Wildcards in Quotes – How to Use Wildcards Inside Quotes

quotingshellwildcards

This will be an easy one, but in my memories, when shell scripting, using double quotes would allow expanding globbing and variables.

But in the following code:

#!/bin/sh

echo *.sh
echo "*.sh"
echo '*.sh'

echo $LANG
echo "$LANG"
echo '$LANG'

I get this result:

bob.sh redeployJboss.sh
*.sh
*.sh
en_US.utf8
en_US.utf8
$LANG

So single quoting prevent glob AND variable expansion but double quoting allows only variable expansion and no globbing?

Can I glob in any quoting pattern?

Best Answer

You are correct: globbing doesn't work in either single- or double-quotes. However, you can interpolate globbing with double-quoted strings:

$ echo "hello world" *.sh "goodbye world"
hello world [list of files] goodbye world

Related Solutions

Shell Environment Variables – Significance of Single and Double Quotes

I think you're confused about terminology.

An "environment variable" is merely a shell variable that any child processes will inherit.

What you're doing in your example is creating a shell variable. It's not in the environment until you export it:

MY_HOME="/home/my_user"
export MY_HOME

puts a variable named "MY_HOME" in almost all shells (csh, tcsh excepted).

In this particular case, the double-quotes are superfluous. They have no effect. Double-quotes group substrings, but allows whatever shell you use to do variable substitution. Single-quoting groups substrings and prevents substitution. Since your example assignment does not have any variables in it, the double-quotes could have appeared as single-quotes.

V='some substrings grouped together'  # assignment
X="Put $V to make a longer string"    # substitution and then assignment
Y=`date`                              # run command, assign its output
Z='Put $V to make a longer string'    # no substition, simple assignment

Nothing is in the environment until you export it.

bash – Why Parameter Expansion with Spaces Works Inside Double Brackets but Not Single Brackets

The single bracket [ is actually an alias for the test command, it's not syntax.

One of the downsides (of many) of the single bracket is that if one or more of the operands it is trying to evaluate return an empty string, it will complain that it was expecting two operands (binary). This is why you see people do [ x$foo = x$blah ], the x guarantees that the operand will never evaluate to an empty string.

The double bracket [[ ]], on the other hand, is syntax and is much more capable than [ ]. As you found out, it does not have the "missing operand" issue and it also allows for more C-like syntax with >, <, >=, <=, !=, ==, &&, || operators.

My recommendation is the following: If your interpreter is #!/bin/bash, then always use [[ ]]

It is important to note that [[ ]] is not supported by all POSIX shells, however many shells do support it such as zsh and ksh in addition to bash

Best Answer

Related Solutions

Shell Environment Variables – Significance of Single and Double Quotes

bash – Why Parameter Expansion with Spaces Works Inside Double Brackets but Not Single Brackets

Related Question