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What does the $( ) notation mean in a Bash script?

On page 329 (Hour 16) of your book Teach Yourself Unix in 24 Hours is the example:

value=3 ; string="my horse Horace"
test $value < $(echo $string | wc –c)
test `wc –l filename` -ge 10000

If I try this with this code:

#!/bin/sh/
value=3;string="my horse Horace"
test $value < $(echo $string | wc -c)
test `wc -l ched4` -ge 10000

I get:-
syntax error at line 3: `(' unexpected

Are you able to quickly explain what the line test $value < $(echo $string | wc -c) is actually trying to do? I don’t quite understand the $(…) part … and is the < performing a ‘redirection for input’ or is it meant to be ‘less than’?

Dave's Answer:

The $( ) notation is a modern replacement for the ` ` (also called the "backtick") notation, both submit the contents to a subshell for execution and replace it with the output of that command. For example:

echo Today is $(date)

would produce the actual date in the string. Make sense? That’s what should be happening above.

The notation then translates step by step:

test $value < $(echo $string | wc -c)

then has the echo $string | wc -c replaced with the number of characters in the variable "string". That's going to be 15, so now we're testing:

test $value < 15

since the variable "value" has been set to 3, we're really testing

test 3 < 15

which should certainly be true!

I notice two additional things in your example, though. First, you have /bin/sh/ with the trailing slash, which might be a problem. But you also might not be finding that you have Bash as your default shell on your system too, so do you want to try

#!/bin/bash

and see if that works.

Let me know how it goes!

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Comments

please let me know ,we type script in editor, i am using vi editor after that at command line we type :wq save and quit enter, we'll go to $ prompt then how to to see output for perticular script? is it shell dependent if yes let me know for borne,c-shell.please

Posted by: mini at December 18, 2007 11:40 PM

put a backslash before the < symbol in the test line, i.e.,
test $value \< $(echo $string | wc -c)

that way the shell will interpret it as a "less than" sign instead of an input redirection

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