$ (dollar-sign)
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The Unix operator “$” can be used to access the value of a variable or command. This can be especially useful in conjunction with the cat command to include the contents of a file in another Unix command.
Example
If you have a search string saved in a text file (searchstring.txt), and would like to use that search string as the value for the esearch -query argument without having to re-type it or copy and paste, you can use the value of cat searchstring.txt (i.e. the contents of the file “searchstring.txt”) as the -query argument for an esearch command.
esearch -db pubmed -query "$(cat searchstring.txt)"