Difference between echo "$shell" and echo '$shell'
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echo is a built-in command in the bash and C shells that writes its arguments to standard output.
A shell is a program that provides the command line (i.e., the all-text display user interface) on Linux and other Unix-like operating systems. It also executes (i.e., runs) commands that are typed into it and displays the results. bash is the default shell on Linux.
A command is an instruction telling a computer to do something. An argument is input data for a command. Standard output is the display screen by default, but it can be redirected to a file, printer, etc.
The syntax for echo is
echo [option(s)] [string(s)]
The items in square brackets are optional. A string is any finite sequence of characters (i.e., letters, numerals, symbols and punctuation marks).
When used without any options or strings, echo returns a blank line on the display screen followed by the command prompt on the subsequent line. This is because pressing the ENTER key is a signal to the system to start a new line, and thus echo repeats this signal.
When one or more strings are provided as arguments, echo by default repeats those stings on the screen. Thus, for example, typing in the following and pressing the ENTER key would cause echo to repeat the phrase This is a pen. on the screen:
echo This is a pen.
It is not necessary to surround the strings with quotes, as it does not affect what is written on the screen. If quotes (either single or double) are used, they are not repeated on the screen.
Fortunately, echo can do more than merely repeat verbatim what follows it. That is, it can also show the value of a particular variable if the name of the variable is preceded directly (i.e., with no intervening spaces) by the dollar character ($), which tells the shell to substitute the value of the variable for its name.
For example, a variable named x can be created and its value set to 5 with the following command:
x=5
The value of x can subsequently be recalled by the following:
echo The number is $x.
Echo is particularly useful for showing the values of environmental variables, which tell the shell how to behave as a user works at the command line or in scripts (short programs).
For example, to see the value of HOME, the environmental value that shows the current user's home directory, the following would be used:
echo $HOME
Likewise, echo can be used to show a user's PATH environmental variable, which contains a colon-separated list of the directories that the system searches to find the executable program corresponding to a command issued by the user:
echo $PATH
echo, by default, follows any output with a newline character. This is a non-printing (i.e., invisible) character that represents the end of one line of text and the start of the next. It is represented by \n in Unix-like operating systems. The result is that the subsequent command prompt begins on a new line rather than on the same line as the output returned by echo.
The -e option is used to enable echo's interpreta