Database Reference
In-Depth Information
Example 2-3. Python script that computes the factorial of an integer (~/book/ch02/
fac.py)
#!/usr/bin/env python
def
factorial
(
x
):
result
=
1
for
i
in
xrange
(
2
,
x
+
1
):
result
*=
i
return
result
if
__name__
==
"__main__"
:
import
sys
x
=
int
(
sys
.
argv
[
1
])
print
factorial
(
x
)
This script computes the factorial of the integer that we pass as a command-line
argument. It can be invoked from the command line as follows:
$
book/ch02/fac.py 5
120
In
Chapter 4
, we'll discuss in great detail how to create reusable command-line
tools using interpreted scripts.
Shell function
A shell function is a function that is executed by the shell itself; in our case, it is
executed by Bash. They provide similar functionality to a Bash script, but they
are usually (though not necessarily) smaller than scripts. They also tend to be
more personal. The following command defines a function called
fac
, which—
just like the interpreted Python script we just looked at—computes the factorial
of the integer we pass as a parameter. It does this by generating a list of numbers
using
seq
, putting those numbers on one line with
*
as the delimiter using
paste
(Ihnat & MacKenzie, 2012), and passing this equation into
bc
(Nelson, 2006),
which evaluates it and outputs the result:
$
fac
()
{
(
echo
1; seq
$1
)
| paste -s -d
\*
| bc;
}
$
fac 5
120
The file
~/.bashrc
, which is a configuration file for Bash, is a good place to define
your shell functions so that they are always available.
Alias
Aliases are like macros. If you often find yourself executing a certain command
with the same parameters (or a part of it), you can define an alias for this. Aliases
are also very useful when you continue to misspell a certain command (see
his
GitHub profile
for a long list of useful aliases). The following commands define
two aliases:
