Shell Programming and Scripting

This might be a simple answer but I can't for my life figure it out. If I input "bat", how do I get the condition to pick it up with the wild card *at*.

Code:

#!/bin/bash

read -p "enter" BOO

if [ $BOO = '*at*' ]; then
        echo "you win"
else
        echo "you lose"
fi

Use double bracket, and no quotes round wild cards. The double brackets are interpreted by Bash (or Kshell if using ksh) and they both interpret the right hand side of the expression as a pattern:

Code:

#!/bin/bash

read -p "enter: " BOO

if [[ $BOO = *"at"* ]]; then   # quotes here optional
        echo "you win"
else
        echo "you lose"
fi

What's the problem if single bracket is used?

The open single bracket is a symlink to the test command. Shells (Ksh, bash) must treat it as an external command, and thus all of the command line is treated as an external command. Therefore, wildcards, unquoted, are expanded in the same manner as would be on any other command: as matching files in the filesystem. Quoted wildcard characters are treated as literals and passed to test which treats them as literals. So executing the following code takes the false branch:

Code:

if [ foo = "*o" ]
then
    echo "seemingly broken because this doesn't echo"
else
    echo "this will echo because * is interpreted literally by test"
fi

While this if statement takes the expected true branch:

Code:

if [[ foo == *"o" ]]

Beyond those differences, the double square bracketed expressions are interpreted by the shell, and no fork/exec overhead is needed, so they are much more efficient than using a single bracket.

Once in a long time does it make sense to use a single bracket expression (except when needing to write a pure bourn shell script), and even then I'd question its use.