Difference between sh and bash

Question

When writing shell programs  we often use  bin sh and  bin bash  I usually use bash  but I don t know what s the difference between them    What s main difference between bash and sh   What do we need to be aware of when programming in bash and sh

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sh  http   man cx sh bash  http   man cx bash  TL DR  bash is a superset of sh with a more elegant syntax and more functionality  It is safe to use a bash shebang line in almost all cases as it s quite ubiquitous on modern platforms   NB  in some environments  sh is bash  Check sh --version

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They re nearly identical but bash has more features     sh is  more or less  an older subset of bash   sh often means the original Bourne shell  which predates bash  Bourne  again  shell   and was created in 1977   But  in practice  it may be better to think of it as a highly-cross-compatible shell compliant with the POSIX standard from 1992   Scripts that start with    bin sh or use the sh shell usually do so for backwards compatibility   Any unix linux OS will have an sh shell   On Ubuntu sh often invokes dash and on MacOS it s a special POSIX version of bash   These shells may be preferred for standard-compliant behavior  speed or backwards compatibility   bash is newer than the original sh  adds more features  and seeks to be backwards compatible with sh   In theory  sh programs should run in bash   bash is available on nearly all linux unix machines and usually used by default     with the notable exception of MacOS defaulting to zsh as of Catalina  10 15   FreeBSD  by default  does not come with bash installed

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Other answers generally pointed out the difference between Bash and a POSIX shell standard  However  when writing portable shell scripts and being used to Bash syntax  a list of typical bashisms and corresponding pure POSIX solutions is very handy  Such list has been compiled when Ubuntu switched from Bash to Dash as default system shell and can be found here  https   wiki ubuntu com DashAsBinSh  Moreover  there is a great tool called checkbashisms that checks for bashisms in your script and comes handy when you want to make sure that your script is portable

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Shell is an interface between a user and OS to access to an operating system s services  It can be either GUI or CLI  Command Line interface    sh  Bourne shell  is a shell command-line interpreter  for Unix Unix-like operating systems  It provides some built-in commands  In scripting language we denote interpreter as    bin sh  It was one most widely supported by other shells like bash  free open   kash  not free    Bash  Bourne again shell  is a shell replacement for the Bourne shell  Bash is superset of sh  Bash supports sh  POSIX is a set of standards defining how POSIX-compliant systems should work   Bash is not actually a POSIX compliant shell  In a scripting language we denote the interpreter as    bin bash   Analogy      Shell is like an interface or specifications or API    sh is a class which implements the Shell interface    Bash is a subclass of the sh

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bin sh may or may not invoke the same program as  bin bash   sh supports at least the features required by POSIX  assuming a correct implementation   It may support extensions as well   bash  the  Bourne Again Shell   implements the features required for sh plus bash-specific extensions  The full set of extensions is too long to describe here  and it varies with new releases  The differences are documented in the bash manual  Type info bash and read the  Bash Features  section  section 6 in the current version   or read the current documentation online

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What is sh  sh  or the Shell Command Language  is a programming language described by the POSIX standard  It has many implementations  ksh88  dash        bash can also be considered an implementation of sh  see below    Because sh is a specification  not an implementation   bin sh is a symlink  or a hard link  to an actual implementation on most POSIX systems   What is bash  bash started as an sh-compatible implementation  although it predates the POSIX standard by a few years   but as time passed it has acquired many extensions  Many of these extensions may change the behavior of valid POSIX shell scripts  so by itself bash is not a valid POSIX shell  Rather  it is a dialect of the POSIX shell language   bash supports a --posix switch  which makes it more POSIX-compliant  It also tries to mimic POSIX if invoked as sh   sh   bash   For a long time   bin sh used to point to  bin bash on most GNU Linux systems  As a result  it had almost become safe to ignore the difference between the two  But that started to change recently   Some popular examples of systems where  bin sh does not point to  bin bash  and on some of which  bin bash may not even exist  are    Modern Debian and Ubuntu systems  which symlink sh to dash by default  Busybox  which is usually run during the Linux system boot time as part of initramfs  It uses the ash shell implementation  BSDs  and in general any non-Linux systems  OpenBSD uses pdksh  a descendant of the Korn shell  FreeBSD s sh is a descendant of the original UNIX Bourne shell   Solaris has its own sh which for a long time was not POSIX-compliant  a free implementation is available from the Heirloom project    How can you find out what  bin sh points to on your system   The complication is that  bin sh could be a symbolic link or a hard link  If it s a symbolic link  a portable way to resolve it is     file -h  bin sh  bin sh  symbolic link to bash   If it s a hard link  try    find -L  bin -samefile  bin sh  bin sh  bin bash   In fact  the -L flag covers both symlinks and hardlinks  but the disadvantage of this method is that it is not portable     POSIX does not require find to support the -samefile option  although both GNU find and FreeBSD find support it   Shebang line  Ultimately  it s up to you to decide which one to use  by writing the   shebang   line as the very first line of the script   E g      bin sh   will use sh  and whatever that happens to point to       bin bash   will use  bin bash if it s available  and fail with an error message if it s not   Of course  you can also specify another implementation  e g      bin dash   Which one to use  For my own scripts  I prefer sh for the following reasons    it is standardized it is much simpler and easier to learn it is portable across POSIX systems     even if they happen not to have bash  they are required to have sh   There are advantages to using bash as well  Its features make programming more convenient and similar to programming in other modern programming languages  These include things like scoped local variables and arrays  Plain sh is a very minimalistic programming language

