Linux Commando

How to insert a file at a specific line and column

2010-04-11T14:42:00.000-07:00

My objective is to insert the complete contents of a text file at a specific row and column of another text file.

If we are merely concerned with inserting after a specific line, it can be readily achieved with a number of Linux tools. For example, to insert file1.txt after the second line of file2.txt, any of the following commands will do:

$ sed -i '2r file1.txt' file2.txt

$ awk '{print} NR==2 {while (getline < "file1.txt") print}' file2.txt

Unlike the previous sed command which modifies file2 in-line, the above awk command writes the desired output to standard output only.

$ emacs -batch +3 file2.txt --insert file1.txt -f save-buffer -kill

This uses the batch capability of the emacs text editor. The command opens file2 at line 3, inserts file1, saves and then exits.

$ vi +2 file2.txt << DELIM
> :r file1.txt
> :wq
> DELIM
Vim: Warning: Input is not from a terminal

Note that after you enter the first line ("vi +2 ..."), you will be prompted for more input. At that time, you enter the next three lines (:r, :wq, DELIM)

While Linux has many utilities to manipulate text (sed, perl, awk, cut, python), the easiest way I can think of to accomplish my objective to insert at a target line and column is using emacs.

$ emacs -batch -Q +2:3 file2.txt --insert file1.txt -f save-buffer -kill 2>/dev/null

The key is +2:3 which directs the editor to open the file at line 2 column 3.

Two other components in the above emacs command warrant some explanation. First, -Q means quick startup. Quick, because emacs won't load any init file, or any splash file. Second, the last part of the command pipes any standard error output from the emacs editor to the null device.

I don't doubt that sed, awk or perl can do the job. If you have a simple solution, please share with us via the comment feature of this web page. Many thanks.

Fun with Date Arithmetic

2009-11-14T22:20:00.000-08:00

We all know that the date command tells you the current time. Occasionally, you use the same command to set the time. That however becomes rarer these days with the advent of the ntp service that automatically synchronizes your computer's time with a super accurate public time server of your choice.

Various implementations of the date command are in use today. This article discusses the date command of the GNU coreutils package.

By default, the date command tells you the time now. The date command also lets you do some basic date addition and subtraction. This is achieved by specifying the -d option which displays a time that you entered as a parameter rather than now.

How many times have you asked yourself what the calendar date is N days ago? Just the other day, I needed to find out the precise date of 30 days ago in order to locate the proper log file.

$ date
Sat Nov 14 17:54:51 PST 2009
$ date -d -30days
Thu Oct 15 18:54:56 PDT 2009

Just like you use subtraction to calculate a back date, you use addition to calculate a forward date. The example below displays the date 30 days from today.

$ date -d +30days
Thu Oct 15 18:54:56 PDT 2009

Besides days as the unit, you can also manipulate years, months, hours, minutes, and seconds.

peter@tiger:~$ date -d +2months
Thu Jan 14 18:48:43 PST 2010

You can combine them together like this:

$ date -d +2months17days
Sun Jan 31 18:49:45 PST 2010

Note that all the above are calculated relative to today's date. What about queries like 10 days tomorrow?

$ date -d tomorrow+10days
date -d tomorrow+10days
Wed Nov 25 18:52:03 PST 2009

10 days yesterday?

$ date -d yesterday-10days
Tue Nov  3 18:53:07 PST 2009

You can also perform your arithmetic relative to a specific day, say January 21, 2010.

$ date -d '2010-01-21 + 2 weeks 3 days'
Sun Feb  7 00:00:00 PST 2010

Lastly, the date command also recognizes the day of weeks (Sunday, Monday, ...) and the 2 keywords "last" and "next".

$ date -d 'next tuesday + 1 day'
Wed Nov 18 00:00:00 PST 2009

I hope you have fun with this versatile tool.

