What does this expression evaluate to? (bash shell script)

Question

I am trying to do a code walk through a badly written bash script.

I have come across this statement:

FOOBAR_NAME=`date +WeekNo.%W`

There are no prior declarations of any of the RHS variables in the script, lines preceding this statement.

So my question is:

What does FOOBAR_NAME resolve to, when it is used a few lines down in the script as $FOOBAR_NAME ?

Answer 1

Try it!

$date +WeekNo.%W
WeekNo.30

Answer 2

There are no variables being referenced in the RHS.

The backtick operator (`) evaluates its contents and returns the output, similar (identical?) to $(). It's a quick way to write an eval (in other languages).

Type date +WeekNo.%W in a shell. What is printed (in stdout, with newlines collapsed) is what will be stored in FOOBAR_NAME.

Note that the evaluation occurs only once, which is during the assignment. date isn't executed each time you reference FOOBAR_NAME.

Answer 3

See man date for a description of the date command and it's formatting options. %W is week number.

Answer 4

This is using a format string to the date command to create the a string that contains the week number.

The backticks execute the command between them; and the line assigns the result to the shell variable FOOBAR_NAME.

So if you really want to know what it does, just cut and paste the text between the `` into a shell and execute it.

Answer 5

You can find the answer in man date: If you specify an argument starting with +, then the rest of that argument is taken as a format string. The Weekno. part is taken literally, the %W does:

%W week number of year, with Monday as first day of week (00..53)

Answer 6

The assignment operator ("=") assigns the value on its right part to a variable on the left part. Here the variable is FOOBAR_NAME.

The right part is a subshell. The backticks ("`") create a subshell. The output of that subshell will go to the variable.

The subshell rurns the Unix date command. The manual page for all Unix commands is on the Internet. There a Unix man page for date. Here, %W will be replaced by the number of the week.

So the variable gets the value "WeekNo" plus the number of the week.