A script is simply a file that contains shell
commands; it can range in complexity from one line to a big program. Before we can
execute a script, we must make the script file executable by using the
chmod command as follows:
% chmod +x filename
where filename is the name of the script (the
chmod command is discussed in the section on file permissions).
We can then run the script by typing its filename at the xterm prompt. If csh fails to
find the executable, then we must specify the script's name in the form
./executable, where
[.] designates the current directory and
executable is the name of the script.
When a script is run, Unix determines which shell the script is written for, and then
starts the appropriate shell to execute the commands in the script. Unix makes this
decision by examining the first line of the script:
- If the first line of the script is just a pound sign
[#], then the script is interpreted by the shell from
which it is executed.
- If the first line of the script is of the form
#!pathName, then
the shell whose path is pathName is used to
interpret the script. The pathname of the csh installed on the undergraduate machines is
/usr/bin/csh.
- If neither of the two cases above applies, then the script is interpreted by a
Bourne shell.
To prevent confusion, it is a good habit to always specify the name of the shell in the
form #!pathName.
Also, just like when we program in any other language, it is important to include
comments in the script to ensure legibility. When the pound sign appears on any other line
apart from the first line, all characters from the pound sign to the end of that line are
treated as a comment.
Redirection and Pipelines
Variables
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