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Bash - The GNU shell*
Chet Ramey
Case Western Reserve University
chet@po.cwru.edu
_1. _I_n_t_r_o_d_u_c_t_i_o_n
_B_a_s_h is the shell, or command language interpreter,
that will appear in the GNU operating system. The name is
an acronym for the "Bourne-Again SHell", a pun on Steve
Bourne, the author of the direct ancestor of the current
UNIX|- shell /_b_i_n/_s_h, which appeared in the Seventh Edition
Bell Labs Research version of UNIX.
Bash is an sh-compatible shell that incorporates useful
features from the Korn shell (ksh) and the C shell (csh),
described later in this article. It is ultimately intended
to be a conformant implementation of the IEEE POSIX Shell
and Utilities specification (IEEE Working Group 1003.2). It
offers functional improvements over sh for both interactive
and programming use.
While the GNU operating system will most likely include
a version of the Berkeley shell csh, Bash will be the
default shell. Like other GNU software, Bash is quite port-
able. It currently runs on nearly every version of UNIX and
a few other operating systems - an independently-supported
port exists for OS/2, and there are rumors of ports to DOS
and Windows NT. Ports to UNIX-like systems such as QNX and
Minix are part of the distribution.
The original author of Bash was Brian Fox, an employee
of the Free Software Foundation. The current developer and
maintainer is Chet Ramey, a volunteer who works at Case
Western Reserve University.
_2. _W_h_a_t'_s _P_O_S_I_X, _a_n_y_w_a_y?
_P_O_S_I_X is a name originally coined by Richard Stallman
_________________________
*An earlier version of this article appeared in The
Linux Journal.
|- UNIX is a trademark of Bell Laboratories.
October 28, 1994
- 2 -
for a family of open system standards based on UNIX. There
are a number of aspects of UNIX under consideration for
standardization, from the basic system services at the sys-
tem call and C library level to applications and tools to
system administration and management. Each area of stan-
dardization is assigned to a working group in the 1003
series.
The POSIX Shell and Utilities standard has been
developed by IEEE Working Group 1003.2 (POSIX.2).|= It con-
centrates on the command interpreter interface and utility
programs commonly executed from the command line or by other
programs. An initial version of the standard has been
approved and published by the IEEE, and work is currently
underway to update it. There are four primary areas of work
in the 1003.2 standard:
o+ Aspects of the shell's syntax and command language. A
number of special builtins such as _c_d and _e_x_e_c are
being specified as part of the shell, since their func-
tionality usually cannot be implemented by a separate
executable;
o+ A set of utilities to be called by shell scripts and
applications. Examples are programs like _s_e_d, _t_r, and
_a_w_k. Utilities commonly implemented as shell builtins
are described in this section, such as _t_e_s_t and _k_i_l_l.
An expansion of this section's scope, termed the User
Portability Extension, or UPE, has standardized
interactive programs such as _v_i and _m_a_i_l_x;
o+ A group of functional interfaces to services provided
by the shell, such as the traditional system() C
library function. There are functions to perform shell
word expansions, perform filename expansion (_g_l_o_b_b_i_n_g),
obtain values of POSIX.2 system configuration vari-
ables, retrieve values of environment variables
(getenv()), _a_n_d _o_t_h_e_r _s_e_r_v_i_c_e_s;
o+ A suite of "development" utilities such as _c_8_9 (the
POSIX.2 version of _c_c), and _y_a_c_c.
Bash is concerned with the aspects of the shell's
behavior defined by POSIX.2. The shell command language has
of course been standardized, including the basic flow con-
trol and program execution constructs, I/O redirection and
pipelining, argument handling, variable expansion, and quot-
ing. The _s_p_e_c_i_a_l builtins, which must be implemented as
part of the shell to provide the desired functionality, are
_________________________
|=IEEE, _I_E_E_E _S_t_a_n_d_a_r_d _f_o_r _I_n_f_o_r_m_a_t_i_o_n _T_e_c_h_n_o_l_o_g_y --
_P_o_r_t_a_b_l_e _O_p_e_r_a_t_i_n_g _S_y_s_t_e_m _I_n_t_e_r_f_a_c_e (_P_O_S_I_X) _P_a_r_t _2:
_S_h_e_l_l _a_n_d _U_t_i_l_i_t_i_e_s, 1992.
October 28, 1994
- 3 -
specified as being part of the shell; examples of these are
_e_v_a_l and _e_x_p_o_r_t. Other utilities appear in the sections of
POSIX.2 not devoted to the shell which are commonly (and in
some cases must be) implemented as builtin commands, such as
_r_e_a_d and _t_e_s_t. POSIX.2 also specifies aspects of the
shell's interactive behavior as part of the UPE, including
job control and command line editing. Interestingly enough,
only _v_i-style line editing commands have been standardized;
_e_m_a_c_s editing commands were left out due to objections.
While POSIX.2 includes much of what the shell has trad-
itionally provided, some important things have been omitted
as being "beyond its scope." There is, for instance, no
mention of a difference between a _l_o_g_i_n shell and any other
interactive shell (since POSIX.2 does not specify a login
program). No fixed startup files are defined, either - the
standard does not mention ._p_r_o_f_i_l_e.
