shell-script headers bin sh vs bin csh

Question

Why do all script files start with      bin sh   or with      bin csh   Is that required  What s the purpose of this  And what s the difference between the two

User · Accepted Answer

This is known as a Shebang   http   en wikipedia org wiki Shebang  Unix        interpreter  optional-arg    A shebang is only relevant when a script has the execute permission  e g  chmod u x script sh    When a shell executes the script it will use the specified interpreter   Example      bin bash   file  foo sh echo 1    chmod u x foo sh     foo sh   1

User · Answer

The    line tells the kernel  specifically  the implementation of the execve system call  that this program is written in an interpreted language  the absolute pathname that follows identifies the interpreter   Programs compiled to machine code begin with a different byte sequence -- on most modern Unixes  7f 45 4c 46    ELF  that identifies them as such   You can put an absolute path to any program you want after the     as long as that program is not itself a    script   The kernel rewrites an invocation of    script arg1 arg2 arg3       where   script starts with  say      usr bin perl  as if the command line had actually been   usr bin perl   script arg1 arg2 arg3   Or  as you have seen  you can use     bin sh to write a script intended to be interpreted by sh   The    line is only processed if you directly invoke the script    script on the command line   the file must also be executable  chmod  x script    If you do sh   script the    line is not necessary  and will be ignored if present   and the file does not have to be executable   The point of the feature is to allow you to directly invoke interpreted-language programs without having to know what language they are written in    Do grep        usr bin   -- you will discover that a great many stock programs are in fact using this feature    Here are some rules for using this feature    The    must be the very first two bytes in the file   In particular  the file must be in an ASCII-compatible encoding  e g  UTF-8 will work  but UTF-16 won t  and must not start with a  byte order mark   or the kernel will not recognize it as a    script  The path after    must be an absolute path  starts with      It cannot contain space  tab  or newline characters  It is good style  but not required  to put a space between the    and the     Do not put more than one space there  You cannot put shell variables on the    line  they will not be expanded  You can put one command-line argument after the absolute path  separated from it by a single space   Like the absolute path  this argument cannot contain space  tab  or newline characters  Sometimes this is necessary to get things to work      usr bin awk -f   sometimes it s just useful      usr bin perl -Tw    Unfortunately  you cannot put two or more arguments after the absolute path  Some people will tell you to use     usr bin env interpreter instead of     absolute path to interpreter  This is almost always a mistake  It makes your program s behavior depend on the  PATH variable of the user who invokes the script   And not all systems have env in the first place  Programs that need setuid or setgid privileges can t use     they have to be compiled to machine code    If you don t know what setuid is  don t worry about this     Regarding csh  it relates to sh roughly as Nutrimat Advanced Tea Substitute does to tea   It has  or rather had  modern implementations of sh have caught up  a number of advantages over sh for interactive usage  but using it  or its descendant tcsh  for scripting is almost always a mistake   If you re new to shell scripting in general  I strongly recommend you ignore it and focus on sh   If you are using a csh relative as your login shell  switch to bash or zsh  so that the interactive command language will be the same as the scripting language you re learning

User · Answer

This defines what shell  command interpreter  you are using for interpreting running your script  Each shell is slightly different in the way it interacts with the user and executes scripts  programs     When you type in a command at the Unix prompt  you are interacting with the shell   E g      bin csh refers to the C-shell   bin tcsh the t-shell   bin bash the bash shell  etc   You can tell which interactive shell you are using the    echo  SHELL   command  or alternatively   env   grep -i shell   You can change your command shell with the chsh command   Each has a slightly different command set and way of assigning variables and its own set of programming constructs  For instance the if-else statement with bash looks different that the one in the C-shell    This page might be of interest as it  translates  between bash and tcsh commands syntax   Using the directive in the shell script allows you to run programs using a different shell  For instance I use the tcsh shell interactively  but often run bash scripts using  bin bash in the script file   Aside   This concept extends to other scripts too  For instance if you program in Python you d put       usr bin python   at the top of your Python program

[shell] shell-script headers (#!/bin/sh vs #!/bin/csh)

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