How to move print jobs from one printer queue to another

2009-07-17T18:41:00.000-07:00

In the olden days, an entire office department shared one printer. At home, an entire household shared one printer. Nowadays, I have 2 printers at home (1 HP LaserJet and 1 Samsung Monochrome Laser printer).

With access to multiple printers come new opportunities and challenges. One of those opportunities is that if one printer goes down for whatever reason, you can route your print jobs to the surviving one. One challenge is that you need to manage multiple printer queues. With more than 1 printer, I often find myself printing to the wrong one. How do you route print jobs from 1 printer to another when the need arises?

Job Submitted to Wrong Printer

Let's first consider this situation: you just submit your print job to the default printer(mySamsung), but you realize after the fact that it is the wrong printer.

First, you should find out the print job number. Run the familiar lpq command.

$ lpq -P mySamsung
mySamsung is ready and printing
Rank    Owner   Job     File(s)              Total Size
active  peter   706     (i)                  100352 bytes
1st     peter   707     MoviesReview         228352 bytes
$

Note that given mySamsung is my default printer, I don't really need to explicitly specify the printer: lpq without any argument will do just fine.

Say job 707 is the one you want to move to the other printer.

The Linux command to do the actual migration is lpmove. Note that you need to run the command as root.


$ sudo lpmove mySamsung-707 myHP
$ lpq -P myHP
myHP is ready
Rank    Owner   Job     File(s)                 Total Size
active  peter   707     MoviesReview            228352  bytes

Note how I specified the job number to lpmove. The format is the printer name(mySamsung) and a dash(-), followed by the job number (707).

Printer Down

Suppose mySamsung is out of service for whatever reason. Then, you need to move all print jobs from mySamsung to myPrinter.


$ sudo lpmove mySamsung myHP
$

Search and replace line feeds using emacs

2009-07-02T16:30:00.000-07:00

As an avid emacs user for a decade plus, I am a little embarrassed to confess that I did not know for the longest time how to search for or replace line feed (newline) characters.

If you are only searching for run-of-the-mill ASCII strings such as abc, you just type C-s (control-S), and then the characters, and hit return.

Unfortunately, if your search string includes a line feed character, e.g., abc followed by a newline, I have some good news and some bad news for you.

The good news is that you use the same shortcut (C-s) just like you would normally to search for anything else.

The bad news is that the representation of what emacs understands to be a newline is not that intuitive ... even to geeks. Instead of something like \n which may mean something to developers, you need to enter C-q C-j (Control-Q, Control-J) for each line feed in the search string.

C-q is the shortcut for the quoted-insert command. The character you type immediately after C-q is escaped, i.e., its normal function is suppressed. C-j is the line feed character. Why the letter j, you ask? It turns out that the line feed character is represented by the decimal value 10 in the ASCII table, and j is the 10th letter in the English alphabet. Similarly, if you are searching for the Tab instead of the newline character, use C-q C-i to denote the Tab character.

If you want to replace occurrences of a string that includes one or more line feeds, the method is similar. Use the usual command replace-string or query-replace(M-%). Enter C-q C-j where you would expect to type the new line.

M-x replace-string (Hit return) abcC-qC-j (Hit return) def (Hit return)

loook: A light-weight text search tool for OpenOffice documents

2009-06-07T17:17:00.000-07:00

If you are happily running a desktop search tool such as Tracker or Beagle, read no further. You already have the search tool you need to search OpenOffice files and more. These desktop search tools build indexes to improve search performance, and are in general quite scalable. Yet, a desktop search tool may be an overkill if you are just searching for certain text string in a directory containing OpenOffice files.

loook is a no-frill, yet very functional text search tool for OpenOffice files. Unlike the aforementioned desktop search engines, it does not build an index of your OpenOffice files. Despite the lack of an index, searching a hierarchy of 75 OpenOffice documents only took a couple of seconds. That is good enough for my use.

loook composes of a single zipped-up python script which you can freely download and install. Unzip the downloaded file into a directory of your choice. On my Debian Etch system, I did not have to install any additional packages for loook to run.

