Ubuntu – Why does redirection to the same file truncate the file

bashcommand line

Normally we use a different file to redirect the output.

For example :

cat < first > temp

In this command the contents of first are redirected to temp instead of the standard output.

Then why does it truncates the file if I use the same file name?

Why can't it overwrite the same file?

cat < first > first

Best Answer

When you use I/O redirection like that, both the "input" & "output" file are opened by the shell before the command is executed. And opening a file for overwriting is the same as truncating it before writing. The result: cat sees an empty file on input...

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Ubuntu – Why does “man x-terminal-emulator” return the output of “man gnome-terminal”

x-terminal-emulator is a virtual package. The terminal emulator is configured by Debian's alternative system.

On Ubuntu you can easily follow this symlink construction, e.g. for gnome-terminal:

$ which x-terminal-emulator
/usr/bin/x-terminal-emulator
$ ls -lF /usr/bin/x-terminal-emulator
/usr/bin/x-terminal-emulator -> /etc/alternatives/x-terminal-emulator*
$ ls -al /etc/alternatives/x-terminal-emulator
/etc/alternatives/x-terminal-emulator -> /usr/bin/gnome-terminal.wrapper*

Now regarding the man pages, update-alternatives also links the man page of gnome-terminal to x-terminal-emulator as explained in the update-alternatives manual:

   It is often useful for a number of alternatives to be synchronized,  so
   that they are changed as a group; for example, when several versions of
   the  vi(1)  editor  are  installed,  the   man   page   referenced   by
   /usr/share/man/man1/vi.1 should correspond to the executable referenced
   by /usr/bin/vi.  update-alternatives handles this by  means  of  master
   and  slave links; when the master is changed, any associated slaves are
   changed too.  A master link and its associated slaves make  up  a  link
   group.

Source

Ubuntu – In what order does the shell execute commands and stream redirection

When you run <command> 2>&1 > file.txt stderr is redirected by 2>&1 to where stdout currently goes, your terminal. After that, stdout is redirected to the file by >, but stderr is not redirected with it, so stays as terminal output.

With <command> > file.txt 2>&1 stdout is first redirected to the file by >, then 2>&1 redirects stderr to where stdout is going, which is the file.

It may seem counter intuitive to start with, but when you think of the redirections in this way, and remember that they are processed from left to right it makes much more sense.

Best Answer

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Ubuntu – Why does “man x-terminal-emulator” return the output of “man gnome-terminal”

Ubuntu – In what order does the shell execute commands and stream redirection

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