One may run or “execute”  shell scripts by typing it’s filename as a command and pressing <Enter> as they would with a regular command. However, the script file must must have executable permission set (see the previous post). Also, scripts may be given arguments and options similar to other programs.

Scripts stored in a directory on one’s path  may be run by simply typing the name of the file and pressing <Enter>. Otherwise one must provide a relative or full path to the location of the script file. Or if one’s current working directory contains the script file it can be run buy adding a “./” to the filename.

Examples…
Run a script named “hello” that is in a directory in your path…
$ hello <Enter>

Run a script named “hello” that is in your current working directory…
$ ./hello <Enter>

Run a script named hello that is in ~/MyScripts/ (a subdirectory of your “home” directory)…
$ ~/MyScripts/hello <Enter>

#!/bin/bash
or
#!/bin/sh

The #! tells the shell that the file contains executable commands and the path that follows tells the shell which program to run the commands with. The program used to run the script can be PERL, SED, AWK, another language or another shell.

The first option indicates that the shell is to use the Bash program itself (/bin/bash) to execute the script. The second option indicates the script is to be executed by (/bin/sh) which is a symbolic link to bash for backwards compatibility with older scripts. sh is the name of the old Bourne shell which Bash has replaced. Bash can run any Bourne script and many people still use /bin/sh to begin their scripts.

Example…
Write a Bash script that outputs the text “Hello world” to stdout…
Step 1. Create a new text document using a text editor and add the following lines…
#!/bin/bash
echo Hello world
then save the file as hello.
Step 2. Using chmod change the permissions on the file to make it executable…
$ chmod a+x hello <Enter>

The next few posts will discuss the basic concepts of shell scripts.

Example…
Remove an alias for ls…
$ unalias ls <Enter>

If the alias is set in one’s .bashrc or .bash_profile the above will remove the alias but only for the current shell session. To remove the alias from all future shell sessions edit said file by removing the line where the alias is defined.

Example…
List aliases in the current shell…
$ alias <Enter>

Example…
Use “bye” as an alias for exit…
$ alias bye=”exit” <Enter>

bye now becomes the alias for exit in the current shell. Thus giving the alias bye will run the exit command.

Arguments and options may also be included in an alias.

Example…
Alias ls such that in a color terminal the output will be color coded…

$ alias ls=”ls –color=auto”

This will alias  the ls command with its option to include color output where it is supported by hardware.

Observe that options and arguments can still be passed to an alias as if they were being typed explicitly without the alias.

To use the original explicit command after an alias has been set one must give the full path to it.

Example…
Run ls -al explicitly even though it has been aliased to another value in the current shell…
$ /bin/ls -al

Observe that if you want an alias to persist in your shell between sessions it must be included in .bashrc located in ones home directory.

When using an alias the command and not the alias will show up in your shell history. This is in contrast to a script which does show up on one’s shell history rather than the commands it is composed of. The scope of the alias is limited to the user that creates it where-as a script can be placed in the local bin so that anyone on the system can use it.

In short an alias can be used to call a command with arguments by a short name. I shall discuss the alias tool in the next few posts.

Example…
See how many seconds the shell has been running…
$ echo $SECONDS

One can use the utility units to convert the output into hours, minutes, ect.

Example…
The PATH variable in .bashrc is set to…
PATH=”/usr/bin:/bin:/usr/bin/X11:/usr/games”
and you want to add /home/oznola/bin.
Subsequently add the new directory to PATH like this..
PATH=”/usr/bin:/bin:/usr/bin/X11:/usr/games:/home/oznola/bin”

Example…
Change your shell prompt to “Genie in a bottle: ” type…
$ PS1=”Genie in a bottle: “ <Enter>
Genie in a bottle:

The new text must be quoted because it has spaces in it. See Quoting Reserved Characters.

One can use special characters in the PS1 variable in order to display special text. “\w” is an example where by adding it to the contents of PS1 will list the current working directory at that point in the shell prompt text.

Example…
Change your prompt to the default Bash prompt or the current working directory plus the “$“…
$ PS1=’\w $ ‘ <Enter>
~ $

Example…
To change back to the default (for a MacBook) type…
~ $ PS1=’\h:\w \u$ ‘ <Enter>
bentley:~ zander10$

Consult your local Bash man page section under “PROMPTING” for the complete list of backslash escaped characters that can be used on your machine.

Here are the ones available on my machine…

\a an ASCII bell character (07)
\d the date in “Weekday Month Date” format (e.g., “Tue May
26″)
\D{format}
the format is passed to strftime(3) and the result is
inserted into the prompt string; an empty format results
in a locale-specific time representation. The braces are
required
\e an ASCII escape character (033)
\h the hostname up to the first `.’
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell’s terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion
following the final slash)

\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated
with a tilde
\W the basename of the current working directory, with $HOME
abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could
be used to embed a terminal control sequence into the
prompt
\] end a sequence of non-printing characters

One can combine any number of these special characters with regular characters to create a value for PS1.

To set your prompt permanently see this page.