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Post from UNIX COM  Shell features  This table below lists most features that I think would make you choose one shell over another  It is not intended to be a definitive list and does not include every single possible feature for every single possible shell  A feature is only considered to be in a shell if in the version that comes with the operating system  or if it is available as compiled directly from the standard distribution  In particular the C shell specified below is that available on SUNOS 4    a considerable number of vendors now ship either tcsh or their own enhanced C shell instead  they don t always make it obvious that they are shipping tcsh   Code                                        sh   csh  ksh  bash tcsh zsh  rc   es Job control                          N    Y    Y    Y    Y    Y    N    N Aliases                              N    Y    Y    Y    Y    Y    N    N Shell functions                      Y 1  N    Y    Y    N    Y    Y    Y  Sensible  Input Output redirection  Y    N    Y    Y    N    Y    Y    Y Directory stack                      N    Y    Y    Y    Y    Y    F    F Command history                      N    Y    Y    Y    Y    Y    L    L Command line editing                 N    N    Y    Y    Y    Y    L    L Vi Command line editing              N    N    Y    Y    Y 3  Y    L    L Emacs Command line editing           N    N    Y    Y    Y    Y    L    L Rebindable Command line editing      N    N    N    Y    Y    Y    L    L User name look up                    N    Y    Y    Y    Y    Y    L    L Login Logout watching                N    N    N    N    Y    Y    F    F Filename completion                  N    Y 1  Y    Y    Y    Y    L    L Username completion                  N    Y 2  Y    Y    Y    Y    L    L Hostname completion                  N    Y 2  Y    Y    Y    Y    L    L History completion                   N    N    N    Y    Y    Y    L    L Fully programmable Completion        N    N    N    N    Y    Y    N    N Mh Mailbox completion                N    N    N    N 4  N 6  N 6  N    N Co Processes                         N    N    Y    N    N    Y    N    N Builtin artithmetic evaluation       N    Y    Y    Y    Y    Y    N    N Can follow symbolic links invisibly  N    N    Y    Y    Y    Y    N    N Periodic command execution           N    N    N    N    Y    Y    N    N Custom Prompt  easily                N    N    Y    Y    Y    Y    Y    Y Sun Keyboard Hack                    N    N    N    N    N    Y    N    N Spelling Correction                  N    N    N    N    Y    Y    N    N Process Substitution                 N    N    N    Y 2  N    Y    Y    Y Underlying Syntax                    sh   csh  sh   sh   csh  sh   rc   rc Freely Available                     N    N    N 5  Y    Y    Y    Y    Y Checks Mailbox                       N    Y    Y    Y    Y    Y    F    F Tty Sanity Checking                  N    N    N    N    Y    Y    N    N Can cope with large argument lists   Y    N    Y    Y    Y    Y    Y    Y Has non-interactive startup file     N    Y    Y 7  Y 7  Y    Y    N    N Has non-login startup file           N    Y    Y 7  Y    Y    Y    N    N Can avoid user startup files         N    Y    N    Y    N    Y    Y    Y Can specify startup file             N    N    Y    Y    N    N    N    N Low level command redefinition       N    N    N    N    N    N    N    Y Has anonymous functions              N    N    N    N    N    N    Y    Y List Variables                       N    Y    Y    N    Y    Y    Y    Y Full signal trap handling            Y    N    Y    Y    N    Y    Y    Y File no clobber ability              N    Y    Y    Y    Y    Y    N    F Local variables                      N    N    Y    Y    N    Y    Y    Y Lexically scoped variables           N    N    N    N    N    N    N    Y Exceptions                           N    N    N    N    N    N    N    Y   Key to the table above   Y      Feature can be done using this shell   N      Feature is not present in the shell   F      Feature can only be done by using the shells function           mechanism   L      The readline library must be linked into the shell to enable           this Feature   Notes to the table above  1  This feature was not in the original version  but has since become    almost standard  2  This feature is fairly new and so is often not found on many    versions of the shell  it is gradually making its way into    standard distribution  3  The Vi emulation of this shell is thought by many to be    incomplete  4  This feature is not standard but unofficial patches exist to    perform this  5  A version called  pdksh  is freely available  but does not have    the full functionality of the AT amp T version  6  This can be done via the shells programmable completion mechanism  7  Only by specifying a file via the ENV environment variable

