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Is it possible to call os system calls like open, close etc from a shell script? I tried googling but it takes me in the wrong direction of using "system()" command. Can some one help on this?
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The shell is a command interpreter that provides Unix users with an interface to the underlying operating system. It interprets user commands and executes them as independent processes. The shell also allows users to code an interpretive script using simple programming constructs such as if, while and for.
By default it will run in the Bourne shell (that would be /bin/sh ). os.
Shells directly make system calls to execute these commands, instead of forking a child process to handle them. This assignment consists of two parts.
$() means: "first evaluate this, and then evaluate the rest of the line". Ex : echo $(pwd)/myFile.txt. will be interpreted as echo /my/path/myFile.txt. On the other hand ${} expands a variable.
Many syscalls are accessible, but only via the native shell mechanisms, rather than being able to directly specify exact parameters. For instance:
exec 4>outfile
calls:
open("outfile", O_WRONLY|O_CREAT|O_APPEND, 0666) = 3
dup2(3, 4)
(with 3 being replaced by the next available descriptor), and
exec 4<&-
calls:
close(4)
Some shells, such as bash, allow additional builtins to be added through loadable modules (see the enable builtin, used to load such modules); if you really needed functionality not provided upstream, you could potentially implement it that way.
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