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@@ -1,4 +1,4 @@ -This is the Bash FAQ, version 3.20, for Bash version 2.05b. +This is the Bash FAQ, version 3.26, for Bash version 2.05b. This document contains a set of frequently-asked questions concerning Bash, the GNU Bourne-Again Shell. Bash is a freely-available command @@ -31,7 +31,7 @@ A6) How can I build bash with gcc? A7) How can I make bash my login shell? A8) I just changed my login shell to bash, and now I can't FTP into my machine. Why not? -A9) What's the `POSIX 1003.2 standard'? +A9) What's the `POSIX Shell and Utilities standard'? A10) What is the bash `posix mode'? Section B: The latest version @@ -75,6 +75,9 @@ E8) Why does the arithmetic evaluation code complain about `08'? E9) Why does the pattern matching expression [A-Z]* match files beginning with every letter except `z'? E10) Why does `cd //' leave $PWD as `//'? +E11) If I resize my xterm while another program is running, why doesn't bash + notice the change? +E12) Why don't negative offsets in substring expansion work like I expect? Section F: Things to watch out for on certain Unix versions @@ -271,7 +274,8 @@ it is invoked as a login shell. Next, add a line similar to the above to ~/.profile: - [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login + [ -f /usr/gnu/bin/bash ] && [ -x /usr/gnu/bin/bash ] && \ + exec /usr/gnu/bin/bash --login This will cause login shells to replace themselves with bash running as a login shell. Once you have this working, you can copy your initialization @@ -325,7 +329,7 @@ this before you can make bash your login shell. Most versions of ftpd use this file to prohibit `special' users such as `uucp' and `news' from using FTP. -A9) What's the `POSIX 1003.2 standard'? +A9) What's the `POSIX Shell and Utilities standard'? POSIX is a name originally coined by Richard Stallman for a family of open system standards based on UNIX. There are a @@ -335,44 +339,50 @@ call and C library level to applications and tools to system administration and management. Each area of standardization 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 concentrates on the command +The POSIX Shell and Utilities standard was originally developed by +IEEE Working Group 1003.2 (POSIX.2). Today it has been merged with +the original 1003.1 Working Group and is maintained by the Austin +Group (a joint working group of the IEEE, The Open Group and +ISO/IEC SC22/WG15). Today the Shell and Utilities are a volume +within the set of documents that make up IEEE Std 1003.1-2001, and +thus now the former POSIX.2 (from 1992) is now part of the current +POSIX.1 standard (POSIX 1003.1-2001). + +The Shell and Utilities volume concentrates 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. +the command line or by other programs. The standard is freely +available on the web at http://www.UNIX-systems.org/version3/ . +Work continues at the Austin Group on maintenance issues; see +http://www.opengroup.org/austin/ to join the discussions. -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 control and program -execution constructs, I/O redirection and pipelining, argument -handling, variable expansion, and quoting. +Bash is concerned with the aspects of the shell's behavior defined +by the POSIX Shell and Utilities volume. The shell command +language has of course been standardized, including the basic flow +control and program execution constructs, I/O redirection and +pipelining, argument handling, variable expansion, and quoting. The `special' builtins, which must be implemented as part of the shell to provide the desired functionality, are specified as being part of the shell; examples of these are `eval' and -`export'. Other utilities appear in the sections of POSIX.2 not +`export'. Other utilities appear in the sections of POSIX not devoted to the shell which are commonly (and in some cases must be) implemented as builtin commands, such as `read' and `test'. -POSIX.2 also specifies aspects of the shell's interactive +POSIX also specifies aspects of the shell's interactive behavior as part of the UPE, including job control and command line editing. Only vi-style line editing commands have been standardized; emacs editing commands were left out due to objections. -The Open Group has made an older version of its Single Unix -Specification (version 2), which is very similar to POSIX.2, -available on the web at - -http://www.opengroup.org/onlinepubs/007908799/ +The latest version of the