1.Introduction
Bourne Shell(即sh)是UNIX最初使用的shell，平且在每种UNIX上都可以使用。BourneShell在shell编程方便相当优秀，但在处理与用户的交互方便作得不如其他几种shell。
LinuxOS默认的是Bourne Again Shell，它是Bourne Shell的扩展，简称bash，与BourneShell完全兼容，并且在BourneShell的基础上增加，增强了很多特性。可以提供命令补全，命令编辑和命令历史等功能。它还包含了很多C Shell和KornShell中的优点，有灵活和强大的编辑接口，同时又很友好的用户界面
C Shell是一种比Bourne Shell更适合的变种Shell，它的语法与C语言很相似。Linux为喜欢使用CShell的人提供了Tcsh。
Tcsh是CShell的一个扩展版本。Tcsh包括命令行编辑，可编程单词补全，拼写校正，历史命令替换，作业控制和类似C语言的语法，他不仅和BashShell提示符兼容，而且还提供比Bash Shell更多的提示符参数。
Korn Shell集合了C Shell和Bourne Shell的优点并且和BourneShell完全兼容。Linux系统提供了pdksh（ksh的扩展），它支持人物控制，可以在命令行上挂起，后台执行，唤醒或终止程序。
Linux还包括了一些流行的Shell如：ash，zsh等。每个Shell都有它的用途，有些Shell是有专利的，有些能从Internet或其他来源获得。
2.How does bash differ from sh, the Bourne shell?
This is a non-comprehensive list of features thatdifferentiate bash from the SVR4.2 shell. Thebash manual page explains these more completely.
Things bash has that sh does not:
long invocation options
[+-]O invocation option
-l invocation option
`!' reserved word to invert pipeline returnvalue
`time' reserved word to time pipelines and shellbuiltins
the `function' reserved word
the `select' compound command and reservedword
arithmetic for command: for ((expr1 ; expr2;expr3 )); do list; done
new $'...' and $"..." quoting
the $(...) form of command substitution
the $(<filename) form of commandsubstitution, equivalent to
$(cat filename)
the ${#param} parameter value lengthoperator
the ${!param} indirect parameter expansionoperator
the ${!param*} prefix expansion operator
the ${param:offset[]} parameter substringoperator
the ${param/pat[/string]} parameter patternsubstitution operator
expansions to perform substring removal(${p%[%]w}, ${p#[#]w})
expansion of positional parameters beyond $9with ${num}
variables: BASH, BASH_VERSION, BASH_VERSINFO,UID, EUID, REPLY,
TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM,SECONDS,
LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,HOSTNAME,
ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE,HISTFILE,
HISTFILESIZE, HISTCONTROL, HISTIGNORE,GLOBIGNORE, GROUPS,
PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF,INPUTRC,
SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME,histchars,
auto_resume
DEBUG trap
ERR trap
variable arrays with new compound assignmentsyntax
redirections: <>,&>, >|,<<<,[n]<&word-,[n]>&word-
prompt string special char translation andvariable expansion
auto-export of variables in initialenvironment
command search finds functions beforebuiltins
bash return builtin will exit a file sourcedwith `.'
builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E,hash -d/-l/-p/-t.
export-n/-f/-p/name=value, pwd -L/-P,
read-e/-p/-a/-t/-n/-d/-s/-u,
readonly-a/-f/name=value, trap -l, set +o,
set-b/-m/-o option/-h/-p/-B/-C/-H/-P,
unset-f/-v, ulimit -m/-p/-u,
type-a/-p/-t/-f/-P, suspend -f, kill -n,
test -ooptname/s1 == s2/s1 < s2/s1 >s2/-nt/-ot/-ef/-O/-G/-S
bash reads ~/.bashrc for interactive shells,$ENV for non-interactive
bash restricted shell mode is moreextensive
bash allows functions and variables with thesame name
brace expansion
tilde expansion
arithmetic expansion with $((...)) and `let'builtin
the `...' extended conditional command
process substitution
aliases and alias/unalias builtins
local variables in functions and `local'builtin
readline and command-line editing withprogrammable completion
command history and history/fc builtins
csh-like history expansion
other new bash builtins: bind, command, compgen,complete, builtin,
declare/typeset, dirs, enable,fc, help,
history, logout, popd, pushd,disown, shopt,
printf
exported functions
filename generation when using outputredirection (command >a*)
POSIX.2-style globbing character classes
POSIX.2-style globbing equivalence classes
POSIX.2-style globbing collating symbols
egrep-like extended pattern matchingoperators
case-insensitive pattern matching andglobbing
variable assignments preceding commands affectonly that command,even for builtins and functions
posix mode
redirection to /dev/fd/N, /dev/stdin,/dev/stdout, /dev/stderr,
/dev/tcp/host/port,/dev/udp/host/port
debugger support, including `caller' builtin andnew variables
RETURN trap
Things sh has that bash does not:
uses variable SHACCT to do shellaccounting
includes `stop' builtin (bash can use aliasstop='kill -s STOP')
`newgrp' builtin
turns on job control if called as `jsh'
$TIMEOUT (like bash $TMOUT)
`^' is a synonym for `|'
new SVR4.2 sh builtins: mldmode, priv
Implementation differences:
redirection to/from compound commands causes shto create a subshell
bash does not allow unbalanced quotes; shsilently inserts them at EOF
bash does not mess with signal 11
sh sets (euid, egid) to (uid, gid) if -p notsupplied and uid < 100
bash splits only the results of expansions onIFS, using POSIX.2
field splitting rules; shsplits all words on IFS
sh does not allow MAILCHECK to be unset(?)
sh does not allow traps on SIGALRM orSIGCHLD
bash allows multiple option arguments wheninvoked (e.g. -x -v);
sh allows only a singleoption argument (`sh -x -v' attempts
to open a file named `-v',and, on SunOS 4.1.4, dumps core.
