bash

class: center, middle

# CSCI 395.86 Open Source Software Development

<br>

## Working in the Linux Command-Line: `bash`

.author[
Stewart Weiss <br>
]

.license[
Unless noted otherwise all content is released under a
 [Creative Commons Attribution-ShareAlike 4.0 International License](https://creativecommons.org/licenses/by/4.0/).
]

---

# A Bit About bash

- bash was written by Brian Fox in 1989 at the request of Richard Stallman,
of the Free Software Foundation. It was added to by Chet Ramey in the early 1990's.

- At the time, there were several other shells, such as the __C-shell__ (`csh`)
written by Bill Joy,
and the __Bourne shell__ (`sh`), written by Stephen Bourne).

- bash was a replacement for the
[Bourne shell](https://en.wikipedia.org/wiki/Bourne_shell). Its name is a pun:

- bash = __b__ourne __a__gain __sh__ell

- It is distributed with most Linux distributions ans is the default shell in
many systems, including most Linux distributions, Apple's macOS, and Solaris.

- It has become very popular.

---

# Shell Scripts

- All common Unix shells are Turing-complete* __scripting__ languages.
Programs written in a shell's language are called __shell scripts__.

.footnote[
* A language is Turing complete if it can be used to simulate any Turing machine.
]

- A _shell script_ is basically a program written in the shell programming language.

- A bash script is written in bash; a C shell script is written using the syntax of the
C shell.

- This is a simple bash script:

```bash
    #!/bin/bash
    # This script does nothing except print "Hello world; code responsibly!"
    # Its purpose is to show what the first line looks like and
    # to introduce comments.
    #
    # Written by Stewart Weiss, Sept. 16, 2014

echo "Hello world; code responsibly!"
   ```

- The first line is required - it tells the shell to run the interpreter
 `/bin/bash` using the rest of the file as its input.  In effect it  says,
"I am a bash script". You must put it at the top of every bash script.

- The comment delimiter is the `#` - anything after it is ignored by bash.

- The only command in this file is the `echo` command.

---

# Running a Shell Script

- Suppose the preceding script is in a file named `helloworld` in the current
working directory. We need to make it an executable file by typing
at the prompt (here assumed to be `$`):

```bash
   $ chmod +x helloworld
   ```

- The `chmod` command changes the permissions on its file arguments. The `+x`
means "add execute permission for all users". The file also needs to be
readable, so it is better to write, instead,

```bash
   $ chmod +rx helloworld
   ```

- We can run the script by typing its name, and the output appears below.
   ```bash
   $ ./helloworld
   Hello world; code responsibly!
   ```

---
# Changing Your PATH Variable

-  To be able to enter
   ```bash
    $ helloworld
   ```
 and have it execute requires that the `PATH` environment variable have
your working directory in it.

- PATH is a colon-separated list of directories that are searched by `bash` when you enter
a command name.

To see its contents, enter
   ```bash
    $ echo $PATH
   ```
 or
   ```bash
    $ printenv PATH
   ```
- You can edit your `.bashrc` file to put "." at the __end__ of `PATH`. The easiest
but not the best way is to edit the file, find `PATH` in it and put ":." at the
end.

---
# Passing Command-line Arguments to `bash`

- `bash` stores the words from the command-line after the command itself
in successive parameters `$1`, `$2`, ...

- The number of parameters on the command line after the command itself
is `$#`. The entire set of words after the command is stored in `$*`:
```bash
 #!/bin/bash
 echo "$*"
```
displays the words after the program name as they were entered.

- The very last parameter can be accessed by the expression `${@: -1}`, whose
meaning will not be explained here.
```bash
 #!/bin/bash
 echo "The last word on the command line is ${@: -1}"
```
---
# Control Flow

- `bash` has control flow, but it is very serious about its syntax and you must
play by its rules, otherwise your scripts will not run. Following are some
examples.

```bash
    #!/bin/bash
    if test $# -eq 0
    then
        echo "This is my directory:"
        ls .
    else
        ls $1
fi

```

---