The Old Monk

A few shell tricks that I picked up.
Variable manipulation:

gera@gera-laptop:~$ BAR=bar
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo $BAR
bar
gera@gera-laptop:~$ echo ${FOO:-baz}
baz
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo ${FOO:-$BAR}
bar
gera@gera-laptop:~$ echo $FOO

gera@gera-laptop:~$ echo ${FOO:=newvalue}
newvalue
gera@gera-laptop:~$ echo $FOO
newvalue
gera@gera-laptop:~$ echo ${BAR:=newvalue}
bar
gera@gera-laptop:~$ echo $BAR
bar

More here.

Here strings - which are like here docs.

gera@gera-laptop:~$ cat <<END
> foo
> bar
> baz spam eggs
> END
foo
bar
baz spam eggs
gera@gera-laptop:~$ cat <<<"foo bar baz spam eggs"
foo bar baz spam eggs

Although you can do the same with a piped echo, the here string saves you a process (the echo). Still, I'm looking for a more convincing use case.

gera@gera-laptop:~$ echo "foo bar baz spam eggs" | cat
foo bar baz spam eggs