As has already been mentioned, zsh is most similar to ksh, while many of the additions are to please csh users. Here are some more detailed notes. See also the article `UNIX shell differences and how to change your shell' posted frequently to the USENET group comp.unix.shell.
Most features of ksh (and hence also of sh) are implemented in zsh; problems can arise because the implementation is slightly different. Note also that not all ksh's are the same either. I have based this on the 11/16/88f version of ksh; differences from ksh93 will be more substantial.
As a summary of the status:
ARGV0=sh zsh will also work);
The classic difference is word splitting, discussed in question 3.1;
this catches out very many beginning zsh users. As explained there,
this is actually a bug in every other shell. The answer is to set
SH_WORD_SPLIT for backward compatibility. The next most classic
difference is that unmatched glob patterns cause the command to abort;
set NO_NOMATCH for those.
Here is a list of various options which will increase ksh
compatibility, though maybe decrease zsh's abilities: see the manual
entries for GLOB_SUBST, IGNORE_BRACES (though brace expansion occurs
in some versions of ksh), KSH_ARRAYS, KSH_GLOB, KSH_OPTION_PRINT,
LOCAL_OPTIONS, NO_BAD_PATTERN, NO_BANG_HIST, NO_EQUALS, NO_HUP,
NO_NOMATCH, NO_RCS, NO_SHORT_LOOPS, PROMPT_SUBST, RM_STAR_SILENT,
POSIX_ALIASES, POSIX_BUILTINS, POSIX_IDENTIFIERS,
SH_FILE_EXPANSION, SH_GLOB, SH_OPTION_LETTERS,
SH_WORD_SPLIT (see question 3.1) and SINGLE_LINE_ZLE.
Note that you can also disable any built-in commands which get in
your way. If invoked as `ksh', the shell will try to set suitable
options.
Here are some differences from ksh which might prove significant for ksh programmers, some of which may be interpreted as bugs; there must be more. Note that this list is deliberately rather full and that most of the items are fairly minor. Those marked `*' perform in a ksh-like manner if the shell is invoked with the name `ksh', or if `emulate ksh' is in effect. Capitalised words with underlines refer to shell options.
array[0] refers to array[1];
$array refers to the whole array, not $array[0];
braces are unnecessary: $a[1] == ${a[1]}, etc.
Set the KSH_ARRAYS option for compatibility.
coproc; |& behaves like
csh. Handling of coprocess file descriptors is also different.
cmd1 && cmd2 &, only cmd2 instead of the whole
expression is run in the background in zsh. The manual implies
this is a bug. Use { cmd1 && cmd2 } & as a workaround.
NO_NOMATCH).
foo="*"; print $foo prints all files in ksh but * in zsh
(use GLOB_SUBST).
$PSn do not do parameter substitution by default (use PROMPT_SUBST).
----------------------------------------------------------------------
ksh zsh Meaning
------ ------ ---------
!(foo) ^foo Anything but foo.
or foo1~foo2 Anything matching foo1 but foo2[1].
@(foo1|foo2|...) (foo1|foo2|...) One of foo1 or foo2 or ...
?(foo) (foo|) Zero or one occurrences of foo.
*(foo) (foo)# Zero or more occurrences of foo.
+(foo) (foo)## One or more occurrences of foo.
----------------------------------------------------------------------
The ^, ~ and # (but not |)forms require EXTENDED_GLOB.
From version 3.1.3, the ksh forms are fully supported when the
option KSH_GLOB is in effect; for previous versions you
must use the table above.
[1] See question 3.27 for more on the mysteries of
~ and ^.
:s (intended for
PATHs).
typeset and integer have special behaviour for
assignments in ksh, but not in zsh. For example, this doesn't
work in zsh:
integer k=$(wc -l ~/.zshrc)
because the return value from wc includes leading
whitespace which causes wordsplitting. Ksh handles the
assignment specially as a single word.
$ENV variable (use /etc/zshrc, ~/.zshrc;
note also $ZDOTDIR).
$PATH is not searched for commands specified
at invocation without -c.
gmacs, viraw are not supported.
