EECS 201 - Class 4: Shells

class: center, middle
# Shells
## feat. Bash
### `:(){ :|:& };:`
#### Do __NOT__ run this
---

# Overview
1. Announcements
2. Review + Exercises
3. Q&A
4. Basic assignment
---

# Announcements
* Unix assignments: February 15
* Shell assignments: February 21
* Unix survey closing today
--

* Reference slides for each week!
--

* Lunar New Year tomorrow!
---

# Review
* Command grouping
  * `(commands)`
  * `{ commands; }`
---

# Expansion
### Parameter expansion ("variable" expansion)
* `$varname`
* `${varname}`
* `${varname:-[value]}`: use default value
--

* Bash substring expansions
  * `${varname:offset}`
  * `${varname:offset:length}`
---

# Expansion
### Filename expansion ("glob"/"wildcards")
* Expand out to filepaths that match the pattern
* `*`, `?`, and `[]`
--

### Command substitution (via subshell)
* `$(command)`: substitute the output of a _command_ in the brackets
--

* Q: `$logname` vs. `$(logname)`?
---

# Expansion
### Arithmetic expansion
* `$((expr))` will expand to an evaluated arithmetic expression _expr_
  * Integer only
--

### Process substitution (Bash)
* `<(command)` will substitute the _command_ output as a filepath, with the output of _command_ being __readable__
* `>(command)` will substitute the _command_ input as a filepath, with the input of _command_ being __writeable__
* `$ diff <(echo hello) <(echo olleh | rev)`
  * `diff` takes in two file names, but we're replacing them with "anonymous" files containing the command outputs
---

## Exercises
1. Assign a variable `greeting` to a string that is concatenation of the string "user:" and the `USER` variable
2. Write a `mv` command that moves all files in the current directory that end in `.txt` into a directory called `text`
3. Use a command substitution (`$(commands here)`) to get the output of `whoami` and save it into a variable `me`
---

## Quoting
* Single quotes (`'`) preserves __all__ of the characters between them
* Double quotes (`"`) preserve all characters except: `$`, `\`, and backtick
---

## Compound commands and control flow
### `if-elif-else`
```bash
# '#' comments out the rest of the line
# elif and else are optional parts
if test-commands; then
  commands
elif more-test-commands; then
  more-commands
else
  alt-commands
fi
```
* _test-commands_ is executed and its __exit status__ is used as the condition
  * ___0___ = success = "true", everything else is "false"
---

### Commands for conditionals
* `test expr`: `test` command
* `[ expr ]` (remember your spaces! `[` is technically a utility name)
* `test $a -eq $b`
* `[ $a -eq $b ]`
* These set the exit status (`?`) to 0 (true) or 1 (false)
* `[ $a -eq $b ] && [ $a -lt 100 ]`
* `test $a -eq $b && test $a -lt 100`
---

### Commands for conditionals (Bash)
* `[[ expr ]]`: __Bash__ conditional
  * Richer set of operators: `==`, `!=`, `<`, `>`, among others
  * __Note__: The symbol operators above operate on strings
* `((expr))`: __Bash__ arithmetic conditional
  * Evaluates as an arithmetic expression
  * `(($a < $b))`: this would evaluate to "false" if a=100, b=2
---

### `while`
```bash
while test-commands; do
  commands
done
```
* Similarly to `if`, the exit status of _test-commands_ is used as the conditional
* Repeats _commands_ until the condition __fails__
--

### `until`
```bash
until test-commands; do
  commands
done
```
* Repeats _commands_ until the condition __succeeds__
---

### `for`
```bash
for var in list; do
  commands
done
```
* Each iteration _var_ will be set to each member of the _list_
* _list_ is simply a list of whitespace-delimited strings
* _list_ will have any necessary expansions performed
* __Note__: if there is no `in list`, it will implicitly iterate over the argument list (i.e. `$@`)
* Example lists:
  * `1 2 3 4 5`
  * `$(ls)`
  * `$(seq 1 5)`
---

### `case`
```bash
case value in
  pattern1 ) commands1 ;;
  pattern2 ) commands2 ;;
  multpat1 | multpat2 ) commands3 ;;
  * ) commands
esac
```
* _value_ is matched against patterns
* When a pattern is matched its command(-list) is run
* A wildcard pattern is often used to represent a "default" case
---

## Exercises
1. Write an `if` statement that prints "success!" if the last command ran successfully
  * Remember the `?` variable?
  * `echo` can print text for you
  * `true` and `false` can give you a success and failure
2.  Write a `for` loop that creates 5 files, named `file1` to `file5`
  * `seq 1 5` can produce a list of integers from 1 to 5
  * `touch` can create empty files for you
---

## Functions
```bash
func-name () compound-command # parens are mandatory
# or
function func-name () compound-command # [Bash]; parens are optional
```
* A __compound command__ is a __command group__ (`()`, `{}`) or a control flow element (`if-elif-else`, `for`)
* Called by invoking them like any other utility, including __passing arguments__
  * Arguments can be accessed via `$n`, where _n_ is the argument number
  * `$@`: list of arguments
  * `$#`: number of arguments
---

### Examples
```bash
hello-world ()
{
  if echo "Hello world!"; then
    echo "This should print"
  fi
}
# calling
hello-world
```

```bash
# Bash
function touch-dir for x in $(ls); do touch $x; done
# calling
touch-dir
```
---

```bash
echo-args ()
{
  for x in $@; do
    echo $x
  done
}
# calling
echo-args a b c d e f g
```

```bash
# Bash
function divide
{
  if (( $2 == 0 )); then
    echo "Error: divide by zero" 1>&2
    # the redirection copies stderr to stdout so when echo
    # outputs it's really going to the caller's stderr
  else
    echo $(($1 / $2))
  fi
}
# calling
divide 10 2
divide 10 0
```
---

# Configuring the shell
* Shells will automatically source certain files to perform configuration
  * `/etc/profile`: system-wide configuration
  * `~/.bashrc`: Bash's user shell configuration file
  * `~/.zshrc`: Zsh's user shell configuration file
--

* You can make your own additions to your `~/.bashrc` or `~/.zshrc` etc.
* Maybe you want to add a directory to `PATH`?
  * `export PATH="newdir:$PATH"`
* Maybe I want to alias a word to a command that navigates to my Windows side?
  * `alias cdw='cd /mnt/c/Users/brandon/'`
* Maybe I want to change up my prompt?...
  * `PS1` variable

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