What is the meaning the $ special character

bodisha · 11-02-2017, 10:13 AM

Hello... and thank you in advanced for any help anyone can offer me

I'm hoping someone can explain what the leading $ is/means (i.e. $PS1, $HOME, etc).... I was having a discussion with someone and was trying to explain it... Which I felt like I came up kind of short with how well I did it.

I understand it's a special character and how to use it if I want to see the value of a variable or if I want to see the status of a command... I'm just unsure what kind of special character its categorized as or the definition of it's exact function.

I got home and googled it... I found plenty of explanations on how to use it but didn't find an adequate explanation of what it is and it's definition. It seems like every special character is well documented except the $... Could someone explain to me how it's categorized and it's extract definition?

Once again... thanks for reading this and any help anyone can offer

michaelk · 11-02-2017, 11:01 AM

You might have to go back to the beginning of Unix to figure that one out. From what little I know it means substitution.

Code:

user=Fred
echo $user

When the shell runs the line "echo $user" Fred is substituted for the variable user and therefore when you run the script it outputs Fred.

I think what you mean by status of a command i.e.

Code:

output=$(ls -l *.log)

Is called command substitution i.e. the variable output is assigned the output of the ls command and not "ls -l *.log"

scasey · 11-02-2017, 11:42 AM

I'm not sure that there is a "meaning" to the $..or even that it is a "special character" in this context. It is an element of the syntax used to define a variable. It's been used historically that way for a long time: BASIC alpha variables [$a, $b, etc] for example.
(How do I remember that? It's been > 40 years since I programmed in BASIC!)

hazel · 11-03-2017, 02:39 AM

The $ character in Linux shells is a "substitution operator". It means "Treat this string as a variable name and substitute the current value". So when you are giving a command that relates to the name of the variable, you don't use $, but when you want the value, you do.

Code:

SAVE_DIR=~/saves
export SAVE_DIR
echo $SAVE_DIR

The export command doesn't use $ because it relates to the variable as such, telling the shell to store it in the environment. But the echo command uses $ because it is the value of the variable that you want to print out.

Shadow_7 · 11-04-2017, 08:53 AM

$ export VAR="bubba"
$ echo $VAR

It can be used get the VALUE of a VARiable.

$ ls /usr/bin/ | grep -i z$

Or used in regex as an end of line and to find something that ends with z like above. And I think there's a special case for $$ as well, but don't recall what that is atm.

David the H. · 11-05-2017, 01:38 AM

To ask what "meaning" a character has depends on the context it's found in, because they can have different uses in different situations.

But in the shell, the "$" nearly always refers to a parameter expansion or substitution of some sort.

The most common use is for variable expansion. If you have a variable set, then placing "$" in front of the variable name substitutes the value of the variable, as shown in the several examples above.

Other common uses include process substitution [$(..)] and math substitution [$((..))].

It is also traditionally used as the default command prompt -- the first character on the line indicating where a command can be typed. You will see this in most shell coding examples on the net and elsewhere. Examples below:

Code:

$ wget "$( grep -Eo 'http://foo.com/[^"]+.jpg' <file.html )"
<wget output here>

$ echo "$(( 12 * 5 + 8 ))"
68

The first command searches "file.html" for all ".jpg" urls in it that start with "http://foo.com", and inserts them into the wget command, which then attempts to download them.

Other commands and programming languages, such as awk or perl, will often use the same characters, but in different ways. That makes learning how to properly use shell quoting very important when running them in the shell.

There are any number of beginning shell scripting tutorials on the net. Try out a few of them. The Bash Guide For Beginners is usually a good place to start:
http://tldp.org/LDP/Bash-Beginners-G...tml/index.html

And finally, man bash is always the first place you should look when you have syntax questions. It's a long document, but well worth reading from end to end at least once (although I recommend first learning the scripting basics elsewhere before doing so).