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                   Bash - The GNU shell*


                         Chet Ramey
              Case Western Reserve University
                      [e-mail]






_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.




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                           - 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.




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                           - 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.




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                           - 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