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Expansion

When we type a command with arguments/inputs and press the enter key, the shell does several things to the arguments/input text before it actually carries out the command. This action is called expansion. With expansion, the arguments expands into something else before the shell acts on it with the command. Let's see an example with echo command which prints out the text arguments given on standard output.

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$ echo hello world!
hello world!

Let's use echo with *:

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$ echo *
hello.c hello.qsub hello.sb README

Instead of printing *, it prints out all file names in the directory because shell expands * into something else before the echo command acts on the argument (in this case *).

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$ ls
hello.c hello.qsub hello.sb README

The * character is called a "wildcard" is replaced by everything in the current directory. We can also modify it so that it's only replaced by certain things. For example, h* is replaced everything that starts with h in the current directory.

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$ echo h*
hello.c hello.qsub hello.sb

~ is another special character with a special meaning. It expands into the name of the home directory of the user:

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$ echo ~
/mnt/home/temp_user_01

The shell will also expand arithmetic. Arithmetic expansion uses the form $((expression)). Look at the example.

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$ echo $((1 + 1))
2

Please keep in mind that arithmetic expansions allows only integers. Arithmetic expression can be nested, and spaces are allowed.

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$ echo $(( 7*( 2 + 2 ) ))
28
$ echo $((7*(2+2)))
28

Brace expansion is useful when you write a shell script or batch script. The brace expression can contain a comma separated list of characters, strings, or integers that will be expanded into multiple expressions. Here are a few examples.

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$ echo srt-{a,b,c}-end
srt-a-end srt-b-end srt-c-end

$ echo number_{1..5}
number_1 number_2 number_3 number_4 number_5

$ echo {A..Z}
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

$ echo a{A{1,5},B{6..10}}b
aA1b aA5b aB6b aB7b aB8b aB9b aB10b

The next example creates multiple directories with a brace expansion.

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$ mkdir Photos
$ cd Photos/
$ mkdir {2020..2021}-{01..12}
$ ls
2020-01/  2020-03/  2020-05/  2020-07/  2020-09/  2020-11/  2021-01/  2021-03/  2021-05/  2021-07/  2021-09/  2021-11/
2020-02/  2020-04/  2020-06/  2020-08/  2020-10/  2020-12/  2021-02/  2021-04/  2021-06/  2021-08/  2021-10/  2021-12/

Expansion also allows us to use the output of a command in different places. The syntax is the same as arithmetic expansion, except the command is placed inside $(...)

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$ echo $(ls | grep 2020)
2020-01/ 2020-02/ 2020-03/ 2020-04/ 2020-05/ 2020-06/ 2020-07/ 2020-08/ 2020-09/ 2020-10/ 2020-11/ 2020-12/

Next, let's learn how to control expansion. Consider the following two examples where automatic expansion doesn't do what we want.

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$ echo This is a      test
This is a test

$ echo The total is $100.00
The total is 00.00

In the first example, the shell removes extra space from the echo command's argument. In the second example, $1 is interpreted as the first input parameter which is not defined here, and therefore, it is replaced as empty string. With quoting, we can suppress unwanted expansions.

First, let's learn about double quotes. If we place text inside double quotes, parameters are not split by white space and all special characters lose their special meaning, and are treated as ordinary characters. However $, \ (backslash), and ` (back quote) are exceptions.

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$ echo "This is a     test"
This is a     test

$ echo "The total is $100.00"
The total is 00.00

Here is another example where the shell splitting the parameters by white space would cause a problem. Suppose that our working directory has the file two words.txt in it, where the filename has a space.

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$ ls two words.txt
ls: cannot access two: No such file or directory
ls: cannot access words.txt: No such file or directory

The shell splits the two words and looks for them each separately with ls. Quoting fixes this.

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$ ls "two words.txt"
two words.txt

If you want to suppress all expansions, you need to use single quotes.

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$ echo 'This is a     test'
This is a     test

$ echo 'The total is $100.00'
The total is $100.00

The next three examples show how quoting gives different results.

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$ echo text ~/*.txt {1..5} $(echo foo) $((2+2)) $(date)
text /mnt/home/user_name/hostfile.txt /mnt/home/user_name/powertools.txt 1 2 3 4 5 foo 4 Tue Jan 19 15:10:00 EST 2021

$ echo "text ~/*.txt {1..5} $(echo foo) $((2+2)) $(date)"
text ~/*.txt {1..5} foo 4 Tue Jan 19 15:10:09 EST 2021
$ echo 'text ~/*.txt {1..5} $(echo foo) $((2+2)) $(date)'
text ~/*.txt {1..5} $(echo foo) $((2+2)) $(date)

A backslash is useful when we want to quote a single character. A backslash is called the escape character. Next example shows how quoting and an escape character work.

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$ echo The balance of $(date) is $100
The balance of Tue Jan 19 15:15:36 EST 2021 is 00

$ echo "The balance of $(date) is $100"
The balance of Tue Jan 19 15:15:48 EST 2021 is 00

$ echo 'The balance of $(date) is $100'
The balance of $(date) is $100

$ echo "The balance of $(date) is \$100"
The balance of Tue Jan 19 15:16:09 EST 2021 is $100

$ echo 'The balance of $(date) is \$100'
The balance of $(date) is \$100

The table show the most frequently used escape characters.

Escape Character Name usage
\n newline Adding blank lines to text
\t tab Inserting horizontal tabs to text
\a alert Making the user terminal beep
\\ backslash Inserting a backslash