Another one: ` backquote char, always used in pair. Command substitution: replace with result (output) of running the command. Replace command with its output.

$ date
Tue  12 Apr  12:20  1995
$ echo date
date
$ echo `date`
Tue  12 Apr  12:21  1995
$ echo Today is `date`
Today is Tue  12 Apr  12:21  1995
$ `date`       is replaced by its output, attempt is made to execute
Tue: command not found


$ ls -l `which cat`    
#long listing of the cat command file, wherever it is

Useful to give list of filenames to command:

$ wc `locate .java | grep wills`  #wc of wills' .java files

$ ls -l `find . -name 'core*'`  # listing of all core dump files

$ file `locate .gif` | egrep -v 'GIF|symbolic'  
#any .gif filename that is not a GIF file nor a symbolic link

Which bash builtins are also the name of commands in your PATH of directories?
$ for i in `cat bash_builtins` ;do which $i 2>/dev/null ;done

$ mail billybob barack hillary rudy mitt <letter1
$ cat >tolist
billybob
barack
hillary
rudy
mitt
$ mail `cat tolist` <letter1

$ cat  alf
a
b
c
d
$ echo `cat alf`                # echo turns newline into space
a b c d

New syntax for command substitution: $(cmd) Allows nested substitutions.
$ echo $(date)

Ex. listing of README files that are larger than the README file in this directory:
$ ls -l $(find . -name README -size +$(echo $(cat ./README |wc -c)c)-print)

$ ps -aux | fgrep "`who | cut -f1 -d' '`"    
#processes of logged on users