_3. _B_a_s_i_c _B_a_s_h _f_e_a_t_u_r_e_s
Since the Bourne shell provides Bash with most of its
philosophical underpinnings, Bash inherits most of its
features and functionality from sh. Bash implements all of
the traditional sh flow control constructs (_f_o_r, _i_f, _w_h_i_l_e,
etc.). All of the Bourne shell builtins, including those
not specified in the POSIX.2 standard, appear in Bash.
Shell _f_u_n_c_t_i_o_n_s, introduced in the SVR2 version of the
Bourne shell, are similar to shell scripts, but are defined
using a special syntax and are executed in the same process
as the calling shell. Bash has shell functions which behave
in a fashion upward-compatible with sh functions. There are
certain shell variables that Bash interprets in the same way
as sh, such as _P_S_1, _I_F_S, and _P_A_T_H. Bash implements essen-
tially the same grammar, parameter and variable expansion
semantics, redirection, and quoting as the Bourne shell.
Where differences appear between the POSIX.2 standard and
traditional sh behavior, Bash follows POSIX.
The Korn Shell (ksh) is a descendent of the Bourne
shell written at AT&T Bell Laboratories by David Korn|-. It
provides a number of useful features that POSIX and Bash
have adopted. Many of the interactive facilities in POSIX.2
have their roots in the ksh: for example, the POSIX and ksh
job control facilities are nearly identical. Bash includes
features from the Korn Shell for both interactive use and
shell programming. For programming, Bash provides variables
such as _R_A_N_D_O_M and _R_E_P_L_Y, the _t_y_p_e_s_e_t builtin, the ability
to remove substrings from variables based on patterns, and
shell arithmetic. _R_A_N_D_O_M expands to a random number each
time it is referenced; assigning a value to _R_A_N_D_O_M seeds the
_________________________
|-Morris Bolsky and David Korn, _T_h_e _K_o_r_n_S_h_e_l_l _C_o_m_m_a_n_d
_a_n_d _P_r_o_g_r_a_m_m_i_n_g _L_a_n_g_u_a_g_e, Prentice Hall, 1989.
October 28, 1994
- 4 -
random number generator. _R_E_P_L_Y is the default variable used
by the _r_e_a_d builtin when no variable names are supplied as
arguments. The _t_y_p_e_s_e_t builtin is used to define variables
and give them attributes such as readonly. Bash arithmetic
allows the evaluation of an expression and the substitution
of the result. Shell variables may be used as operands, and
the result of an expression may be assigned to a variable.
Nearly all of the operators from the C language are avail-
able, with the same precedence rules:
9 $ echo $((3 + 5 * 32))
163
9
For interactive use, Bash implements ksh-style aliases and
builtins such as _f_c (discussed below) and _j_o_b_s. Bash
aliases allow a string to be substituted for a command name.
They can be used to create a mnemonic for a UNIX command
name (alias del=rm), to expand a single word to a complex
command (alias news='xterm -g 80x45 -title trn -e trn -e -S1
-N &'), or to ensure that a command is invoked with a basic
set of options (alias ls="/bin/ls -F").
The C shell (csh)|-, originally written by Bill Joy
while at Berkeley, is widely used and quite popular for its
interactive facilities. Bash includes a csh-compatible his-
tory expansion mechanism ("! history"), brace expansion,
access to a stack of directories via the _p_u_s_h_d, _p_o_p_d, and
_d_i_r_s builtins, and tilde expansion, to generate users' home
directories. Tilde expansion has also been adopted by both
the Korn Shell and POSIX.2.
There were certain areas in which POSIX.2 felt stan-
dardization was necessary, but no existing implementation
provided the proper behavior. The working group invented
and standardized functionality in these areas, which Bash
implements. The _c_o_m_m_a_n_d builtin was invented so that shell
functions could be written to replace builtins; it makes the
capabilities of the builtin available to the function. The
reserved word "!" was added to negate the return value of a
command or pipeline; it was nearly impossible to express "if
not x" cleanly using the sh language. There exist multiple
incompatible implementations of the _t_e_s_t builtin, which
tests files for type and other attributes and performs
arithmetic and string comparisons. POSIX considered none of
these correct, so the standard behavior was specified in
terms of the number of arguments to the command. POSIX.2
dictates exactly what will happen when four or fewer argu-
ments are given to _t_e_s_t, and leaves the behavior undefined
when more arguments are supplied. Bash uses the POSIX.2
_________________________
|-Bill Joy, An Introduction to the C Shell, _U_N_I_X _U_s_e_r'_s
_S_u_p_p_l_e_m_e_n_t_a_r_y _D_o_c_u_m_e_n_t_s, University of California at
Berkeley, 1986.
October 28, 1994
- 5 -
algorithm, which was conceived by David Korn.