Running the tool cannot be any simpler. Open a bash shell, and enter:

$ python somedir/loook.py

The Viewer field has the default value: soffice. This is the name of the OpenOffice executable. You don't need to change this field unless your version of OpenOffice has a different name or the executable is not in your default PATH.

What you do want to change are the following fields:

Search Path
Search Terms
Mode

Search Path

You navigate or type in the path of the directory containing the OpenOffice files you want to search. loook will recursively search the hierarchy starting from the specified directory. Unfortunately, you are limited to specifying one starting directory only.

Search Terms

You enter your search terms in this field. Note that loook supports Unicode. Non-English language users can search using UTF-8 characters.

Mode

The Mode determines whether a match requires all the search terms to be present in a document (AND search), any of the terms to be present (OR search), or the terms must appear together (PHRASE search).

You can pre-populate the search path and the search terms. Simply enter them as arguments to loook on the command line. For example,

$ python somedir/loook.py /home/peter/mydocs "Cisco Routers"

The search results are returned in the Matches field. Note that only the full path of the files are returned. It does not display the actual matches. You can double click the file path in the Matches field to directly open the file with OpenOffice.

loook does not have all the bells and whistles that you expect of a desktop search tool, with respect to both features and supported file formats. However, if all you want is a light-weight tool for searching OpenOffice documents in a directory structure, it is worth your time to try it out.

Is PHP supported on my Web Server?

2009-05-18T04:28:00.000-07:00

Recently, I had to install a PHP-based web application on my Apache web server.

My first question was: "Is PHP enabled on the web server?"

You can always ask the systems guy. But if you happen to be the systems guy, and you don't quite remember if you have installed PHP, here is how to find out.

Create a new file with a php suffix (say myphp.php)
Copy and paste the following code into the php file:
Save the file and upload it to a location on your web server that is accessible to the World Wide Web.
For expedience, upload it to the Apache document root directory which is /var/www on my Debian Etch server.
Open your web browser to the URL for that PHP file (http://localhost/myphp.php).

If your web server supports PHP, the browser will return with a page full of information about the PHP version and configuration. In addition, it will display information about your web server and your system in general.

On the contrary, if all you see is a blank page, then you know PHP is not supported.

Note that you must delete the myphp.php file from your web server after you obtain the necessary info. You don't want people to run the PHP script, and obtain sensitive information about your system.

More on Inserting Arguments from Previous Commands

2009-05-16T12:03:00.001-07:00

I previously blogged on using the shortcut Alt + dot to insert the last argument from the previous command.

Suppose you don't want the last argument. Instead, you want to insert the first, second, or third argument of a previous command.

No problem!

Alt + 1 + dot inserts the first argument of the last command. To key it in properly, hold the alt key, press the 1 key, and then the dot (".") key.

Similarly, alt + 2 + dot inserts the second command argument.

For example, you just executed this command.

$ ls -l /home/peter/somefile.txt   secondfile.txt
-rw-r--r-- 1 peter peter 8115 2007-12-19 21:03 somefile.txt
$

If you now type in cat and then alt + 2 + dot, it will insert the second argument from the last command (-l is the first).

$ cat /home/peter/somefile.txt

You can repeat the key sequence, and this will go back 1 command at a time, and insert the specified argument of that command.

If you want just the command itself, not an argument, type in alt + 0 + dot .

P.S. Related articles from this blog:

A Surefire Shortcut to Insert the Last Argument of the Last Command

Open a file from the command line using its default application

2009-05-08T21:59:00.000-07:00

Windows users are familiar with the concept of file association. When you double click a file (say, cisco.doc), Windows examines the file name extension (doc), and opens the file using the default program associated with that extension(Office).

Linux users can open files in a similar way in their X Window graphical user interface. But, if you want to open the file from the command-line, you need to type out at least the program name, oowriter, or do you?

