Scripts
A method of performing several commands, one after the other, is
to create a file containing those commands, one per line, and then
'execute' that file. As an example, create a file using Vi, called
(say) whenandwho, and containing two lines:
date
who
Now, type sh whenandwho and the commands
date and who mentioned in the file
whenandwho will be executed, and you will get
$ sh whenandwho
Tue Dec 4 20:10:39 GMT 2001
chris pts/1 Dec 3 07:21 (console)
sam pts/3 Dec 3 08:38 (console)
jo pts/4 Dec 3 14:58 (console)
A file such as whenandwho, which contains commands,
is called a shell script ('script' for short).
At this stage it is important to understand how UNIX executes
commands in a script. When you are logged into your system, you
will be communicating with it via a program called a shell. The
shell that is run from the moment you log in to the end of your
session is your login shell. It is possible to run
further copies of the shell by typing the command sh.
In order to execute a shell script, the program sh is
run, but instead of taking standard input from your terminal, the
input comes from the file. Thus while the script is executing there
are two copies of sh running. The login shell runs one
command, namely sh, and this shell executes the
commands in the file.
When writing scripts, the command echo is very
useful. This command takes arguments and simply copies them to the
standard output, thus:
$ echo Hello there
Hello there
Worked example
Write a script called niceday to display today's
time and date together with a user-friendly message, thus:
$ sh niceday
The time and date are now:
Tue Dec 4 20:10:39 GMT 2001
Have a nice day!
Solution: We can use date to
output the date and time, and echo to output the
messages on lines 1 and 3. To create the file niceday,
use either Vi, or if your typing is good, simply use
cat:
$ cat >niceday
echo The time and date are now:
date
echo Have a nice day!
ctrl-D
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