_3._1. _F_e_a_t_u_r_e_s _n_o_t _i_n _t_h_e _B_o_u_r_n_e _S_h_e_l_l
There are a number of minor differences between Bash
and the version of sh present on most other versions of
UNIX. The majority of these are due to the POSIX standard,
but some are the result of Bash adopting features from other
shells. For instance, Bash includes the new "!" reserved
word, the _c_o_m_m_a_n_d builtin, the ability of the _r_e_a_d builtin
to correctly return a line ending with a backslash, symbolic
arguments to the _u_m_a_s_k builtin, variable substring removal,
a way to get the length of a variable, and the new algorithm
for the _t_e_s_t builtin from the POSIX.2 standard, none of
which appear in sh.
Bash also implements the "$(...)" command substitution
syntax, which supersedes the sh `...` construct. The
"$(...)" construct expands to the output of the command con-
tained within the parentheses, with trailing newlines
removed. The sh syntax is accepted for backwards compati-
bility, but the "$(...)" form is preferred because its quot-
ing rules are much simpler and it is easier to nest.
The Bourne shell does not provide such features as
brace expansion, the ability to define a variable and a
function with the same name, local variables in shell func-
tions, the ability to enable and disable individual builtins
or write a function to replace a builtin, or a means to
export a shell function to a child process.
Bash has closed a long-standing shell security hole by
not using the $_I_F_S variable to split each word read by the
shell, but splitting only the results of expansion (ksh and
the 4.4 BSD sh have fixed this as well). Useful behavior
such as a means to abort execution of a script read with the
"." command using the return builtin or automatically
exporting variables in the shell's environment to children
is also not present in the Bourne shell. Bash provides a
much more powerful environment for both interactive use and
programming.
_4. _B_a_s_h-_s_p_e_c_i_f_i_c _F_e_a_t_u_r_e_s
This section details a few of the features which make
Bash unique. Most of them provide improved interactive use,
but a few programming improvements are present as well.
Full descriptions of these features can be found in the Bash
documentation.
_4._1. _S_t_a_r_t_u_p _F_i_l_e_s
Bash executes startup files differently than other
shells. The Bash behavior is a compromise between the csh
October 28, 1994
- 6 -
principle of startup files with fixed names executed for
each shell and the sh "minimalist" behavior. An interactive
instance of Bash started as a login shell reads and executes
~/._b_a_s_h__p_r_o_f_i_l_e (the file .bash_profile in the user's home
directory), if it exists. An interactive non-login shell
reads and executes ~/._b_a_s_h_r_c. A non-interactive shell (one
begun to execute a shell script, for example) reads no fixed
startup file, but uses the value of the variable $_E_N_V, if
set, as the name of a startup file. The ksh practice of
reading $_E_N_V for every shell, with the accompanying diffi-
culty of defining the proper variables and functions for
interactive and non-interactive shells or having the file
read only for interactive shells, was considered too com-
plex. Ease of use won out here. Interestingly, the next
release of ksh will change to reading $_E_N_V only for interac-
tive shells.
_4._2. _N_e_w _B_u_i_l_t_i_n _C_o_m_m_a_n_d_s
There are a few builtins which are new or have been
extended in Bash. The _e_n_a_b_l_e builtin allows builtin com-
mands to be turned on and off arbitrarily. To use the ver-
sion of _e_c_h_o found in a user's search path rather than the
Bash builtin, enable -n echo suffices. The _h_e_l_p builtin
provides quick synopses of the shell facilities without
requiring access to a manual page. _B_u_i_l_t_i_n is similar to
_c_o_m_m_a_n_d in that it bypasses shell functions and directly
executes builtin commands. Access to a csh-style stack of
directories is provided via the _p_u_s_h_d, _p_o_p_d, and _d_i_r_s buil-
tins. _P_u_s_h_d and _p_o_p_d insert and remove directories from the
stack, respectively, and _d_i_r_s lists the stack contents. On
systems that allow fine-grained control of resources, the
_u_l_i_m_i_t builtin can be used to tune these settings. _U_l_i_m_i_t
allows a user to control, among other things, whether core
dumps are to be generated, how much memory the shell or a
child process is allowed to allocate, and how large a file
created by a child process can grow. The _s_u_s_p_e_n_d command
will stop the shell process when job control is active; most
other shells do not allow themselves to be stopped like
that. _T_y_p_e, the Bash answer to _w_h_i_c_h and _w_h_e_n_c_e, shows what
will happen when a word is typed as a command:
9 $ type export
export is a shell builtin
$ type -t export
builtin
$ type bash
bash is /bin/bash
$ type cd
cd is a function
cd ()
{
builtin cd ${1+"$@"} && xtitle $HOST: $PWD
}
9
October 28, 1994
- 7 -
Various modes tell what a command word is (reserved word,
alias, function, builtin, or file) or which version of a
command will be executed based on a user's search path.
Some of this functionality has been adopted by POSIX.2 and
folded into the _c_o_m_m_a_n_d utility.
_4._3. _E_d_i_t_i_n_g _a_n_d _C_o_m_p_l_e_t_i_o_n
One area in which Bash shines is command line editing.
Bash uses the _r_e_a_d_l_i_n_e library to read and edit lines when
interactive. Readline is a powerful and flexible input
facility that a user can configure to individual tastes. It
allows lines to be edited using either emacs or vi commands,
where those commands are appropriate. The full capability
of emacs is not present - there is no way to execute a named
command with M-x, for instance - but the existing commands
are more than adequate. The vi mode is compliant with the
command line editing standardized by POSIX.2.
