Why do you need to put bin bash at the beginning of a script file

Question

I have made Bash scripts before and they all ran fine without    bin bash at the beginning   What s the point of putting it in  Would things be any different   Also  how do you pronounce    I know that   is pronounced as  bang    How is    pronounced

User · Answer

It can be useful to someone that uses a different system that does not have that library readily available   If that is not declared and you have some functions in your script that are not supported by that system  you should declare   bin bash  I ve ran into this problem before at work and now I just include it as a practice

User · Answer

The operating system takes default shell to run your shell script  so mentioning shell path at the beginning of script  you are asking the OS to use that particular shell  It is also useful for portability

User · Answer

To be more precise the shebang     when it is the first two bytes of an executable  x mode  file  is interpreted by the execve 2  system call  which execute programs   But POSIX specification for execve don t mention the shebang   It must be followed by a file path of an interpreter executable  which BTW could even be relative  but most often is absolute    A nice trick  or perhaps not so nice one  to find an interpreter  e g  python  in the user s  PATH is to use the env program  always at  usr bin env on all Linux  like e g       usr bin env python   Any ELF executable can be an interpreter  You could even use    bin cat or    bin true if you wanted to   but that would be often useless

User · Answer

Every distribution has a default shell  Bash is the default on the majority of the systems  If you happen to work on a system that has a different default shell  then the scripts might not work as intended if they are written specific for Bash    Bash has evolved over the years taking code from ksh and sh    Adding    bin bash as the first line of your script  tells the OS to invoke the specified shell to execute the commands that follow in the script       is often referred to as a  hash-bang    she-bang  or  sha-bang

User · Answer

The shebang is a directive to the loader to use the program which is specified after the    as the interpreter for the file in question when you try to execute it  So  if you try to run a file called foo sh which has    bin bash at the top  the actual command that runs is  bin bash foo sh  This is a flexible way of using different interpreters for different programs  This is something implemented at the system level and the user level API is the shebang convention    It s also worth knowing that the shebang is a magic number - a human readable one that identifies the file as a script for the given interpreter    Your point about it  working  even without the shebang is only because the program in question is a shell script written for the same shell as the one you are using  For example  you could very well write a javascript file and then put a     usr bin js  or something similar  to have a javascript  Shell script

User · Answer

Also you will see some other parameters after    bin bash   for example    bin bash -v -x read this to get more idea  https   unix stackexchange com questions 124272 what-do-the-arguments-v-and-x-mean-to-bash

User · Answer

It s called a shebang  In unix-speak    is called sharp  like in music  or hash  like hashtags on twitter   and   is called bang   You can actually reference your previous shell command with     called bang-bang   So when put together  you get haSH-BANG  or shebang   The part after the    tells Unix what program to use to run it  If it isn t specified  it will try with bash  or sh  or zsh  or whatever your  SHELL variable is  but if it s there it will use that program  Plus    is a comment in most languages  so the line gets ignored in the subsequent execution

User · Answer

It s a convention so the  nix shell knows what kind of interpreter to run   For example  older flavors of ATT defaulted to sh  the Bourne shell   while older versions of BSD defaulted to csh  the C shell    Even today  where most systems run bash  the  Bourne Again Shell    scripts can be in bash  python  perl  ruby  PHP  etc  etc   For example  you might see    bin perl or    bin perl5   PS  The exclamation mark     is affectionately called  bang    The shell comment symbol     is sometimes called  hash    PPS  Remember - under  nix  associating a suffix with a file type is merely a convention  not a  rule    An executable can be a binary program  any one of a million script types and other things as well  Hence the need for    bin bash

User · Answer

It is called a shebang  It consists of a number sign and an exclamation point character       followed by the full path to the interpreter such as  bin bash  All scripts under UNIX and Linux execute using the interpreter specified on a first line

[linux] Why do you need to put #!/bin/bash at the beginning of a script file?

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