Touch files in a directory recursively

Using the following command is able to touch files recursively:

find . -print0 | xargs -r0 touch

where . is the current directory and the option r of xargs is specific for GNU xargs.

Bash Script-exec

The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

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NAME

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

SYNOPSIS

exec [command [argument ...]]

DESCRIPTION

The exec utility shall open, close, and/or copy file descriptors as specified by any redirections as part of the command.
If exec is specified without command or arguments, and any file descriptors with numbers greater than 2 are opened with associated redirection statements, it is unspecified whether those file descriptors remain open when the shell invokes another utility. Scripts concerned that child shells could misuse open file descriptors can always close them explicitly, as shown in one of the following examples.
If exec is specified with command, it shall replace the shell with command without creating a new process. If arguments are specified, they shall be arguments to command. Redirection affects the current shell execution environment.

OPTIONS

None.

OPERANDS

See the DESCRIPTION.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

None.

ASYNCHRONOUS EVENTS

Default.

STDOUT

Not used.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

If command is specified, exec shall not return to the shell; rather, the exit status of the process shall be the exit status of the program implementing command, which overlaid the shell. If command is not found, the exit status shall be 127. If command is found, but it is not an executable utility, the exit status shall be 126. If a redirection error occurs (see Consequences of Shell Errors ), the shell shall exit with a value in the range 1-125. Otherwise, exec shall return a zero exit status.

CONSEQUENCES OF ERRORS

Default.

---

The following sections are informative.

APPLICATION USAGE

None.

EXAMPLES

Open readfile as file descriptor 3 for reading:

exec 3< readfile

Open writefile as file descriptor 4 for writing:

exec 4> writefile

Make file descriptor 5 a copy of file descriptor 0:

exec 5<&0

Close file descriptor 3:

exec 3<&-

Cat the file maggie by replacing the current shell with the cat utility:

exec cat maggie

RATIONALE

Most historical implementations were not conformant in that:

foo=bar exec cmd

did not pass foo to cmd.

FUTURE DIRECTIONS

None.

SEE ALSO

Special Built-In Utilities

CHANGE HISTORY

Issue 6

IEEE Std 1003.1-2001/Cor 1-2002, item XCU/TC1/D6/5 is applied so that the reference page sections use terms as described in the Utility Description Defaults ( Utility Description Defaults ). No change in behavior is intended.

End of informative text.

 

Refer to: http://pubs.opengroup.org/onlinepubs/009695399/utilities/exec.html

Shell Script

When I was using Bash Shell script to extract the available free space in the hard driver, there are several things that are worth to be noticed.

1. The importance to declare variables.

There is a segment fault when I was trying to invoke the C program from the script if the arguments that are forwarded to the C program is not declared at the beginning of the script. Next step should be to make clear what is the meaning of the parameters when the declaration was made. Those are called "variable attributes" (Burth, 73)

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Just found that the arguments can only be declared to be read-only......

       If I did not use the declaration, there will be a segment fault. However, today I confirm that if I lost a argument of shell script such as the path of the experiment it will also lead to a segment fault!


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I just found the main problem within this issue. The structure has problem:

df | while read content
    do
        echo ${content}
        temp=${content}
   done

echo ${content}
echo ${temp}

The situation is the same with the following snippet of code:

df | read content

echo ${content}

It can not  read anything after "df".