Readline is fully customizable. In addition to the
basic commands and key bindings, the library allows users to
define additional key bindings using a startup file. The
_i_n_p_u_t_r_c file, which defaults to the file ~/._i_n_p_u_t_r_c, is read
each time readline initializes, permitting users to maintain
a consistent interface across a set of programs. Readline
includes an extensible interface, so each program using the
library can add its own bindable commands and program-
specific key bindings. Bash uses this facility to add bind-
ings that perform history expansion or shell word expansions
on the current input line.
Readline interprets a number of variables which further
tune its behavior. Variables exist to control whether or
not eight-bit characters are directly read as input or con-
verted to meta-prefixed key sequences (a meta-prefixed key
sequence consists of the character with the eighth bit
zeroed, preceded by the _m_e_t_a-_p_r_e_f_i_x character, usually
escape, which selects an alternate keymap), to decide
whether to output characters with the eighth bit set
directly or as a meta-prefixed key sequence, whether or not
to wrap to a new screen line when a line being edited is
longer than the screen width, the keymap to which subsequent
key bindings should apply, or even what happens when read-
line wants to ring the terminal's bell. All of these vari-
ables can be set in the inputrc file.
The startup file understands a set of C preprocessor-
like conditional constructs which allow variables or key
bindings to be assigned based on the application using read-
line, the terminal currently being used, or the editing
mode. Users can add program-specific bindings to make their
lives easier: I have bindings that let me edit the value of
$_P_A_T_H and double-quote the current or previous word:
9 # Macros that are convenient for shell interaction
9 October 28, 1994
- 8 -
$if Bash
# edit the path
"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
# prepare to type a quoted word -- insert open and close double
# quotes and move to just after the open quote
"\C-x\"": "\"\"\C-b"
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
9
There is a readline command to re-read the file, so users
can edit the file, change some bindings, and begin to use
them almost immediately.
Bash implements the _b_i_n_d builtin for more dyamic con-
trol of readline than the startup file permits. _B_i_n_d is
used in several ways. In _l_i_s_t mode, it can display the
current key bindings, list all the readline editing direc-
tives available for binding, list which keys invoke a given
directive, or output the current set of key bindings in a
format that can be incorporated directly into an inputrc
file. In _b_a_t_c_h mode, it reads a series of key bindings
directly from a file and passes them to readline. In its
most common usage, _b_i_n_d takes a single string and passes it
directly to readline, which interprets the line as if it had
just been read from the inputrc file. Both key bindings and
variable assignments may appear in the string given to _b_i_n_d.
The readline library also provides an interface for
_w_o_r_d _c_o_m_p_l_e_t_i_o_n. When the _c_o_m_p_l_e_t_i_o_n character (usually
TAB) is typed, readline looks at the word currently being
entered and computes the set of filenames of which the
current word is a valid prefix. If there is only one possi-
ble completion, the rest of the characters are inserted
directly, otherwise the common prefix of the set of
filenames is added to the current word. A second TAB char-
acter entered immediately after a non-unique completion
causes readline to list the possible completions; there is
an option to have the list displayed immediately. Readline
provides hooks so that applications can provide specific
types of completion before the default filename completion
is attempted. This is quite flexible, though it is not com-
pletely user-programmable. Bash, for example, can complete
filenames, command names (including aliases, builtins, shell
reserved words, shell functions, and executables found in
the file system), shell variables, usernames, and hostnames.
It uses a set of heuristics that, while not perfect, is gen-
erally quite good at determining what type of completion to
attempt.
_4._4. _H_i_s_t_o_r_y
Access to the list of commands previously entered (the
_c_o_m_m_a_n_d _h_i_s_t_o_r_y) is provided jointly by Bash and the
9 October 28, 1994
- 9 -
readline library. Bash provides variables ($HISTFILE,
$HISTSIZE, and $HISTCONTROL) and the _h_i_s_t_o_r_y and _f_c builtins
to manipulate the history list. The value of $_H_I_S_T_F_I_L_E
specifes the file where Bash writes the command history on
exit and reads it on startup. $_H_I_S_T_S_I_Z_E is used to limit
the number of commands saved in the history. $_H_I_S_T_C_O_N_T_R_O_L
provides a crude form of control over which commands are
saved on the history list: a value of _i_g_n_o_r_e_s_p_a_c_e means to
not save commands which begin with a space; a value of
_i_g_n_o_r_e_d_u_p_s means to not save commands identical to the last
command saved. $HISTCONTROL was named $history_control in
earlier versions of Bash; the old name is still accepted for
backwards compatibility. The _h_i_s_t_o_r_y command can read or
write files containing the history list and display the
current list contents. The _f_c builtin, adopted from POSIX.2
and the Korn Shell, allows display and re-execution, with
optional editing, of commands from the history list. The
readline library offers a set of commands to search the his-
tory list for a portion of the current input line or a
string typed by the user. Finally, the _h_i_s_t_o_r_y library,
generally incorporated directly into the readline library,
implements a facility for history recall, expansion, and
re-execution of previous commands very similar to csh ("bang
history", so called because the exclamation point introduces
a history substitution):
9 $ echo a b c d e
a b c d e
$ !! f g h i
echo a b c d e f g h i
a b c d e f g h i
$ !-2
echo a b c d e
a b c d e
$ echo !-2:1-4
echo a b c d
a b c d
9
The command history is only saved when the shell is interac-
tive, so it is not available for use by shell scripts.
