article.txt

android-w.song.android.widget

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