article.txt

android-w.song.android.widget

     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



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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



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                           - 11 -


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



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                           - 12 -


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



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                           - 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-