What does the command $() do in UNIX? Could someone explain what is the significance of $ in the below examples;
$(echo var=10)
and
eval $(echo var=10)
1 Answer
This isn't a command itself, rather this is an example of command substitution. It takes the value of what is in parentheses and uses it in context.
For example, I will frequently make a backup of a file before editing it. I'll use command substitution to add the date to the end of the filename in ISO 8601 format (for sorting.) Of course, I could just type the date, but this is great for scripts where the date may be different each time it runs.
$ cp -p /path/to/file /path/to/file.$(date +%F)
$ ls /path/to/file.*
/path/to/file.2013-09-10 /path/to/file
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This
$(echo something)was always written`echo something`before, and you easily overlook those backticks, especially on a cheap printer.ott--– ott--2013-09-10 20:40:34 +00:00Commented Sep 10, 2013 at 20:40 -
5Not to mention the nightmare it creates for nesting!Aaron Copley– Aaron Copley2013-09-10 21:22:42 +00:00Commented Sep 10, 2013 at 21:22
man bashfor “$(”. (Depending on your man pager, you may need to write the search expression as “\$\(”.)$()in the title -- which I don't see in the "Related" sidebar, and that is what people will look for if they don't know it is about command substitution. So thank you for asking that way!