On Solaris 2.4 and earlierversions, sh goes into an infinite
loop.)
sh exits a script if any builtin fails; bashexits only if one of
the POSIX.2 `special'builtins fails
3.How does bash differ from the Korn shell, versionksh88?
Things bash has or uses that ksh88 does not:
long invocation options
[-+]O invocation option
-l invocation option
`!' reserved word
arithmetic for command: for ((expr1 ; expr2;expr3 )); do list; done
arithmetic in largest machine-supported size(intmax_t)
posix mode and posix conformance
command hashing
tilde expansion for assignment statements thatlook like $PATH
process substitution with named pipes if /dev/fdis not available
the ${!param} indirect parameter expansionoperator
the ${!param*} prefix expansion operator
the ${param:offset[]} parameter substringoperator
the ${param/pat[/string]} parameter patternsubstitution operator
variables: BASH, BASH_VERSION, BASH_VERSINFO,UID, EUID, SHLVL,
TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE,MACHTYPE,
HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE,DIRSTACK,
PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS,GLOBIGNORE,
GROUPS, FUNCNAME, histchars, auto_resume
prompt expansion with backslash escapes andcommand substitution
redirection: &>(stdout and stderr),<<<,[n]<&word-,[n]>&word-
more extensive and extensible editing andprogrammable completion
builtins: bind, builtin, command, declare, dirs,echo -e/-E, enable,
exec-l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
jobs-x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
read-e/-p/-a/-t/-n/-d/-s, readonly -a/-n/-f/-p,
set -obraceexpand/-o histexpand/-o interactive-comments/
-onotify/-o physical/-o posix/-o hashall/-o onecmd/
-h/-B/-C/-b/-H/-P, set +o, suspend, trap -l, type,
typeset-a/-F/-p, ulimit -u, umask -S, alias -p, shopt,
disown,printf, complete, compgen
`!' csh-style history expansion
POSIX.2-style globbing character classes
POSIX.2-style globbing equivalence classes
POSIX.2-style globbing collating symbols
egrep-like extended pattern matchingoperators
case-insensitive pattern matching andglobbing
`**' arithmetic operator to doexponentiation
redirection to /dev/fd/N, /dev/stdin,/dev/stdout, /dev/stderr
arrays of unlimited size
TMOUT is default timeout for `read' and`select'
debugger support, including the `caller'builtin
RETURN trap
Timestamps in history entries
{x..y} brace expansion
Things ksh88 has or uses that bash does not:
tracked aliases (alias -t)
variables: ERRNO, FPATH, EDITOR, VISUAL
co-processes (|&,>&p,<&p)
weirdly-scoped functions
typeset +f to list all function names withoutdefinitions
text of command history kept in a file, notmemory
builtins: alias -x, cd old new, newgrp,print,
read-p/-s/var?prompt, set -A/-o gmacs/
-obgnice/-o markdirs/-o trackall/-o viraw/-s,
typeset-H/-L/-R/-Z/-A/-ft/-fu/-fx/-l/-u/-t, whence
using environment to pass attributes of exportedvariables
arithmetic evaluation done on arguments to somebuiltins
reads .profile from $PWD when invoked as loginshell
Implementation differences:
ksh runs last command of a pipeline in parentshell context
bash has brace expansion by default (ksh88compile-time option)
bash has fixed startup file for all interactiveshells; ksh reads $ENV
bash has exported functions
bash command search finds functions beforebuiltins
bash waits for all commands in pipeline to exitbefore returning status
emacs-mode editing has some slightly differentkey bindings
Which new features in ksh-93 are not in bash, and whichare?
New things in ksh-93 not in bash-3.0:
associative arrays
floating point arithmetic and variables
math library functions
${!name[sub]} name of subscript for associativearray
`.' is allowed in variable names to create ahierarchical namespace
more extensive compound assignment syntax
discipline functions
`sleep' and `getconf' builtins (bash hasloadable versions)
typeset -n and `nameref' variables
KEYBD trap
variables: .sh.edchar, .sh.edmode, .sh.edcol,.sh.edtext, .sh.version,
.sh.name, .sh.subscript, .sh.value, .sh.match,HISTEDIT
backreferences in pattern matching (\N)
`&' operator in pattern listsfor matching
print -f (bash uses printf)
`fc' has been renamed to `hist'
`.' can execute shell functions
exit statuses between 0 and 255
`+=' variable assignment operator
FPATH and PATH mixing
getopts -a
-I invocation option
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-3.0:
[n]<&word- and[n]>&word- redirections (combinationdup and close)
for (( expr1; expr2; expr3 )) ; do list; done -arithmetic for command
:, ++, --, `expr1 , expr2' arithmeticoperators
expansions: ${!param}, ${param:offset[]},${param/pat[/str]},
${!param*}
compound array assignment
the `!' reserved word
loadable builtins -- but ksh uses `builtin'while bash uses `enable'
`command', `builtin', `disown' builtins
new $'...' and $"..." quoting
FIGNORE (but bash uses GLOBIGNORE),HISTCMD
set -o notify/-C
changes to kill builtin
read -A (bash uses read -a)
read -t/-d
trap -p
exec -c/-a
`.' restores the positional parameters when itcompletes
POSIX.2 `test'
umask -S
unalias -a
command and arithmetic substitution performed onPS1, PS4, and ENV
command name completion
ENV processed only for interactive shells
set -o pipefail
