CODEWITHSUNDEEP
linuxbashubuntu
morpheous
morpheous

Reputation: 16966

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

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 ?

Upvotes: 0

Views: 519

Answers (6)

eje211
eje211

Reputation: 2429

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.

Upvotes: 0

Carl Smotricz
Carl Smotricz

Reputation: 67760

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)

Upvotes: 0

Recurse
Recurse

Reputation: 3585

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.

Upvotes: 0

heijp06
heijp06

Reputation: 11788

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

Upvotes: 0

strager
strager

Reputation: 90012

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 aneval` (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.

Upvotes: 2

Karussell
Karussell

Reputation: 17375

Try it!

$date +WeekNo.%W
WeekNo.30

Upvotes: 3

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