Shell Script – Redirecting to /dev/null

nullshell-script

I'm reading an example bash shell script:

#!/bin/bash

# This script makes a backup of my home directory.

cd /home

# This creates the archive
tar cf /var/tmp/home_franky.tar franky > /dev/null 2>&1

# First remove the old bzip2 file.  Redirect errors because this generates some if the archive
# does not exist.  Then create a new compressed file.
rm /var/tmp/home_franky.tar.bz2 2> /dev/null
bzip2 /var/tmp/home_franky.tar

# Copy the file to another host - we have ssh keys for making this work without intervention.
scp /var/tmp/home_franky.tar.bz2 bordeaux:/opt/backup/franky > /dev/null 2>&1

# Create a timestamp in a logfile.
date >> /home/franky/log/home_backup.log
echo backup succeeded >> /home/franky/log/home_backup.log

I'm trying to understand the use of /dev/null 2>&1 here. At first, I thought this script uses /dev/null in order to gracefully ignore errors, without causing the script to crash (kind of like try catch exception handling in programming languages). Because I don't see how using tar to compress a directory into a tar file could possibly cause any type of errors.

Best Answer

No, this will not prevent the script from crashing. If any errors occur in the tar process (e.g.: permission denied, no such file or directory, ...) the script will still crash.

This is because of using > /dev/null 2>&1 will redirect all your command output (both stdout and stderr) to /dev/null, meaning no outputs are printed to the terminal.

By default:

stdin  ==> fd 0
stdout ==> fd 1
stderr ==> fd 2

In the script, you use > /dev/null causing:

stdin  ==> fd 0
stdout ==> /dev/null
stderr ==> fd 2

And then 2>&1 causing:

stdin  ==> fd 0
stdout ==> /dev/null
stderr ==> stdout

Related Solutions

Mv a file to /dev/null breaks dev/null

Looking at the source code for mv, http://www.opensource.apple.com/source/file_cmds/file_cmds-220.7/mv/mv.c :

/*
 * If rename fails because we're trying to cross devices, and
 * it's a regular file, do the copy internally; otherwise, use
 * cp and rm.
 */
if (lstat(from, &sb)) {
    warn("%s", from);
    return (1);
}
return (S_ISREG(sb.st_mode) ?
    fastcopy(from, to, &sb) : copy(from, to));

...

int
fastcopy(char *from, char *to, struct stat *sbp)
{
...
while ((to_fd =
    open(to, O_CREAT | O_EXCL | O_TRUNC | O_WRONLY, 0)) < 0) {
        if (errno == EEXIST && unlink(to) == 0)
            continue;
        warn("%s", to);
        (void)close(from_fd);
        return (1);
}

In the first pass through the while loop, open(to, O_CREAT | O_EXCL | O_TRUNC | O_WRONLY, 0) will fail with EEXIST. Then /dev/null will be unlinked, and the loop repeated. But as you pointed out in your comment, regular files can't be created in /dev, so on the next pass through the loop, open(to, O_CREAT | O_EXCL | O_TRUNC | O_WRONLY, 0) is still going to fail.

I'd file a bug report with Apple. The mv source code is mostly unchanged from the FreeBSD version, but because OSX's devfs has that non-POSIX behavior with regular files, Apple should fix their mv.

Bash – cd to a name-unknown directory in a known path

Solution

If you know for sure there's exactly one directory and nothing else (no other folder and no other file) in the current directory, you can enter that directory with

cd *

If you know that there is only one directory, but there might be non-directory and non-symlink-to-directory files in the current directory, you can use

cd */.

to select only the directory.

Explanation

A single * is expanded by the shell to a list of all file names (which includes subdirectories) in the current directory (excluding hidden files, especially the directories . and .. which are always there). If the only thing in the current directory is the subdirectory to enter, this will expand to

cd unknown_folder_name

and thus do what you want.

The trick with */. is that this expands to the "self directory" (named .) in any subdirectory, which is of course the subdirectory itself. Since file names can never contain a /, and by assumption there's only one directory, the only name it expands to will be unknown_folder_name/., which is of course the same directory as unknown_folder_name.