_4._5. _N_e_w _S_h_e_l_l _V_a_r_i_a_b_l_e_s
There are a number of convenience variables that Bash
interprets to make life easier. These include _F_I_G_N_O_R_E,
which is a set of filename suffixes identifying files to
exclude when completing filenames; _H_O_S_T_T_Y_P_E, which is
automatically set to a string describing the type of
hardware on which Bash is currently executing;
_c_o_m_m_a_n_d__o_r_i_e_n_t_e_d__h_i_s_t_o_r_y, which directs Bash to save all
lines of a multiple-line command such as a _w_h_i_l_e or _f_o_r loop
in a single history entry, allowing easy re-editing; and
_I_G_N_O_R_E_E_O_F, whose value indicates the number of consecutive
EOF characters that an interactive shell will read before
October 28, 1994
- 10 -
exiting - an easy way to keep yourself from being logged out
accidentally. The _a_u_t_o__r_e_s_u_m_e variable alters the way the
shell treats simple command names: if job control is active,
and this variable is set, single-word simple commands
without redirections cause the shell to first look for and
restart a suspended job with that name before starting a new
process.
_4._6. _B_r_a_c_e _E_x_p_a_n_s_i_o_n
Since sh offers no convenient way to generate arbitrary
strings that share a common prefix or suffix (filename
expansion requires that the filenames exist), Bash imple-
ments _b_r_a_c_e _e_x_p_a_n_s_i_o_n, a capability picked up from csh.
Brace expansion is similar to filename expansion, but the
strings generated need not correspond to existing files. A
brace expression consists of an optional _p_r_e_a_m_b_l_e, followed
by a pair of braces enclosing a series of comma-separated
strings, and an optional _p_o_s_t_a_m_b_l_e. The preamble is
prepended to each string within the braces, and the postam-
ble is then appended to each resulting string:
9 $ echo a{d,c,b}e
ade ace abe
9
As this example demonstrates, the results of brace expansion
are not sorted, as they are by filename expansion.
_4._7. _P_r_o_c_e_s_s _S_u_b_s_t_i_t_u_t_i_o_n
On systems that can support it, Bash provides a facil-
ity known as _p_r_o_c_e_s_s _s_u_b_s_t_i_t_u_t_i_o_n. Process substitution is
similar to command substitution in that its specification
includes a command to execute, but the shell does not col-
lect the command's output and insert it into the command
line. Rather, Bash opens a pipe to the command, which is
run in the background. The shell uses named pipes (FIFOs)
or the /_d_e_v/_f_d method of naming open files to expand the
process substitution to a filename which connects to the
pipe when opened. This filename becomes the result of the
expansion. Process substitution can be used to compare the
outputs of two different versions of an application as part
of a regression test:
9 $ cmp <(old_prog) <(new_prog)
9
_4._8. _P_r_o_m_p_t _C_u_s_t_o_m_i_z_a_t_i_o_n
One of the more popular interactive features that Bash
provides is the ability to customize the prompt. Both $_P_S_1
and $_P_S_2, the primary and secondary prompts, are expanded
before being displayed. Parameter and variable expansion is
performed when the prompt string is expanded, so any shell
variable can be put into the prompt (e.g., $_S_H_L_V_L, which
October 28, 1994
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indicates how deeply the current shell is nested). Bash
specially interprets characters in the prompt string pre-
ceded by a backslash. Some of these backslash escapes are
replaced with the current time, the date, the current work-
ing directory, the username, and the command number or his-
tory number of the command being entered. There is even a
backslash escape to cause the shell to change its prompt
when running as root after an _s_u. Before printing each pri-
mary prompt, Bash expands the variable $_P_R_O_M_P_T__C_O_M_M_A_N_D and,
if it has a value, executes the expanded value as a command,
allowing additional prompt customization. For example, this
assignment causes the current user, the current host, the
time, the last component of the current working directory,
the level of shell nesting, and the history number of the
current command to be embedded into the primary prompt:
9 $ PS1='\u@\h [\t] \W($SHLVL:\!)\$ '
chet@odin [21:03:44] documentation(2:636)$ cd ..
chet@odin [21:03:54] src(2:637)$
9
The string being assigned is surrounded by single quotes so
that if it is exported, the value of $_S_H_L_V_L will be updated
by a child shell:
9 chet@odin [21:17:35] src(2:638)$ export PS1
chet@odin [21:17:40] src(2:639)$ bash
chet@odin [21:17:46] src(3:696)$
9
The \$ escape is displayed as "$" when running as a normal
user, but as "#" when running as root.
_4._9. _F_i_l_e _S_y_s_t_e_m _V_i_e_w_s
Since Berkeley introduced symbolic links in 4.2 BSD,
one of their most annoying properties has been the "warping"
to a completely different area of the file system when using
_c_d, and the resultant non-intuitive behavior of "cd ..".