$ oowriter cisco.doc

If GNOME is your window manager, use the gnome-open command as follow:

$ gnome-open cisco.doc

oowriter is automatically started up to open cisco.doc, if oowriter is indeed associated with the .doc file name extension.

For KDE users, use kde-open instead.

Alternatively, you can run the window-manager-neutral program called xdg-open. xdg-open is part of the xdg-utils package.


$ xdg-open cisco.doc

Find all (non-)empty files in a directory

2009-05-05T09:46:00.000-07:00

Creating an empty file in Linux is easy. If a-file does not exist, create the file and make it empty by simply:

$ touch a-file
$ ls -l a-file
-rw-r--r-- 1 peter peter 0 2009-05-02 20:15 a-file

Finding all empty files in a directory can also be done using a single command. Ditto for non-empty files.

Suppose you want to find all empty files in the directory /home/peter. The command is:

 $ find -L /home/peter -maxdepth 1  -type f -size 0

By default, the find command excludes symbolic files. Use the -L option to include them.

The expression -maxdepth 1 specifies that the maximum depth to which the search will drill is one only. By default, the find command will recursively go down the directory. A maximum depth of 1 means that you only want the files directly under /home/peter. Keep in mind that depth 0 is the level of the command line argument (/home/peter). You can use -maxdepth and -mindepth to finely control the depth levels you want included.

-type f means include regular files only. This is not strictly necessary for empty files (as opposed to non-empty ones) because all directories, even those with no files, are of non-empty size.

-size 0 is self-explanatory.

To find all non-empty files in the same directory, simply add ! before -size 0:

 $ find -L /home/peter -maxdepth 1  -type f ! -size 0

Note that -type f becomes necessary. Without this expression, subdirectories in /home/peter such as /home/peter/.Trash will appear in the output.

Most of the time, you don't just want to find the files: you want to do something with them. Suppose you want to remove all empty files in the directory. For safety, first run the find command by itself to list the files. Then with the xargs command as below.

 $ find -L /home/peter -maxdepth 1  -type f  -size 0 
/home/peter/file1.txt
/home/peter/file2.txt
$ find -L /home/peter -maxdepth 1  -type f  -size 0 -print0 |xargs -0 -r rm -f

For more information about how to use the find and xargs commands, see my earlier blog entry.

A surefire shortcut to Insert the Last Argument of the Last Command

2009-05-02T14:05:00.000-07:00

Sometimes, in the Linux command-line world, a seemingly trivial technique can turn out to be tremendously useful. Before I discover the Alt-dot(.) shortcut, I type !$ to insert the last argument of the previous command.


peter@tiger:~$ ls -l Windows_20081110102654.log
-rw-r--r-- 1 peter peter 808 2008-11-10 10:26 Windows_20081110102654.log
peter@tiger:~$ cat !$
cat Windows_20081110102654.log
.....
.....
peter@tiger:~$

Then, I discovered that typing Alt-dot achieves the same result. That is, press (and hold) the Alt key, then the dot key.

There are some advantages of using Alt-dot over !$. First, you can actually see the argument immediately and interactively. You can verify that is indeed what you want, edit it if necessary, before you continue the command-line input, and eventually hit Enter to execute the command. With !$, you better have a pretty good memory.

Another advantage is that you can repeatedly type Alt-dot. The net effect is that you scroll back in command history, and display the last argument of each successive command.

After assimilating the shortcut into my command-line work habit, I found that I have been using it a lot. Give it a try!

PS Related entries from this blog:

More on Inserting Arguments from Previous Commands

Auto Start Applications at Login to GNOME Desktop

2008-12-06T17:30:00.001-08:00

It may sound trivial, but for the longest time, I, being an avid CLI fan, did not configure GNOME to auto-start certain applications after login to desktop.

A couple of times, I actually went looking for some entry similar to Microsoft Windows Startup menu in the GNOME menu structures, but each time, I came up empty and frustrated.

Finally, I found out how, and I wrote it down. The following works for the GNOME Desktop 2.14.3 using the Debian Etch Linux distribution.

