Bash – The files with the extension bash and sh

bashshellshell-script

I made some researches about the files with bash and sh extensions. Most of the people and resources say that if a file has bash extension, then it contains bash scripts. Likewise, the file with sh extensions contains sh scripts.

However, I cannot find the differences between bash and sh scripting. There are some courses and articles which aim to teach the people to write scripts on shell, and all of them has the title shell scripting.

In this point, which one does shell scripting correspond to ?

Bash Scripting or Sh scripting.

What I try to understand is what is the difference between bash and sh scripting.

Best Answer

File names in POSIXland don't have "extensions". A . in a filename is no different from any other character and has no specific meaning other than those that might be attributed to them by meatbags such as ourselves.

One could hope that any file with a name ending in .bash would be a script meant to be executed via the bash shell, but there is no guarantee of this.

Indeed, it's quite common to give all shell scripts a suffix of .sh no matter which interpreter is intended for their use, as the shebang line should properly specify which shell should be used to execute such a file.

sh and bash are two different, but related, shells; two amongst many others such as ksh, csh, zsh, fish, ash, dash, and yet more others.

Each shell has its own syntax, capabilities, mannerisms, and foibles; some shells are largely compatible with each other (generally any script written for sh can also be run in bash or many other shells), but some are not.

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Bash – In Bash, are if [ -z “$1” ] and if [ “$1” = “” ] the same

[ "$1" = "" ] and [ -z "$1" ] are exactly equivalent in bash and other POSIX-compliant shells. (Note that there must be a space on each side of the brackets, unless there is a non-word-constituent character like ;.)

[ is a shell built-in like any other; in fact it can also be spelled test (the only difference between the two is that [ requires a ] as the last argument). So if you run [ "$1" = "" ] with $1 expanding to -z, the test operator sees three arguments: -z, = and the empty string. Some older Bourne shells sometimes threw parse errors when an operand looked like an operator in this way, even if there was no ambiguity in the complete expression. I don't know if any version did in fact have trouble with this particular expression, but more complex expressions could throw them off. There may also have been versions that had trouble with empty words; these would not have supported [ -z "$1" ] either. A common shell idiom is [ x"$1" = x"" ]. It avoids any risk of having operands parsed as operators because no operator starts with a letter.

In ksh, bash and zsh, you can use the double bracket syntax, [[ -z $1 ]]. This newer syntax (it's from the late 1980s rather than the mid-1970s) eliminates the risk of having operands parsed as operators by using a special syntactic construct rather than an ordinary built-in. Operators must appear literally, unquoted within the double brackets, and you don't need to double quote variable expansions.

Bash – the difference between the commands builtin cd and cd

The cd command is a built-in, so normally builtin cd will do the same thing as cd. But there is a difference if cd is redefined as a function or alias, in which case cd will call the function/alias but builtin cd will still change the directory (in other words, will keep the built-in accessible even if clobbered by a function.)

For example:

user:~$ cd () { echo "I won't let you change directories"; }
user:~$ cd mysubdir
I won't let you change directories
user:~$ builtin cd mysubdir
user:~/mysubdir$ unset -f cd  # undefine function

Or with an alias:

user:~$ alias cd='echo Trying to cd to'
user:~$ cd mysubdir
Trying to cd to mysubdir
user:~$ builtin cd mysubdir
user:~/mysubdir$ unalias cd  # undefine alias

Using builtin is also a good way to define a cd function that does something and changes directory (since calling cd from it would just keep calling the function again in an endless recursion.)

For example:

user:~ $ cd () { echo "Changing directory to ${1-home}"; builtin cd "$@"; }
user:~ $ cd mysubdir
Changing directory to mysubdir
user:~/mysubdir $ cd
Changing directory to home
user:~ $ unset -f cd  # undefine function

Best Answer

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Bash – In Bash, are if [ -z “$1” ] and if [ “$1” = “” ] the same

Bash – the difference between the commands builtin cd and cd

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