The UNIX kernel treats symbolic links _p_h_y_s_i_c_a_l_l_y. When the
kernel is translating a pathname in which one component is a
symbolic link, it replaces all or part of the pathname while
processing the link. If the contents of the symbolic link
begin with a slash, the kernel replaces the pathname
entirely; if not, the link contents replace the current com-
ponent. In either case, the symbolic link is visible. If
the link value is an absolute pathname, the user finds him-
self in a completely different part of the file system.
Bash provides a _l_o_g_i_c_a_l view of the file system. In
this default mode, command and filename completion and buil-
tin commands such as _c_d and _p_u_s_h_d which change the current
working directory transparently follow symbolic links as if
they were directories. The $_P_W_D variable, which holds the
shell's idea of the current working directory, depends on
the path used to reach the directory rather than its
October 28, 1994
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physical location in the local file system hierarchy. For
example:
9 $ cd /usr/local/bin
$ echo $PWD
/usr/local/bin
$ pwd
/usr/local/bin
$ /bin/pwd
/net/share/sun4/local/bin
$ cd ..
$ pwd
/usr/local
$ /bin/pwd
/net/share/sun4/local
$ cd ..
$ pwd
/usr
$ /bin/pwd
/usr
9
One problem with this, of course, arises when programs that
do not understand the shell's logical notion of the file
system interpret ".." differently. This generally happens
when Bash completes filenames containing ".." according to a
logical hierarchy which does not correspond to their physi-
cal location. For users who find this troublesome, a
corresponding _p_h_y_s_i_c_a_l view of the file system is available:
9 $ cd /usr/local/bin
$ pwd
/usr/local/bin
$ set -o physical
$ pwd
/net/share/sun4/local/bin
9
_4._1_0. _I_n_t_e_r_n_a_t_i_o_n_a_l_i_z_a_t_i_o_n
One of the most significant improvements in version
1.13 of Bash was the change to "eight-bit cleanliness".
Previous versions used the eighth bit of characters to mark
whether or not they were quoted when performing word expan-
sions. While this did not affect the majority of users,
most of whom used only seven-bit ASCII characters, some
found it confining. Beginning with version 1.13, Bash
implemented a different quoting mechanism that did not alter
the eighth bit of characters. This allowed Bash to manipu-
late files with "odd" characters in their names, but did
nothing to help users enter those names, so version 1.13
introduced changes to readline that made it eight-bit clean
as well. Options exist that force readline to attach no
special significance to characters with the eighth bit set
(the default behavior is to convert these characters to
meta-prefixed key sequences) and to output these characters
October 28, 1994
- 13 -
without conversion to meta-prefixed sequences. These
changes, along with the expansion of keymaps to a full eight
bits, enable readline to work with most of the ISO-8859 fam-
ily of character sets, used by many European countries.
_4._1_1. _P_O_S_I_X _M_o_d_e
Although Bash is intended to be POSIX.2 conformant,
there are areas in which the default behavior is not compa-
tible with the standard. For users who wish to operate in a
strict POSIX.2 environment, Bash implements a _P_O_S_I_X _m_o_d_e.
When this mode is active, Bash modifies its default opera-
tion where it differs from POSIX.2 to match the standard.
POSIX mode is entered when Bash is started with the -_p_o_s_i_x
option. This feature is also available as an option to the
set builtin, set -o posix. For compatibility with other GNU
software that attempts to be POSIX.2 compliant, Bash also
enters POSIX mode if the variable $_P_O_S_I_X_L_Y__C_O_R_R_E_C_T is set
when Bash is started or assigned a value during execution.
$_P_O_S_I_X__P_E_D_A_N_T_I_C is accepted as well, to be compatible with
some older GNU utilities. When Bash is started in POSIX
mode, for example, it sources the file named by the value of
$_E_N_V rather than the "normal" startup files, and does not
allow reserved words to be aliased.
_5. _N_e_w _F_e_a_t_u_r_e_s _a_n_d _F_u_t_u_r_e _P_l_a_n_s
There are several features introduced in the current
version of Bash, version 1.14, and a number under considera-
tion for future releases. This section will briefly detail
the new features in version 1.14 and describe several
features that may appear in later versions.
_5._1. _N_e_w _F_e_a_t_u_r_e_s _i_n _B_a_s_h-_1._1_4
The new features available in Bash-1.14 answer several
of the most common requests for enhancements. Most notably,
there is a mechanism for including non-visible character
sequences in prompts, such as those which cause a terminal
to print characters in different colors or in standout mode.
There was nothing preventing the use of these sequences in
earlier versions, but the readline redisplay algorithm
assumed each character occupied physical screen space and
would wrap lines prematurely.
Readline has a few new variables, several new bindable
commands, and some additional emacs mode default key bind-
ings. A new history search mode has been implemented: in
this mode, readline searches the history for lines beginning
with the characters between the beginning of the current
line and the cursor. The existing readline incremental
search commands no longer match identical lines more than
once. Filename completion now expands variables in direc-
tory names. The history expansion facilities are now nearly
October 28, 1994
- 14 -
completely csh-compatible: missing modifiers have been added
and history substitution has been extended.
