Shell Programming and Scripting

At a basic level the curly braces enclose the name of the variable such that it stands alone as an entity.

If I want to substitute the contents of the string into a variable or a command we can get problems:

file1="myfilename"
echo "$file1MYSTRING"

sh: file1MYSTRING: Parameter not set

This is because the shell doesn' know when the parameter name ends and the string starts.

file1="myfilename"
echo "${file1}MYSTRING"

myfilenameMYSTRING

Similarly distinguishing variables $file1 from $file11 which cropped up on this board recently.

Get it?

In the example shown, they are unnecessary.

They are only needed when the variable name is followed by a character that could be part of a variable name.

To cfajohnson. I did not expect this response from you who I recognise as an expert in shell syntax. The O/P script did not contain the problem, my initial response did not contain the problem, but the example was designed to teach.

Please explain the error message in my example in your own words.

I was not commenting on any error message. I was answering the question, "Can you briefly explain the curly brackets."

My comment was about this:

Code:

mv "${file1}" "${file1}.TMP"
mv "${file2}" "${file2}.TMP"

The braces are not necessary in that example. They are not wrong, but they do nothing.

If the example had been:

Code:

mv "${file1}" "${file1}_TMP"
mv "${file2}" "${file2}_TMP"

...then they would have been necessary but only in the second instance on each line.

Thanks cfajohnson. No offence taken or meant, but pedandtic or what? One day the O/P will appreciate this input and we may have to do a contract on that site.

lol thanks guys. I greatly appreciate the clarification.

Curly brackets separate the variable name from other text surrounding it. Eg.

Code:

echo ${foo}bar

means fetch variable foo's value and append it with text bar. Without curly brackets the shell would be searching for a variable called foobar. One could also write

Code:

echo $foo bar

, but that would produce a space between.

Regards,

pen