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This question has frequently been nominated as a canonical for people who try to use sh and are surprised that it s not behaving the same as bash   Here s a quick rundown of common misunderstandings and pitfalls  First off  you should understand what to expect   If you run your script with sh scriptname  or run it with scriptname and have    bin sh in the shebang line  you should expect POSIX sh behavior  If you run your script with bash scriptname  or run it with scriptname and have    bin bash  or the local equivalent  in the shebang line  you should expect Bash behavior   Having a correct shebang and running the script by typing just the script name  possibly with a relative or full path  is generally the preferred solution   In addition to a correct shebang  this requires the script file to have execute permission  chmod a x scriptname   So  how do they actually differ  The Bash Reference manual has a section which attempts to enumerate the differences but some common sources of confusion include     is not available in sh  only   which is more clunky and limited   See also Difference between single and double square brackets in Bash sh does not have arrays  Some Bash keywords like local  source  function  shopt  let  declare  and select are not portable to sh   Some sh implementations support e g  local   Bash has many C-style syntax extensions like the three-argument for  i 0 i lt  3 i     loop     increment assignment  etc  The   string nwith tC aescapes  feature is tentatively accepted for POSIX  meaning it works in Bash now  but will not yet be supported by sh on systems which only adhere to the current POSIX specification  and likely will not for some time to come   Bash supports  lt  lt  lt  here strings   Bash has    png jpg  and  0  12  brace expansion    refers to  HOME only in Bash  and more generally  username to the home directory of username  This is in POSIX  but may be missing from some pre-POSIX  bin sh implementations  Bash has process substitution with  lt  cmd  and  gt  cmd   Bash has Csh-style convenience redirection aliases like  amp   for 2 gt  amp 1   and  amp  gt  for  gt      2 gt  amp 1 Bash supports coprocesses with  lt  gt  redirection  Bash features a rich set of expanded non-standard parameter expansions such as   substring 1 2     variable pattern replacement   case conversion  etc  Bash has significantly extended facilities for shell arithmetic  though still no floating-point support   There is an obsolescent legacy   expression  syntax which however should be replaced with POSIX arithmetic    expression   syntax   Some legacy pre-POSIX sh implementations may not support that  though   Several built-in commands have options which are not portable  like type -a  printf -v  and the perennial echo -e  Magic variables like  RANDOM   SECONDS   PIPESTATUS    and  FUNCNAME are Bash extensions  Syntactic differences like export variable value and    quot x quot      quot y quot    which are not portable  export variable should be separate from variable assignment  and portable string comparison in         uses a single equals sign   Many  many Bash-only extensions to enable or disable optional behavior and expose internal state of the shell  Many  many convenience features for interactive use which however do not affect script behavior   Remember  this is an abridged listing   Refer to the reference manual for the full scoop  and http   mywiki wooledge org Bashism for many good workarounds  and or try http   shellcheck net  which warns for many Bash-only features  A common error is to have a    bin bash shebang line  but then nevertheless using sh scriptname to actually run the script  This basically disables any Bash-only functionality  so you get syntax errors e g  for trying to use arrays   The shebang line is syntactically a comment  so it is simply ignored in this scenario   Unfortunately  Bash will not warn when you try to use these constructs when it is invoked as sh  It doesn t completely disable all Bash-only functionality  either  so running Bash by invoking it as sh is not a good way to check if your script is properly portable to ash dash POSIX sh or variants like Heirloom sh

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The Differences in as easy as much possible  After having a basic understanding  the other comments posted above will be easier to catch   Shell -  Shell  is a program  which facilitates the interaction between the user and the operating system  kernel   There are many shell implementation available  like sh  bash  csh  zsh   etc   Using any of the Shell programs  we will be able to execute commands that are supported by that shell program   Bash - It derived from Bourne-again Shell  Using this program  we will be able to execute all the commands specified by Shell  Also  we will be able to execute some commands that are specifically added to this program  Bash has backward compatibility with sh   Sh - It derived from Bourne Shell   sh  supports all the commands specified in the shell  Means  Using this program  we will be able to execute all the commands specified by Shell    For more information  do  - https   man cx sh  - https   man cx bash