Several of the features described earlier, such as _s_e_t
-_o _p_o_s_i_x and $_P_O_S_I_X__P_E_D_A_N_T_I_C, are new in version 1.14.
There is a new shell variable, _O_S_T_Y_P_E, to which Bash assigns
a value that identifies the version of UNIX it's running on
(great for putting architecture-specific binary directories
into the $PATH). Two variables have been renamed: $_H_I_S_T_C_O_N_-
_T_R_O_L replaces $_h_i_s_t_o_r_y__c_o_n_t_r_o_l, and $_H_O_S_T_F_I_L_E replaces
$_h_o_s_t_n_a_m_e__c_o_m_p_l_e_t_i_o_n__f_i_l_e. In both cases, the old names are
accepted for backwards compatibility. The ksh _s_e_l_e_c_t con-
struct, which allows the generation of simple menus, has
been implemented. New capabilities have been added to
existing variables: $_a_u_t_o__r_e_s_u_m_e can now take values of
_e_x_a_c_t or _s_u_b_s_t_r_i_n_g, and $_H_I_S_T_C_O_N_T_R_O_L understands the value
_i_g_n_o_r_e_b_o_t_h, which combines the two previously acceptable
values. The _d_i_r_s builtin has acquired options to print out
specific members of the directory stack. The $_n_o_l_i_n_k_s vari-
able, which forces a physical view of the file system, has
been superseded by the -_P option to the _s_e_t builtin
(equivalent to set -o physical); the variable is retained
for backwards compatibility. The version string contained
in $_B_A_S_H__V_E_R_S_I_O_N now includes an indication of the patch
level as well as the "build version". Some little-used
features have been removed: the _b_y_e synonym for _e_x_i_t and
the $_N_O__P_R_O_M_P_T__V_A_R_S variable are gone. There is now an
organized test suite that can be run as a regression test
when building a new version of Bash.
The documentation has been thoroughly overhauled: there
is a new manual page on the readline library and the _i_n_f_o
file has been updated to reflect the current version. As
always, as many bugs as possible have been fixed, although
some surely remain.
_5._2. _O_t_h_e_r _F_e_a_t_u_r_e_s
There are a few features that I hope to include in
later Bash releases. Some are based on work already done in
other shells.
In addition to simple variables, a future release of
Bash will include one-dimensional arrays, using the ksh
implementation of arrays as a model. Additions to the ksh
syntax, such as _v_a_r_n_a_m_e=( ... ) to assign a list of words
directly to an array and a mechanism to allow the _r_e_a_d buil-
tin to read a list of values directly into an array, would
be desirable. Given those extensions, the ksh _s_e_t -_A syntax
may not be worth supporting (the -_A option assigns a list of
values to an array, but is a rather peculiar special case).
Some shells include a means of _p_r_o_g_r_a_m_m_a_b_l_e word com-
pletion, where the user specifies on a per-command basis how
October 28, 1994
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the arguments of the command are to be treated when comple-
tion is attempted: as filenames, hostnames, executable
files, and so on. The other aspects of the current Bash
implementation could remain as-is; the existing heuristics
would still be valid. Only when completing the arguments to
a simple command would the programmable completion be in
effect.
It would also be nice to give the user finer-grained
control over which commands are saved onto the history list.
One proposal is for a variable, tentatively named _H_I_S_T_I_G_-
_N_O_R_E, which would contain a colon-separated list of com-
mands. Lines beginning with these commands, after the res-
trictions of $_H_I_S_T_C_O_N_T_R_O_L have been applied, would not be
placed onto the history list. The shell pattern-matching
capabilities could also be available when specifying the
contents of $_H_I_S_T_I_G_N_O_R_E.
One thing that newer shells such as _w_k_s_h (also known as
_d_t_k_s_h) provide is a command to dynamically load code imple-
menting additional builtin commands into a running shell.
This new builtin would take an object file or shared library
implementing the "body" of the builtin (_x_x_x__b_u_i_l_t_i_n() for
those familiar with Bash internals) and a structure contain-
ing the name of the new command, the function to call when
the new builtin is invoked (presumably defined in the shared
object specified as an argument), and the documentation to
be printed by the _h_e_l_p command (possibly present in the
shared object as well). It would manage the details of
extending the internal table of builtins.
A few other builtins would also be desirable: two are
the POSIX.2 _g_e_t_c_o_n_f command, which prints the values of sys-
tem configuration variables defined by POSIX.2, and a _d_i_s_o_w_n
builtin, which causes a shell running with job control
active to "forget about" one or more background jobs in its
internal jobs table. Using _g_e_t_c_o_n_f, for example, a user
could retrieve a value for $_P_A_T_H guaranteed to find all of
the POSIX standard utilities, or find out how long filenames
may be in the file system containing a specified directory.
There are no implementation timetables for any of these
features, nor are there concrete plans to include them. If
anyone has comments on these proposals, feel free to send me
electronic mail.
