Shell – exec < filename - what does this do

io-redirectionshellshell-script

I have found the following code snippet (sh):

    TESTFILE=$TEST_PATH/test.out
    if [ -f $TESTFILE ]
    then
        exec < $TESTFILE
    else
        echo "$TEST_PATH: no test.out file"
    fi

the focus of my question is, what is this line doing? (the rest i understand!)

        exec < $TESTFILE

The file is not executable, nor is another process invoked, and i am having no luck finding what exec does on a plain text file.

Best Answer

From the POSIX description of exec:

exec - execute commands and open, close, or copy file descriptors

In this case, there is no command, so only file descriptors are modified. Normally, redirections you write on the command line affect the command on that line. exec is how you do redirections on the currently executing shell.

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Shell – printing colored text using echo

They are part of the 'Sequence elements' of ANSI escape sequences also known as ECMA-48 CSI sequences which were originally adopted in 1976

More specifically the [ is the 'Control Sequence Introducer'

Type man console_codes in almost any *nix distro to see the codes and some docs.

Shell – What Happens When Executing a File

The definitive answer to "how programs get run" on Linux is the pair of articles on LWN.net titled, surprisingly enough, How programs get run and How programs get run: ELF binaries. The first article addresses scripts briefly. (Strictly speaking the definitive answer is in the source code, but these articles are easier to read and provide links to the source code.)

A little experimentation show that you pretty much got it right, and that the execution of a file containing a simple list of commands, without a shebang, needs to be handled by the shell. The execve(2) manpage contains source code for a test program, execve; we'll use that to see what happens without a shell. First, write a testscript, testscr1, containing

#!/bin/sh

pstree

and another one, testscr2, containing only

pstree

Make them both executable, and verify that they both run from a shell:

chmod u+x testscr[12]
./testscr1 | less
./testscr2 | less

Now try again, using execve (assuming you built it in the current directory):

./execve ./testscr1
./execve ./testscr2

testscr1 still runs, but testscr2 produces

execve: Exec format error

This shows that the shell handles testscr2 differently. It doesn't process the script itself though, it still uses /bin/sh to do that; this can be verified by piping testscr2 to less:

./testscr2 | less -ppstree

On my system, I get

    |-gnome-terminal--+-4*[zsh]
    |                 |-zsh-+-less
    |                 |     `-sh---pstree

As you can see, there's the shell I was using, zsh, which started less, and a second shell, plain sh (dash on my system), to run the script, which ran pstree. In zsh this is handled by zexecve in Src/exec.c: the shell uses execve(2) to try to run the command, and if that fails, it reads the file to see if it has a shebang, processing it accordingly (which the kernel will also have done), and if that fails it tries to run the file with sh, as long as it didn't read any zero byte from the file:

        for (t0 = 0; t0 != ct; t0++)
            if (!execvebuf[t0])
                break;
        if (t0 == ct) {
            argv[-1] = "sh";
            winch_unblock();
            execve("/bin/sh", argv - 1, newenvp);
        }

bash has the same behaviour, implemented in execute_cmd.c with a helpful comment (as pointed out by taliezin):

Execute a simple command that is hopefully defined in a disk file somewhere.

fork ()

connect pipes

look up the command

do redirections

execve ()

If the execve failed, see if the file has executable mode set. If so, and it isn't a directory, then execute its contents as a shell script.

POSIX defines a set of functions, known as the exec(3) functions, which wrap execve(2) and provide this functionality too; see muru's answer for details. On Linux at least these functions are implemented by the C library, not by the kernel.

Best Answer

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