# Shell Features

This document gives an overview of common shell features (the same in every shell flavour) and differing shell features (shell specific features).

## Common features

The following features are standard in every shell. Note that the stop, suspend, jobs, bg and fg commands are only available on systems that support job control.

### Common Shell Features

Command | Meaning
--- | ---
> | Redirect output
>> | Append to file
< | Redirect input
<< | "Here" document (redirect input)
| | Pipe output
& | Run process in background.
; | Separate commands on same line
* | Match any character(s) in filename
? | Match single character in filename
[ ] | Match any characters enclosed
( ) | Execute in subshell
` ` | Substitute output of enclosed command
" " | Partial quote (allows variable and command expansion)
' ' | Full quote (no expansion)
\ | Quote following character
$var | Use value for variable
$$ | Process id
$0 | Command name
$n | nth argument (n from 0 to 9)
$* | All arguments as a simple word
`#` | Begin comment
bg | Background execution
break | Break from loop statements
cd | Change directories
continue | Resume a program loop
echo | Display output
eval | Evaluate arguments
exec | Execute a new shell
fg | Foreground execution
jobs | Show active jobs
kill | Terminate running jobs
newgrp | Change to a new group
shift | Shift positional parameters
stop | Suspend a background job
suspend | Suspend a foreground job
time | Time a command
umask | Set or list file permissions
unset | Erase variable or function definitions
wait | Wait for a background job to finish

## Differing features

The table below shows major differences between the standard shell (`sh`), Bourne Again SHell (`bash`), Korn shell (`ksh`) and the C shell (`csh`).

### Shell compatibility

Since the Bourne Again SHell is a superset of `sh`, all `sh` commands will also work in `bash` - but not vice versa. `bash` has many more features of its own, and, as the table below demonstrates, many features incorporated from other shells.

Since the Turbo C shell is a superset of `csh`, all `csh` commands will work in `tcsh`, but not the other way round.

### Differing Shell Features

`sh` | `bash` | `ksh` | `csh` | Meaning/Action
--- | --- | --- | --- | ---
$ | $ | $ | % | Default user prompt
| `>|` | `>|` | `>!` | Force redirection
`> `file` 2>&1` | `&> `file`` or `> `file` 2>&1` | `> `file` 2>&1` | `>& `file`` | Redirect stdout and stderr to `file`
| `{ }` |  | `{ }` | Expand elements in list
``command`` | ``command`` or `$(command)` | `$(command)` | ``command`` | Substitute output of enclosed `command`
`$HOME` | `$HOME` | `$HOME` | `$home` | Home directory
| ~ | ~ | ~ | Home directory symbol
| `~+`, `~-`, `dirs` | `~+`, `~-` | `=-`, `=N` | Access directory stack
``var`=value` | ``VAR`=value` | ``var`=value` | `set `var`=value` | Variable assignment
`export `var`` | `export `VAR`=value` | `export `var`=val` | `setenv `var` `val`` | Set environment variable
| `${nnnn}` | `${nn}` |  | More than 9 arguments can be referenced
"`$@`" | "`$@`" | "`$@`" |  | All arguments as separate words
`$#` | `$#` | `$#` | `$#argv` | Number of arguments
`$?` | `$?` | `$?` | `$status` | Exit status of the most recently executed command
`$!` | `$!` | `$!` |  | PID of most recently backgrounded process
`$-` | `$-` | `$-` |  | Current options
`. `file`` | `source `file`` or `. `file`` | `. `file`` | `source `file`` | Read commands in file
| `alias x='y'` | `alias x=y` | `alias x y` | Name `x` stands for command `y`
`case` | `case` | `case` | `switch` or `case` | Choose alternatives
`done` | `done` | `done` | `end` | End a loop statement
`esac` | `esac` | `esac` | `endsw` | End `case` or `switch`
`exit `n`` | `exit `n`` | `exit `n`` | `exit `(expr)`` | Exit with a status
`for`/`do` | `for`/`do` | `for`/`do` | `foreach` | Loop through variables
| `set `-f`` , `set `-o nullglob|dotglob|nocaseglob|noglob`` |  | `noglob` | Ignore substitution characters for filename generation
`hash` | `hash` | `alias `-t`` | `hashstat` | Display hashed commands (tracked aliases)
`hash `cmds`` | `hash `cmds`` | `alias `-t` `cmds`` | `rehash` | Remember command locations
`hash `-r`` | `hash `-r`` |  | `unhash` | Forget command locations
| `history` | `history` | `history` | List previous commands
| ArrowUp+Enter or `!!` | `r` | `!!` | Redo previous command
| `!`str`` | `r `str`` | `!`str`` | Redo last command that starts with str
| `!`cmd`:s/`x`/`y`/` | `r `x`=`y` `cmd`` | `!`cmd`:s/`x`/`y`/` | Replace x with y in most recent command starting with cmd, then execute.
`if [ `$i` -eq `5` ]` | `if [ `$i` -eq `5` ]` | `if ((`i`==`5`))` | `if (`$i`==`5`)` | Sample condition test
`fi` | `fi` | `fi` | `endif` | End `if` statement
`ulimit` | `ulimit` | `ulimit` | `limit` | Set resource limits
`pwd` | `pwd` | `pwd` | `dirs` | Print working directory
`read` | `read` | `read` | `$<` | Read from terminal
`trap `2`` | `trap `2`` | `trap `2`` | `onintr` | Ignore interrupts
| `unalias` | `unalias` | `unalias` | Remove aliases
`until` | `until` | `until` |  | Begin `until` loop
`while`/`do` | `while`/`do` | `while`/`do` | `while` | Begin `while` loop

The Bourne Again SHell has many more features not listed here. This table is just to give you an idea of how this shell incorporates all useful ideas from other shells: there are no blanks in the column for `bash`. More information on features found only in Bash can be retrieved from the Bash info pages, in the Bash Features section.

More information:

You should at least read one manual, being the manual of your shell. The preferred choice would be `info bash`, `bash` being the GNU shell and easiest for beginners. Print it out and take it home, study it whenever you have 5 minutes.

See if you are having difficulties to assimilate shell commands.