3. Differences from other Unix shells

3.1 How does bash differ from sh, the Bourne shell?

This is a non-comprehensive list of features that differentiate bash from the SVR4.2 shell. The bash manual page explains these more completely.

Things bash has that sh does not:

• long invocation options
• `!' reserved word to invert pipeline return value
• `time' reserved word to time pipelines and shell builtins
• the `function' reserved word
• the `select' compound command and reserved word
• arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
• new $'...' and $"..." quoting
• the $(...) form of command substitution
• the $( form of command substitution, equivalent to $(cat filename)
• the ${#param} parameter value length operator
• the ${!param} indirect parameter expansion operator
• the ${!param*} prefix expansion operator
• the ${param:length:offset]} parameter substring operator
• the ${param/pat[/string]} parameter pattern substitution operator

        expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
        expansion of positional parameters beyond $9 with ${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
• variable arrays with new compound assignment syntax
• redirections:
• , &>, >|
• prompt string special char translation and variable expansion
• auto-export of variables in initial environment
• command search finds functions before builtins
• bash return builtin will exit a file sourced with `.'

  ---

          builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p.
                    export -n/-f/-p/name=value, pwd -L/-P,
                    read -e/-p/-a/-t/-n/-d/-s,
                    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, suspend -f, kill -n,
                    test -o optname/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 more extensive
• bash allows functions and variables with the same 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 with programmable 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 output redirection (command >a*)
• POSIX.2-style globbing character classes
• POSIX.2-style globbing equivalence classes
• POSIX.2-style globbing collating symbols
• egrep-like extended pattern matching operators
• case-insensitive pattern matching and globbing
• variable assignments preceding commands affect only 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

Things sh has that bash does not: uses variable SHACCT to do shell accounting includes `stop' builtin (bash can use alias stop='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 sh to create a subshell
• bash does not allow unbalanced quotes; sh silently inserts them at EOF
• bash does not mess with signal 11
• sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
• bash splits only the results of expansions on IFS, using POSIX.2 field splitting rules; sh splits all words on IFS
• sh does not allow MAILCHECK to be unset (?)
• sh does not allow traps on SIGALRM or SIGCHLD
• bash allows multiple option arguments when invoked (e.g. -x -v); sh allows only a single option argument (`sh -x -v' attempts to open a file named `-v', and, on SunOS 4.1.4, dumps core. On Solaris 2.4 and earlier versions, sh goes into an infinite loop.)
• sh exits a script if any builtin fails; bash exits only if one of the POSIX.2 `special' builtins fails

3.2 How does bash differ from the Korn shell, version ksh88?

Things bash has or uses that ksh88 does not:

• long invocation options
• `!' reserved word
• arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
• posix mode and posix conformance
• command hashing
• tilde expansion for assignment statements that look like $PATH
• process substitution with named pipes if /dev/fd is not available
• the ${!param} indirect parameter expansion operator
• the ${!param*} prefix expansion operator
• the ${param:length:offset]} parameter substring operator
• the ${param/pat[/string]} parameter pattern substitution 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 and command substitution
• redirection: &> (stdout and stderr)
• more extensive and extensible editing and programmable 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 -o braceexpand/-o histexpand/-o interactive-comments/ -o notify/-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 matching operators
• case-insensitive pattern matching and globbing
• `**' arithmetic operator to do exponentiation
• redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr

Things ksh88 has or uses that bash does not:

• tracked aliases
• variables: ERRNO, FPATH, EDITOR, VISUAL
• co-processes (|&, >, <)
• weirdly-scoped functions
• typeset +f to list all function names without definitions
• text of command history kept in a file, not memory
• builtins: alias -x, cd old new, fc -e -, newgrp, print, read -p/-s/-u/var?prompt, set -A/-o gmacs/ -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s, typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence

Implementation differences:

• ksh runs last command of a pipeline in parent shell context
• bash has brace expansion by default (ksh88 compile-time option)
• bash has fixed startup file for all interactive shells; ksh reads $ENV
• bash has exported functions
• bash command search finds functions before builtins

3.3 Which new features in ksh-93 are not in bash, and which are?

New things in ksh-93 not in bash-2.04:

• associative arrays
• floating point arithmetic
• math library functions
• ${!namesub]} name of subscript for associative array
• `.' is allowed in variable names to create a hierarchical namespace
• more extensive compound assignment syntax

• discipline functions
• `sleep' and `getconf' builtins (bash has loadable 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, HISTEDIT
• backreferences in pattern matching
• print -f (bash uses printf)
• `fc' has been renamed to `hist'
• `.' can execute shell functions

New things in ksh-93 present in bash-2.04:

        for (( expr1; expr2; expr3 )) ; do list; done - arithmetic for command
        ?:, ++, --, `expr1 , expr2' arithmetic operators
        expansions: ${!param}, ${param:offset[:len]}, ${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 it completes
        POSIX.2 `test'
        umask -S
        unalias -a
        command and arithmetic substitution performed on PS1, PS4, and ENV
        command name completion
        ENV processed only for interactive shells