Use bindkey to change the editing behaviour: set -o {emacs,vi}
becomes `bindkey -{e,v}', although `set -o emacs' and `set -o vi'
are supported for compatibility; for gmacs, go to emacs mode and
use `bindkey \^t gosmacs-transpose-characters'.
keyword option does not exist and -k is instead
interactivecomments. (keyword is not in recent versions
of ksh either.)
SHARE_HISTORY appeared in 3.1.6 and is set in ksh
compatibility mode to remedy this.
\ does not escape editing chars (use ^V).
<ESC>#; try <ESC>q).
# in an interactive shell is not treated as a comment by
default.
bindkey -M vicmd 'k' vi-up-line-or-history
bindkey -M vicmd 'j' vi-down-line-or-history
r, autoload, history, integer ...)
were aliases in ksh.
alias
newgrp="exec newgrp"
jobs has no -n flag.
select always redisplays the list of selections on each loop.
Although certain features aim to ease the withdrawal symptoms of csh (ab)users, the syntax is in general rather different and you should certainly not try to run scripts without modification. The c2z script is provided with the source (in Misc/c2z) to help convert .cshrc and .login files; see also the next question concerning aliases, particularly those with arguments.
Csh-compatibility additions include:
logout, rehash, source, (un)limit built-in commands.
*rc file for interactive shells.
cshjunkie*, ignoreeof options.
CSH_NULL_GLOB option.
>&, |& etc. redirection.
(Note that >file 2>&1 is the standard Bourne shell command for
csh's >&file.)
foreach ... loops; alternative syntax for other loops.
if ( ... ) ..., though this still doesn't
work like csh: it expects a command in the parentheses. Also
for, which.
$PROMPT as well as $PS1, $status as well as $?,
$#argv as well as $#, ....
% for prompts.
$PATH etc. are colon-separated, $path
are arrays.
!-type history (which may be turned off via setopt
nobanghist).
First of all, check you are using the syntax
alias newcmd='list of commands'
and not
alias newcmd 'list of commands'
which won't work. (It tells you if `newcmd' and `list of commands' are
already defined as aliases.)
Otherwise, your aliases probably contain references to the command
line of the form \!*, etc. Zsh does not handle this behaviour as it
has shell functions which provide a way of solving this problem more
consistent with other forms of argument handling. For example, the
csh alias
alias cd 'cd \!*; echo $cwd'
can be replaced by the zsh function,
cd() { builtin cd "$@"; echo $PWD; }
(the `builtin' tells zsh to use its own `cd', avoiding an infinite loop)
or, perhaps better,
cd() { builtin cd "$@"; print -D $PWD; }
(which converts your home directory to a ~). In fact, this problem is
better solved by defining the special function chpwd() (see
the manual). Note also that the ; at the end of the function is
optional in zsh, but not in ksh or sh (for sh's where it exists).
Here is Bart Schaefer's guide to converting csh aliases for zsh.
\!:1, \!* etc.),
then in zsh you need a function (referencing $1, $* etc.).
In recent versions of zsh this can be done by defining an anonymous
function within the alias. Otherwise, a simple zsh alias suffices.
$* in the body (inside the { }). Parameters don't magically
appear inside the { } the way they get appended to an alias.
alias rm "rm -i"),
then in a zsh function you need the "command" or "builtin" keyword
(function rm() { command rm -i "$@" }), but in a zsh alias
you don't (alias rm="rm -i").
alias ls "ls -C";
alias lf "ls -F" ==> lf == ls -C -F) then you must either:
Those first four are all you really need, but here are four more for heavy csh alias junkies:
SH_WORD_SPLIT and KSH_ARRAYS are NOT set in zsh):
csh zsh
===== ==========
\!* $* (or $argv)
\!^ $1 (or $argv[1])
\!:1 $1
\!:2 $2 (or $argv[2], etc.)
\!$ $*[$#] (or $argv[$#], or $*[-1])
\!:1-4 $*[1,4]
\!:1- $*[1,$#-1] (or $*[1,-2])
\!^- $*[1,$#-1]
\!*:q "$@"
\!*:x $=* ($*:x doesn't work (yet))
$1, $2, etc.) greater than the number of
parameters. (E.g., in a csh alias, a reference to \!:5 will
cause an error if 4 or fewer arguments are given; in a zsh
function, $5 is the empty string if there are 4 or fewer
parameters. Force an error in this example by using ${5?}.)
alias --:
csh zsh
=============== ==================
alias - "fg %-" alias -- -="fg %-"
alias -g in zsh until you REALLY know what
you're doing.