To configure GNOME to auto start an application on login,

Mouse to Desktop -> Preferences -> Sessions.

Click to select the Startup Programs Tab.
Click Add.
Enter the command, e.g., skype, and click OK.

Note: You may need to enter the full path for the command, (e.g., /etc/bin/skype) if the command is not in the command path.
Click Close to exit Sessions dialog box.

How to increase number of disk mounts before next fsck at system boot

2008-11-29T11:48:00.000-08:00

Many home users often power off their computers when they are not being used. Some do it to be green: turning idle computers off saves electricity and $$. Others do it for the extra security. To those who are hard-core Linux geeks, machine uptime is sacred, and voluntarily rebooting the machine is nothing but sacrilegious.

If you do reboot your machine from time to time, you most definitely have encountered a most annoying experience. Once in a while, while the computer is booting up, you see the message /dev/hdaN has reached maximal mount count, check forced. The check seems to take forever, and the system boot won't resume until the check is over.

The check refers to a file system check performed using the fsck command. For many Linux distributions, by default, the system will do a fsck check on a file system after it has been mounted 30 times, which means after 30 reboots. This is the maximum mount count before fsck is performed on the file system.

You can specify a maximum mount count for each individual partition on your hard drive. To find out the maximum mount count for /dev/hda1, execute this command as root:

$ tune2fs -l /dev/hda1 | grep 'Maximum mount count'
Maximum mount count:      30

Note that the tune2fs command is only applicable for ext2 and ext3 file systems.

The tune2fs command can also tell you how many times a file system has actually been mounted since the last fsck check.

$ tune2fs -l /dev/hda1 |grep 'Mount count'
Mount count:    17

To increase the maximum mount count, you will use the same tune2fs command but with the -c option.

Note that you should not modify the maximum mount count which is a file system parameter while the file system is mounted. The recommended way is to boot your system using a Linux Live CD, and then run tune2fs.

For me, I happened to have a Ubuntu 7.10 Live CD at my desk. I inserted the Live CD to boot up my system. Then, I opened a Terminal window, sudo -s, and executed the following commands.

First, I reminded myelf of how the /dev/hda disk is partitioned:

$ fdisk -l /dev/hda
Disk /dev/hda: 82.3 GB, 82348277760 bytes
255 heads, 63 sectors/track, 10011 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1   *           1          31      248976   83  Linux
/dev/hda2              32       10011    80164350    5  Extended
/dev/hda5              32       10011    80164318+  8e  Linux LVM

To increase the maximum mount count to 50 for /dev/hda1:

$ tune2fs -c 50 /dev/hda1
tune2fs 1.40.2 (12-Jul-2007)
Setting maximal mount count to 50

If there are more than 1 file system on your hard drive, you should stagger the maximum mount count for the different file systems so that they don't all trigger the lengthy fsck at the same time. For example, set the maximum mount count to 40, 50 and 60 for /dev/hda1, hda2, and hda3 respectively.

In the above case, /dev/hda5 is a physical LVM volume. You cannot run tune2fs on physical LVM volumes directly.

$ tune2fs -l /dev/hda5
tune2fs 1.40.2 (12-Jul-2007)
tune2fs: Bad magic number in super-block while trying to open /dev/hda5
Couldn't find valid filesystem superblock.

You need to run tune2fs against each logical LVM volume. To find out their names, cat /etc/fstab.

$ tune2fs -c 60 /dev/mapper/myhost-root 
tune2fs 1.40.2 (12-Jul-2007)
Setting maximal mount count to 60

How to open and close the CD DVD tray

2008-11-08T19:21:00.000-08:00

You can open/close the CD, DVD disk tray from the command line.

Many Linux users already know about the eject command for opening the disk tray:

 $ eject

How do you close the tray?

Turns out that you can use the same eject command but with an additional -t option to close the tray.

$ eject -t

Before I found out about the -t option, I always had to reach and press the open/close button on the drive to close the tray. With the -t option, you can now both open and close the tray from the command line.

