Advanced Bash-Scripting HOWTO: A guide to shell scripting, using Bash
PrevChapter 3. Tutorial / ReferenceNext

3.12. Backticks (`COMMAND`)

Command substitution

Commands within backticks generate command line text.

The output of commands within backticks can be used as arguments to another command or to load a variable. 
rm `cat filename`
# "filename" contains a list of files to delete.

textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
#
textfile_listing2=$(ls *.txt)
echo $textfile_listing
# Also works.

Note: Using backticks for command substitution has been superseded by the $(COMMAND) form.

Arithmetic expansion (commonly used with expr)

z=`expr $z + 3`
Note that this particular use of backticks has been superseded by double parentheses $((...)) or the very convenient let construction. 
z=$(($z+3))
# $((EXPRESSION)) is arithmetic expansion.
# Not to be confused with command substitution.

let z=z+3
let "z += 3"  #If quotes, then spaces and special operators allowed.
All these are equivalent. You may use whichever one "rings your chimes".

PrevHomeNext
System and Administrative CommandsUpI/O Redirection