There is one other serious problem with aliases: consider
alias l='/bin/ls -F'
l() { /bin/ls -la "$@" | more }
l in the function definition is in command position and is expanded
as an alias, defining /bin/ls and -F as functions which call
/bin/ls, which gets a bit recursive. Recent versions of zsh treat
this as an error, but older versions silently create the functions.
One workaround for this is to use the "function" keyword instead:
alias l='/bin/ls -F'
function l { /bin/ls -la "$@" | more }
The l after function is not expanded. Note you don't need
the () in this case, although it's harmless.
You need to be careful if you are defining a function with multiple names; most people don't need to do this, so it's an unusual problem, but in case you do you should be aware that in versions of the shell before 5.1 names after the first were expanded:
function a b c { ... }
Here, b and c, but not a, have aliases expanded.
This oddity was fixed in version 5.1.
The rest of this item assumes you use the (more common,
but equivalent) () definitions.
Bart Schaefer's rule is: Define first those aliases you expect to use in the body of a function, but define the function first if the alias has the same name as the function.
If you aware of the problem, you can always escape part or all of the name of the function:
'l'() { /bin/ls -la "$@" | more }
Adding the quotes has no effect on the function definition, but
suppresses alias expansion for the function name. Hence this is
guaranteed to be safe---unless you are in the habit of defining
aliases for expressions such as 'l', which is valid, but probably
confusing.
(The sections on csh apply too, of course.) Certain features have
been borrowed from tcsh, including $watch, run-help, $savehist,
periodic commands etc., extended prompts, sched and which built-ins.
Programmable completion was inspired by, but is entirely different to,
tcsh's complete. (There is a perl script called lete2ctl in the
Misc directory of the source distribution to convert complete to compctl
statements.) This list is not definitive: some features have gone in
the other direction.
If you're missing the editor function run-fg-editor, try something
with bindkey -s (which binds a string to a keystroke), e.g.
bindkey -s '^z' '\eqfg %$EDITOR:t\n'
which pushes the current line onto the stack and tries to bring a job
with the basename of your editor into the foreground. bindkey -s
allows limitless possibilities along these lines. You can execute
any command in the middle of editing a line in the same way,
corresponding to tcsh's -c option:
bindkey -s '^p' '\eqpwd\n'
In both these examples, the \eq saves the current input line to
be restored after the command runs; a better effect with multiline
buffers is achieved if you also have
bindkey '\eq' push-input
to save the entire buffer. In version 4 and recent versions of zsh 3.1,
you have the following more sophisticated option,
run-fg-editor() {
zle push-input
BUFFER="fg %$EDITOR:t"
zle accept-line
}
zle -N run-fg-editor
and can now bind run-fg-editor just like any other editor function.
The Bourne-Again Shell, bash, is another enhanced Bourne-like shell; the most obvious difference from zsh is that it does not attempt to emulate the Korn shell. Since both shells are under active development it is probably not sensible to be too specific here. Broadly, bash has paid more attention to standards compliance (i.e. POSIX) for longer, and has so far avoided the more abstruse interactive features (programmable completion, etc.) that zsh has.
In recent years there has been a certain amount of crossover in the
extensions, however. Zsh (as of 3.1.6) has bash's `${var/old/new}'
feature for replacing the text old with the text new in the
parameter $var. Note one difference here: while both shells
implement the syntax `${var/#old/new}' and `${var/%old/new}' for
anchoring the match of old to the start or end of the parameter text,
respectively, in zsh you can't put the `#' or `%' inside a
parameter: in other words `{var/$old/new}' where old begins with
a `#' treats that as an ordinary character in zsh, unlike bash. To
do this sort of thing in zsh you can use (from 3.1.7) the new syntax
for anchors in any pattern, `(#s)' to match the start of a string,
and `(#e)' to match the end. These require the option
EXTENDED_GLOB to be set.
