Linux provides us with some expansions for duplicate, print current user’s directory, and parameter operations.

Brace Expansion

We can use curly braces to output something similar to the screen:

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echo a{d,c,b}e    # an official example

Another example:

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echo M{ike,ichelle,ichael}

Tilde Expansion

We can get our home directory through the tilde symbol:

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echo ~

We can also use the tilde symbol to see what files are in our home directory:

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ll ~

Parameter Expansion

This expansion has difference presentations:

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${param:[operator]word}

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${[operator]param}

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${param[operator]word}

If the param is null, use the word

The presentation of this expansion is as follows:

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${param:-word}

Let’s demonstrate:

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MY_NAME_HYPHEN='Dong'
echo My name is ${MY_NAME_HYPHEN:-DONG}

If the param is null, replace with word and use

The presentation of this expansion is as follows:

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${param:=word}

If there is a variable, use the value of the variable:

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MY_NAME_EQUAL='Dong'
echo My name is ${MY_NAME_EQUAL:=DONG}

If there is no variable, use the word and assign the value to the variable:

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echo $MY_NAME_EQUAL_NULL    # We haven't create this variable, so it's null
echo My name is ${MY_NAME_EQUAL_NULL:=DONG}    # := will assign DONG to MY_NAME_EQUAL_NULL variable because it's null
echo $MY_NAME_EQUAL_NULL    # Now it's been created

If the param is not null, use the word

The presentation of this expansion is as follows:

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${param:+word}

If the variable exists, use the word:

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MY_NAME_PLUS='Dong'
echo My name is ${MY_NAME_PLUS:+DONG}

If the variable does not exist, nothing is used:

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echo MY_NAME_PLUS_NULL    # We haven't create this variable, so it's null
echo My name is ${MY_NAME_PLUS_NULL:+DONG}    # Use nothing
echo MY_NAME_PLUS_NULL    # The MY_NAME_PLUS_NULL variable is null

Use a value as a variable name

The presentation of this expansion is as follows:

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${!param}

We can use the ! symbol to use the value of the variable as the name of another variable:

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MY_FIRST_NAME="Dong"
MY_NAME='MY_FIRST_NAME'
echo My name is ${!MY_NAME}

Delete the content before the first or the last appearance of a specified word and output the remaining content

The presentation of this expansion is as follows:

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${param#*word}

We can use the # symbol to delete the content before the specified word’s first or last appearance include the word itself, but this won’t change the value of the variable.

Delete to the first appearance:

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NAMES='Jack:Rose:Marry:John:Anna:Mike'
echo $NAMES
echo ${NAMES#*:}
echo $NAMES

Delete to the last appearance:

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NAMES='Jack:Rose:Marry:John:Anna:Mike'
echo $NAMES
echo ${NAMES##*:}
echo $NAMES

References 3.5.1 Brace Expansion, 3.5.2 Tilde Expansion, 3.5.3 Shell Parameter Expansion

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