EECS 201 - Class 3: Unix and You

# Overview
1. Announcements
2. Review
3. Q&A
4. Exercises
5. Basic assignment
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# Announcements
* Basic/Adv - Git 1 due May 18
* Basic/Adv - Unix due May 22
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# Review
* Programs vs. processes
* Processes have extra info
  * Process ID (PID)
  * User running the process
  * Current working directory
  * Program image
  * Working memory (stack and heap)
  * Environment variables
  * File descriptor table
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# Review
* Signals are another communication mechanism
  * ^C: SIGINT
  * ^Z: SIGTSTP
  * `kill`
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# Review
* Everything is a "file"
  * Files are a read/write interface
  * Become a general communication mechanism
  * Can be used to represent anything that'd work with such an interface
  * Terminals are files as well!
--

* Files have metadata including permissions
  * Represented in decimal/octal, with three bits
  * `rwx`
```
user  group  other
rwx   rwx    rwx
110   100    100
 6     4      4
```
  * `ugo`
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# Help/manual pages
* Most commands have a flag for showing a brief help message,
  typically `-h` or `--help`
* Many commands and programs have more extensive "man" pages
  * `man ls`
  * `man git`
--

* More concise man pages available through the `tlrc` package
  * MacOS: `brew install tlrc`
  * WSL/Linux: do a web search for "tlrc", find the GitHub page, use
    one of the options there (nixpkgs or a prebuilt `.deb` file frome
    GitHub Releases)
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# Shell operation
1. Receive a command from a file or terminal input
  * `ls -l $HOME > some_file`
--

2. Splits it into tokens separated by __white-space__
  * Takes into account _"quoting"_ rules
  * `ls`, `-l`, __`$HOME`__, `>`, `some_file`
--

3. Expands/substitutes special tokens
  * `ls`, `-l`, __`/home/akamil`__, `>`, `some_file`
--

4. Perform file redirections (and making sure they don't end up as command args)
  * `ls`, `-l`, `/home/akamil`; __(set standard output to `some_file`)__
--

5. Execute command (remember our friend `exec()`?)
  * `argc` = 3
  * `argv` = [`"ls"`, `"-l"`, `"/home/akamil"`]
  * Standard output redirected to `some_file`
  * First "normal" token is the command/utility to run
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# Shell operation
* File redirection
  * `<`, `>`, `>>`
* Variable assignment
  * `varname=value`
* Variable expansion
  * Text substitution before execution
  * `echo $HOME`
  * `$` indicates variable expansion
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# Shell scripts
* Just files with shell commands in them
* Same stuff that you'd type at terminal
* Shebang to specify interpreter when you execute the file
  * `#!/bin/bash`
  * Applicable to other interpreted languages e.g. Python
---

### [signals - turnoff.us](http://turnoff.us/geek/signals/)
<a href="http://turnoff.us/geek/signals/">
<img src="http://turnoff.us/image/en/signals.png" class="center" style="position:fixed; left: 28%; height: 75%;">
</a>
---

### [the real reason not to use sigkill - turnoff.us](http://turnoff.us/geek/dont-sigkill/)
<a href="http://turnoff.us/geek/dont-sigkill/">
<img src="http://turnoff.us/image/en/dont-sigkill.png" class="center" style="position:fixed; left: 36%; height: 75%;" title="I put this outside my cubicle at one of my first internships and it stayed there until the building got demolished two years later">
</a>
---

# Q&A
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# Exercises
1. Write a command that saves the output of `ls` to a file `listing`
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2. The command `rev` reverses a line of text and `sort` takes lines of input and outputs them in a sorted manner
  * Write a command that takes the output of `ls`, reverses the name of each file, sorts these reversed names and saves it to a file called `gnitsil`
--

3. Write a command that runs `git status` and saves the standard output to `out.txt` and standard error to `err.txt`
--

4. The command `date` outputs a timestamp
  * Write a command that appends the current timestamp to a file called `timestamps.log`
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# Exercises
* Write a shell script that appends an ISO 8601 format timestamp, then appends the first argument to a file named `log`
  * `date -Isec` can get this timestamp for you
  * `date "+%Y-%m-%dT%H:%M:%S%z"` for macOS (or if you want to be cross compatible)
  * `$1` expands the first argument
  * Make sure to give it a shebang
  * Make sure to `chmod` it so it's executable
  * Run it with an argument e.g. `$ ./myscript this-is-an-argument`
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