POSIX Shell and Utilities standard is +available (now updated to the 2003 Edition incorporating the +Technical Corrigendum 1), as part of the Single UNIX Specification +Version 3 at -The Single Unix Specification, version 3, is available on the web at - -http://www.opengroup.org/onlinepubs/007904975/ +http://www.UNIX-systems.org/version3/ A10) What is the bash `posix mode'? -Although bash is an implementation of the POSIX.2 shell +Although bash is an implementation of the POSIX shell specification, there are areas where the bash default behavior differs from that spec. The bash `posix mode' changes the bash behavior in these areas so that it obeys the spec more closely. @@ -821,6 +831,8 @@ New things in ksh-93 not in bash-2.05b: -I invocation option DEBUG trap now executed before each simple command, instead of after printf %H, %P, %T, %Z modifiers, output base for %d + lexical scoping for local variables in `ksh' functions + no scoping for local variables in `POSIX' functions New things in ksh-93 present in bash-2.05b: [n]<&word- and [n]>&word- redirections (combination dup and close) @@ -1305,6 +1317,49 @@ This is, I presume, for historical compatibility. Certain versions of Unix, and early network file systems, used paths of the form //hostname/path to access `path' on server `hostname'. +E11) If I resize my xterm while another program is running, why doesn't bash + notice the change? + +This is another issue that deals with job control. + +The kernel maintains a notion of a current terminal process group. Members +of this process group (processes whose process group ID is equal to the +current terminal process group ID) receive terminal-generated signals like +SIGWINCH. (For more details, see the JOB CONTROL section of the bash +man page.) + +If a terminal is resized, the kernel sends SIGWINCH to each member of +the terminal's current process group (the `foreground' process group). + +When bash is running with job control enabled, each pipeline (which may be +a single command) is run in its own process group, different from bash's +process group. This foreground process group receives the SIGWINCH; bash +does not. Bash has no way of knowing that the terminal has been resized. + +There is a `checkwinsize' option, settable with the `shopt' builtin, that +will cause bash to check the window size and adjust its idea of the +terminal's dimensions each time a process stops or exits and returns control +of the terminal to bash. Enable it with `shopt -s checkwinsize'. + +E12) Why don't negative offsets in substring expansion work like I expect? + +When substring expansion of the form ${param:offset[:length} is used, +an `offset' that evaluates to a number less than zero counts back from +the end of the expanded value of $param. + +When a negative `offset' begins with a minus sign, however, unexpected things +can happen. Consider + + a=12345678 + echo ${a:-4} + +intending to print the last four characters of $a. The problem is that +${param:-word} already has a well-defined meaning: expand to word if the +expanded value of param is unset or null, and $param otherwise. + +To use negative offsets that begin with a minus sign, separate the +minus sign and the colon with a space. + Section F: Things to watch out for on certain Unix versions F1) Why can't I use command line editing in my `cmdtool'? @@ -1671,6 +1726,12 @@ A second edition of this book is available, published in January, 1998. The ISBN number is 1-56592-347-2. Look for it in the same fine bookstores or on the web. +The GNU Bash Reference Manual has been published as a printed book by +Network Theory Ltd (Paperback, ISBN: 0-9541617-7-7, Feb 2003). It covers +bash-2.0 and is available from most online bookstores (see +http://www.network-theory.co.uk/bash/manual/ for details). The publisher +will donate $1 to the Free Software Foundation for each copy sold. + H3) What's coming in future versions? These are features I hope to include in a future version of bash. @@ -1698,10 +1759,10 @@ Some of the new ksh93 pattern matching operators, like backreferencing H5) When will the next release appear? -The next version will appear sometime in 2002. Never make predictions. +The next version will appear sometime in 2003. Never make predictions. -This document is Copyright 1995-2002 by Chester Ramey. +This document is Copyright 1995-2003 by Chester Ramey. Permission is hereby granted, without written agreement and without license or royalty fees, to use, copy, and distribute |