wiki.bash-hackers.org/bash4.md

# Bash 4 - a rough overview

| :shell: Attention: Since Bash 4 has been around for quite some time now (4.3 will come soon), I consider it to be "standard". This page is not maintained anymore and is left here to keep your links working. See the [bashchanges](/scripting/bashchanges.md) page for new stuff introduced. |
| --- |

Besides many bugfixes since Bash 3.2, Bash 4 will bring some interesting
new features for shell users and scripters. See also
[bashchanges](/scripting/bashchanges.md) for a small general overview with
more details.

Not all of the changes and news are included here, just the biggest or
most interesting ones. The changes to completion, and the readline
component are not covered. **Though, if you're familiar with these parts
of Bash (and Bash 4), feel free to write a chapter here.**

The complete list of fixes and changes is in the CHANGES or NEWS file of
your Bash 4 distribution.

| :floppy_disk: The current available **stable** version is 4.4.18 release (February 03, 2018): |
| --- |
| - <ftp://ftp.cwru.edu/pub/bash/bash-4.4.18.tar.gz> |
| - <ftp://ftp.gnu.org/pub/gnu/bash/bash-4.4.18.tar.gz> |

## New or changed commands and keywords

### The new "coproc" keyword

Bash 4 introduces the concepts of coprocesses, a well known feature of
other shells. The basic concept is simple: It will start any command in
the background and set up an array that is populated with accessible
files that represent the filedescriptors of the started process.

In other words: It lets you start a process in background and
communicate with its input and output data streams.

See [The coproc keyword](/syntax/keywords/coproc.md)

### The new "mapfile" builtin

The `mapfile` builtin is able to map the lines of a file directly into
an array. This avoids having to fill an array yourself using a loop. It
enables you to define the range of lines to read, and optionally call a
callback, for example to display a progress bar.

See: [mapfile](/commands/builtin/mapfile.md)

### Changes to the "case" keyword

The `case` construct understands two new action list terminators:

The `;&` terminator causes execution to continue with the next action
list (rather than terminate the `case` construct).

The `;;&` terminator causes the `case` construct to test the next given
pattern instead of terminating the whole execution.

See [case](/syntax/ccmd/case.md)

### Changes to the "declare" builtin

The `-p` option now prints all attributes and values of declared
variables (or functions, when used with `-f`). The output is fully
re-usable as input.

The new option `-l` declares a variable in a way that the content is
converted to lowercase on assignment. For uppercase, the same applies to
`-u`. The option `-c` causes the content to be capitalized before
assignment.

`declare -A` declares associative arrays (see below).

### Changes to the "read" builtin

The `read` builtin command has some interesting new features.

The `-t` option to specify a timeout value has been slightly tuned. It
now accepts fractional values and the special value 0 (zero). When
`-t 0` is specified, `read` immediately returns with an exit status
indicating if there's data waiting or not. However, when a timeout is
given, and the `read` builtin times out, any partial data recieved up to
the timeout is stored in the given variable, rather than lost. When a
timeout is hit, `read` exits with a code greater than 128.

A new option, `-i`, was introduced to be able to preload the input
buffer with some text (when Readline is used, with `-e`). The user is
able to change the text, or press return to accept it.

See [read](/commands/builtin/read.md)

### Changes to the "help" builtin

The builtin itself didn't change much, but the data displayed is more
structured now. The help texts are in a better format, much easier to
read.

There are two new options: `-d` displays the summary of a help text,
`-m` displays a manpage-like format.

### Changes to the "ulimit" builtin

Besides the use of the 512 bytes blocksize everywhere in POSIX mode,
`ulimit` supports two new limits: `-b` for max socket buffer size and
`-T` for max number of threads.

## Expansions

### Brace Expansion

The brace expansion was tuned to provide expansion results with leading
zeros when requesting a row of numbers.

See [brace](/syntax/expansion/brace.md)

### Parameter Expansion

Methods to modify the case on expansion time have been added.

On expansion time you can modify the syntax by adding operators to the
parameter name.

See [Case modification on parameter
expansion](/syntax/pe.md#case_modification)

### Substring expansion

When using substring expansion on the positional parameters, a starting
index of 0 now causes \$0 to be prepended to the list (if the positional
parameters are used). Before, this expansion started with \$1:

    # this should display $0 on Bash v4, $1 on Bash v3
    echo ${@:0:1}

### Globbing

There's a new shell option `globstar`. When enabled, Bash will perform
recursive globbing on `**` -- this means it matches all directories and
files from the current position in the filesystem, rather than only the
current level.

The new shell option `dirspell` enables spelling corrections on
directory names during globbing.

See [globs](/syntax/expansion/globs.md)

## Associative Arrays

Besides the classic method of integer indexed arrays, Bash 4 supports
associative arrays.

An associative array is an array indexed by an arbitrary string,
something like

    declare -A ASSOC

    ASSOC[First]="first element"
    ASSOC[Hello]="second element"
    ASSOC[Peter Pan]="A weird guy"

See [arrays](/syntax/arrays.md)

## Redirection

There is a new `&>>` redirection operator, which appends the standard
output and standard error to the named file. This is the same as the
good old `>>FILE 2>&1` notation.

The parser now understands `|&` as a synonym for `2>&1 |`, which
redirects the standard error for a command through a pipe.

See [redirection](/syntax/redirection.md)

## Interesting new shell variables

| Variable                                           | Description                                                                    |
|----------------------------------------------------|--------------------------------------------------------------------------------|
| [BASHPID](/syntax/shellvars.md#BASHPID)               | contains the PID of the current shell (this is different than what `$$` does!) |
| [PROMPT_DIRTRIM](/syntax/shellvars.md#PROMPT_DIRTRIM) | specifies the max. level of unshortened pathname elements in the prompt        |
| [FUNCNEST](/syntax/shellvars.md#FUNCNEST)             | control the maximum number of shell function recursions                        |

See [shellvars](/syntax/shellvars.md)

## Interesting new Shell Options

The mentioned shell options are **off by default** unless otherwise
mentioned.

| Option      | Description                                                                                            |
|-------------|--------------------------------------------------------------------------------------------------------|
| `checkjobs` | check for and report any running jobs at shell exit                                                    |
| `compat*`   | set compatiblity modes for older shell versions (influences regular expression matching in `[[ ... ]]` |
| `dirspell`  | enables spelling corrections on directory names during globbing                                        |
| `globstar`  | enables recursive globbing with `**`                                                                   |
| `lastpipe`  | (4.2) to execute the last command in a pipeline in the current environment                             |

See [shell_options](/internals/shell_options.md)

## Misc

- If a command is not found, the shell attempts to execute a shell
  function named `command_not_found_handle`, supplying the command words
  as the function arguments. This can be used to display userfriendly
  messages or perform different command searches
- The behaviour of the `set -e` (`errexit`) mode was changed, it now
  acts more intuitive (and is better documented in the manpage).
- The output target for the `xtrace` (`set -x`/`set +x`) feature is
  configurable **since Bash 4.1** (previously, it was fixed to
  `stderr`): a variable named
  [BASH_XTRACEFD](/syntax/shellvars.md#BASH_XTRACEFD) can be set to the
  filedescriptor that should get the output
- Bash 4.1 is able to log the history to syslog (only to be enabled at
  compile time in `config-top.h`)