_6. _R_e_f_l_e_c_t_i_o_n_s _a_n_d _L_e_s_s_o_n_s _L_e_a_r_n_e_d
The lesson that has been repeated most often during
Bash development is that there are dark corners in the
Bourne shell, and people use all of them. In the original
description of the Bourne shell, quoting and the shell gram-
mar are both poorly specified and incomplete; subsequent
descriptions have not helped much. The grammar presented in
October 28, 1994
- 16 -
Bourne's paper describing the shell distributed with the
Seventh Edition of UNIX|- is so far off that it does not
allow the command who|wc. In fact, as Tom Duff states:
Nobody really knows what the Bourne shell's gram-
mar is. Even examination of the source code is
little help.|=
The POSIX.2 standard includes a _y_a_c_c grammar that comes
close to capturing the Bourne shell's behavior, but it
disallows some constructs which sh accepts without complaint
- and there are scripts out there that use them. It took a
few versions and several bug reports before Bash implemented
sh-compatible quoting, and there are still some "legal" sh
constructs which Bash flags as syntax errors. Complete sh
compatibility is a tough nut.
The shell is bigger and slower than I would like,
though the current version is substantially faster than pre-
viously. The readline library could stand a substantial
rewrite. A hand-written parser to replace the current
_y_a_c_c-generated one would probably result in a speedup, and
would solve one glaring problem: the shell could parse com-
mands in "$(...)" constructs as they are entered, rather
than reporting errors when the construct is expanded.
As always, there is some chaff to go with the wheat.
Areas of duplicated functionality need to be cleaned up.
There are several cases where Bash treats a variable spe-
cially to enable functionality available another way
($notify vs. set -o notify and $nolinks vs. set -o physi-
cal, for instance); the special treatment of the variable
name should probably be removed. A few more things could
stand removal; the $_a_l_l_o_w__n_u_l_l__g_l_o_b__e_x_p_a_n_s_i_o_n and
$_g_l_o_b__d_o_t__f_i_l_e_n_a_m_e_s variables are of particularly question-
able value. The $[...] arithmetic evaluation syntax is
redundant now that the POSIX-mandated $((...)) construct has
been implemented, and could be deleted. It would be nice if
the text output by the _h_e_l_p builtin were external to the
shell rather than compiled into it. The behavior enabled by
$_c_o_m_m_a_n_d__o_r_i_e_n_t_e_d__h_i_s_t_o_r_y, which causes the shell to attempt
to save all lines of a multi-line command in a single his-
tory entry, should be made the default and the variable
removed.
_________________________
|-S. R. Bourne, "UNIX Time-Sharing System: The UNIX
Shell", _B_e_l_l _S_y_s_t_e_m _T_e_c_h_n_i_c_a_l _J_o_u_r_n_a_l, 57(6), July-
August, 1978, pp. 1971-1990.
|=Tom Duff, "Rc - A Shell for Plan 9 and UNIX systems",
_P_r_o_c. _o_f _t_h_e _S_u_m_m_e_r _1_9_9_0 _E_U_U_G _C_o_n_f_e_r_e_n_c_e, London, July,
1990, pp. 21-33.
October 28, 1994
- 17 -
_7. _A_v_a_i_l_a_b_i_l_i_t_y
As with all other GNU software, Bash is available for
anonymous FTP from _p_r_e_p._a_i._m_i_t._e_d_u:/_p_u_b/_g_n_u and from other
GNU software mirror sites. The current version is in _b_a_s_h-
_1._1_4._1._t_a_r._g_z in that directory. Use _a_r_c_h_i_e to find the
nearest archive site. The latest version is always avail-
able for FTP from _b_a_s_h._C_W_R_U._E_d_u:/_p_u_b/_d_i_s_t. Bash documenta-
tion is available for FTP from _b_a_s_h._C_W_R_U._E_d_u:/_p_u_b/_b_a_s_h.
The Free Software Foundation sells tapes and CD-ROMs
containing Bash; send electronic mail to gnu@prep.ai.mit.edu
or call +1-617-876-3296 for more information.
Bash is also distributed with several versions of
UNIX-compatible systems. It is included as /bin/sh and
/bin/bash on several Linux distributions (more about the
difference in a moment), and as contributed software in
BSDI's BSD/386* and FreeBSD.
The Linux distribution deserves special mention. There
are two configurations included in the standard Bash distri-
bution: a "normal" configuration, in which all of the stan-
dard features are included, and a "minimal" configuration,
which omits job control, aliases, history and command line
editing, the directory stack and _p_u_s_h_d/_p_o_p_d/_d_i_r_s, process
substitution, prompt string special character decoding, and
the _s_e_l_e_c_t construct. This minimal version is designed to
be a drop-in replacement for the traditional UNIX /bin/sh,
and is included as the Linux /bin/sh in several packagings.
_8. _C_o_n_c_l_u_s_i_o_n
Bash is a worthy successor to sh. It is sufficiently
portable to run on nearly every version of UNIX from 4.3 BSD
to SVR4.2, and several UNIX workalikes. It is robust enough
to replace sh on most of those systems, and provides more
functionality. It has several thousand regular users, and
their feedback has helped to make it as good as it is today
- a testament to the benefits of free software.
_________________________
*BSD/386 is a trademark of Berkeley Software Design,
Inc.
October 28, 1994