People often ask why zsh has all these `unnecessary' csh-like features, or alternatively why zsh doesn't understand more csh syntax. This is far from a definitive answer and the debate will no doubt continue.
Paul's object in writing zsh was to produce a ksh-like shell which
would have features familiar to csh users. For a long time, csh was
the preferred interactive shell and there is a strong resistance to
changing to something unfamiliar, hence the additional syntax and
CSH_JUNKIE options. This argument still holds. On the other hand,
the arguments for having what is close to a plug-in replacement for ksh
are, if anything, even more powerful: the deficiencies of csh as a
programming language are well known (look in any Usenet FAQ archive, e.g.
http://www.cis.ohio-state.edu/hypertext/faq/usenet/unix-faq/\
shell/csh-whynot/faq.html
if you are in any doubt) and zsh is able to run many standard
scripts such as /etc/rc.
Of course, this makes zsh rather large and feature-ridden so that it seems to appeal mainly to hackers. The only answer, perhaps not entirely satisfactory, is that you have to ignore the bits you don't want. The introduction of loadable in modules in version 3.1 should help.
`Unicode', or UCS for Universal Character Set, is the modern way of specifying character sets. It replaces a large number of ad hoc ways of supporting character sets beyond ASCII. `UTF-8' is an encoding of Unicode that is particularly natural on Unix-like systems.
The production branch of zsh, 4.2, has very limited support: the built-in printf command supports "\u" and "\U" escapes to output arbitrary Unicode characters; ZLE (the Zsh Line Editor) has no concept of character encodings, and is confused by multi-octet encodings.
However, the 4.3 branch has much better support, and furthermore this is now fairly stable. (Only a few minor areas need fixing before this becomes a production release.) This is discussed more fully below, see `Multibyte input and output'.
tl;dr: bash is not the reference implementation of zsh, and zsh is not a bug-for-bug compatible reimplementation of bash.
bash and zsh are different programming languages. They are not interchangeable; programs written for either of these languages will, in general, not run under the other. (The situation is similar with many other pairs of closely-related languages, such as Python 2 and Python 3; C and C++; and even C89 and C11.)
When bash and zsh behave differently on the same input, whether zsh's
behaviour is a bug does not depend on what bash does on the same
input; rather, it depends on what zsh's user manual specifies.
(By way of comparison, it's not a bug in Emacs that :q! doesn't
cause it to exit.)
That being said, the bash and zsh languages do have a common subset, and it is feasible to write non-trivial pieces of code that would run under either of them, if one is sufficiently familiar with both of them. However, a difference between bash's behaviour and zsh's does not imply that zsh has a bug. The difference might be a bug in zsh, a bug in bash, or a bug in neither shell (see 3.1 for an example).
The recommended way to deal with these differences depends on what kind of piece of code is in question: a script or a plugin.
For scripts --- external commands that
are located in $PATH, or located elsewhere and are executed by
giving their path explicitly (as in ls, /etc/rc.d/sshd,
and ./configure) --- the answer is simple:
Don't run bash scripts under zsh. If the scripts were written for
bash, run them in bash. There's absolutely no problem with having
#!/usr/bin/env bash scripts even if zsh is your shell for
interactive sessions.
In fact, if you've recently changed to zsh, we recommend that
you keep your scripts as #!/usr/bin/env bash, at least for
a while: this would make the change more gradual and flatten your
learning curve. Once you're used to zsh, you can decide for each
script whether to port it to zsh or keep it as-is.
For plugins --- pieces of code
executed within the shell itself, loaded via the .,
source, or autoload builtins, added to .zshrc, or
pasted interactively at the shell prompt --- one may consider it
worthwhile to invest the effort to make them runnable under either shell.
However, as mentioned above, doing so requires one to be familiar with both
shells, and either steer clear of their differences or handle them explicitly
with conditional code (such as if test -n "$ZSH_VERSION").
In summary, if you'd like to run a bash script or plugin under zsh, you must port the script or plugin properly, reviewing it line by line for differences between the two languages and adjusting it accordingly, just like you would when translating a book from American English to British English.