There is another option -T (CAPITAL T, that is) you should know.

eject -T basically closes the tray if it is open, and opens the tray if it is closed.

The man page for eject has detailed explanation about the options.

Two additional ways to tail a log file

2008-11-02T21:05:00.000-08:00

I want to revisit a topic: how to tail a log file. My earlier posts discussed the use of tail and less commands to tail a log file, and multitail if you need to tail multiple files at once.

This followup article discussed two other ways to tail a log file: the use of the most command, and to tail within emacs.

The most command bills itself as a replacement for less. Like its predecessors less and more, most is a file pager program.

To install most on my Debian Etch system:

$ apt-get update && apt-get install most

To page the log file say /var/log/messages:

$ most /var/log/messages

This opens the log file, and displays the first page.

To tail the file, put most into Tail Mode by pressing the F key (capital F).

You should see the following line in the status area at the bottom of the tail window.

Most Tail Mode--  MOST keys are still active.

If a new line is appended to the log file, most will automatically reposition the file to the end and display the new line.

Note that you can scroll the file or do a search when you are in Tail Mode. For example, you can press the PageUp/PageDown, UpArrow/DownArrow keys to scroll the file. Also, you can press / to search forward or ? to search backward. However, when you press a key in Tail Mode, you break out of Tail mode in the sense that newly appended lines are not automatically displayed in the window. Yet, you can hit F again to re-enter Tail Mode, or simply use the DownArrow scroll key to scroll past the end of file to fetch the new lines.

To quit most, press the q key.

For my fellow emacs users out there, another way to tail a log file is to do it right within emacs.

You need the tail-file elisp function which is in the emacs-goodies-el package. Debian users can install this package like this:

$ apt-get update && apt-get install emacs-goodies.el

To tail a file in emacs: start emacs, hit M-x (Alt and x keys together), and type tail-file. Then, enter the filename to tail. The net result is that this will spawn an external tail -f process.

Note that you can't read the entire file under emacs using tail-file, only the tail initially and the newly appended lines portion afterwards.

How to disable SSH host key checking

2008-10-18T20:36:00.001-07:00

Remote login using the SSH protocol is a frequent activity in today's internet world. With the SSH protocol, the onus is on the SSH client to verify the identity of the host to which it is connecting. The host identify is established by its SSH host key. Typically, the host key is auto-created during initial SSH installation setup.

By default, the SSH client verifies the host key against a local file containing known, rustworthy machines. This provides protection against possible Man-In-The-Middle attacks. However, there are situations in which you want to bypass this verification step. This article explains how to disable host key checking using OpenSSH, a popular Free and Open-Source implementation of SSH.

When you login to a remote host for the first time, the remote host's host key is most likely unknown to the SSH client. The default behavior is to ask the user to confirm the fingerprint of the host key.

$ ssh peter@192.168.0.100
The authenticity of host '192.168.0.100 (192.168.0.100)' can't be established.
RSA key fingerprint is 3f:1b:f4:bd:c5:aa:c1:1f:bf:4e:2e:cf:53:fa:d8:59.
Are you sure you want to continue connecting (yes/no)?

If your answer is yes, the SSH client continues login, and stores the host key locally in the file ~/.ssh/known_hosts. You only need to validate the host key the first time around: in subsequent logins, you will not be prompted to confirm it again.

Yet, from time to time, when you try to remote login to the same host from the same origin, you may be refused with the following warning message:

$ ssh peter@192.168.0.100
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
3f:1b:f4:bd:c5:aa:c1:1f:bf:4e:2e:cf:53:fa:d8:59.
Please contact your system administrator.
Add correct host key in /home/peter/.ssh/known_hosts to get rid of this message.
Offending key in /home/peter/.ssh/known_hosts:3
RSA host key for 192.168.0.100 has changed and you have requested strict checking.
Host key verification failed.$

There are multiple possible reasons why the remote host key changed. A Man-in-the-Middle attack is only one possible reason. Other possible reasons include:

OpenSSH was re-installed on the remote host but, for whatever reason, the original host key was not restored.
The remote host was replaced legitimately by another machine.

