcheck are there to help, and will warn about missing quotes in all the cases above.