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Linux operating system offers different types of shell  Though shells have many commands in common  each type has unique features  Let   s study different kind of mostly used shells   Sh shell   Sh shell is also known as Bourne Shell  Sh shell is the first shell developed for Unix computers by Stephen Bourne at AT amp T s Bell Labs in 1977  It include many scripting tools   Bash shell    Bash shell stands for Bourne Again Shell  Bash shell is the default shell in most linux distribution and substitute for Sh Shell  Sh shell will also run in the Bash shell    Bash Shell can execute the vast majority of Sh shell scripts without modification and provide commands line editing feature also

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bash and sh are two different shells  Basically bash is sh  with more features and better syntax  Most commands work the same  but they are different Bash  bash  is one of many available  yet the most commonly used  Unix shells  Bash stands for  Bourne Again SHell  and is a replacement improvement of the original Bourne shell  sh    Shell scripting is scripting in any shell  whereas Bash scripting is scripting specifically for Bash  In practice  however   shell script  and  bash script  are often used interchangeably  unless the shell in question is not Bash   Having said that  you should realize  bin sh on most systems will be a symbolic link and will not invoke sh  In Ubuntu  bin sh used to link to bash  typical behavior on Linux distributions  but now has changed to linking to another shell called dash  I would use bash  as that is pretty much the standard  or at least most common  from my experience   In fact  problems arise when a bash script will use    bin sh because the script-maker assumes the link is to bash when it doesn t have to be

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TERMINAL    program s  that put a window up xterm  rxvt  konsole  kvt  gnome-terminal  nxterm  and eterm    SHELL   Is a program that runs in the terminal Shell is both a command interpreter and a programming language Shell is simply a macro processor that executes commands   Macro processor means functionality where text and symbols are expanded to create larger expressions     SH Vs  BASH  SH    SHell  Is a specific shell a command interpreter and a programming language Predecessor of BASH   BASH    Bourne-Again SHell  Is a specific shell a command interpreter and a programming language Has sh functionality and more Successor of SH BASH is the default SHELL   REFERENCE MATERIAL   SHELL gnu org      At its base  a shell is simply a macro processor that executes   commands  The term macro processor means functionality where text and   symbols are expanded to create larger expressions       A Unix shell is both a command interpreter and a programming language    As a command interpreter  the shell provides the user interface to the   rich set of GNU utilities  The programming language features allow   these utilities to be combined  Files containing commands can be   created  and become commands themselves  These new commands have the   same status as system commands in directories such as  bin  allowing   users or groups to establish custom environments to automate their   common tasks       Shells may be used interactively or non-interactively  In interactive   mode  they accept input typed from the keyboard  When executing   non-interactively  shells execute commands read from a file       A shell allows execution of GNU commands  both synchronously and   asynchronously  The shell waits for synchronous commands to complete   before accepting more input  asynchronous commands continue to execute   in parallel with the shell while it reads and executes additional   commands  The redirection constructs permit fine-grained control of   the input and output of those commands  Moreover  the shell allows   control over the contents of commands    environments       Shells also provide a small set of built-in commands  builtins    implementing functionality impossible or inconvenient to obtain via   separate utilities  For example  cd  break  continue  and exec cannot   be implemented outside of the shell because they directly manipulate   the shell itself  The history  getopts  kill  or pwd builtins  among   others  could be implemented in separate utilities  but they are more   convenient to use as builtin commands  All of the shell builtins are   described in subsequent sections       While executing commands is essential  most of the power  and   complexity  of shells is due to their embedded programming languages    Like any high-level language  the shell provides variables  flow   control constructs  quoting  and functions       Shells offer features geared specifically for interactive use rather   than to augment the programming language  These interactive features   include job control  command line editing  command history and   aliases  Each of these features is described in this manual    BASH gnu org      Bash is the shell  or command language interpreter  for the GNU   operating system  The name is an acronym for the    Bourne-Again SHell       a pun on Stephen Bourne  the author of the direct ancestor of the   current Unix shell sh  which appeared in the Seventh Edition Bell Labs   Research version of Unix       Bash is largely compatible with sh and incorporates useful features   from the Korn shell ksh and the C shell csh  It is intended to be a   conformant implementation of the IEEE POSIX Shell and Tools portion of   the IEEE POSIX specification  IEEE Standard 1003 1   It offers   functional improvements over sh for both interactive and programming   use       While the GNU operating system provides other shells  including a   version of csh  Bash is the default shell  Like other GNU software    Bash is quite portable  It currently runs on nearly every version of   Unix and a few other operating systems - independently-supported ports   exist for MS-DOS  OS 2  and Windows platforms

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