If you are sure that this is harmless, you can use either 1 of 2 methods below to trick openSSH to let you login. But be warned that you have become vulnerable to man-in-the-middle attacks.

The first method is to remove the remote host from the ~/.ssh/known_hosts file. Note that the warning message already tells you the line number in the known_hosts file that corresponds to the target remote host. The offending line in the above example is line 3("Offending key in /home/peter/.ssh/known_hosts:3")

You can use the following one liner to remove that one line (line 3) from the file.

$ sed -i 3d ~/.ssh/known_hosts

Note that with the above method, you will be prompted to confirm the host key fingerprint when you run ssh to login.

The second method uses two openSSH parameters:

StrictHostKeyCheckin, and

UserKnownHostsFile.

This method tricks SSH by configuring it to use an empty known_hosts file, and NOT to ask you to confirm the remote host identity key.

$ ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no peter@192.168.0.100
Warning: Permanently added '192.168.0.100' (RSA) to the list of known hosts.
peter@192.168.0.100's password:

The UserKnownHostsFile parameter specifies the database file to use for storing the user host keys (default is ~/.ssh/known_hosts).

The /dev/null file is a special system device file that discards anything and everything written to it, and when used as the input file, returns End Of File immediately.

By configuring the null device file as the host key database, SSH is fooled into thinking that the SSH client has never connected to any SSH server before, and so will never run into a mismatched host key.

The parameter StrictHostKeyChecking specifies if SSH will automatically add new host keys to the host key database file. By setting it to no, the host key is automatically added, without user confirmation, for all first-time connection. Because of the null key database file, all connection is viewed as the first-time for any SSH server host. Therefore, the host key is automatically added to the host key database with no user confirmation. Writing the key to the /dev/null file discards the key and reports success.

Please refer to this excellent article about host keys and key checking.

By specifying the above 2 SSH options on the command line, you can bypass host key checking for that particular SSH login. If you want to bypass host key checking on a permanent basis, you need to specify those same options in the SSH configuration file.

You can edit the global SSH configuration file (/etc/ssh/ssh_config) if you want to make the changes permanent for all users.

If you want to target a particular user, modify the user-specific SSH configuration file (~/.ssh/config). The instructions below apply to both files.

Suppose you want to bypass key checking for a particular subnet (192.168.0.0/24).

Add the following lines to the beginning of the SSH configuration file.

Host 192.168.0.*
   StrictHostKeyChecking no
   UserKnownHostsFile=/dev/null

Note that the configuration file should have a line like Host * followed by one or more parameter-value pairs. Host *means that it will match any host. Essentially, the parameters following Host * are the general defaults. Because the first matched value for each SSH parameter is used, you want to add the host-specific or subnet-specific parameters to the beginning of the file.

As a final word of caution, unless you know what you are doing, it is probably best to bypass key checking on a case by case basis, rather than making blanket permanent changes to the SSH configuration files.

Upgrade individual packages for Debian-based systems

2008-09-27T18:28:00.000-07:00

If you are using Debian-based distributions (Debian, Ubuntu, etc), you are probably familiar with the apt-get update followed by the apt-get upgrade routine. That is what I regularly use to upgrade ALL packages that have an update available.

But what if you only want to upgrade certain individual packages?

apt-get upgrade will upgrade ALL or nothing. So, that is out of the question.

What you need is apt-get install. apt-get install serves dual purposes: install and upgrade. It installs a package if it is not already installed. If it is already installed, apt-get install will upgrade the package to the latest version.

You should still run apt-get update before apt-get install. You can specify one or more package names as arguments to the apt-get install command.

For example, to upgrade these 2 packages: libfreetype6, libtiff4:

$ sudo apt-get update
$ sudo apt-get install libfreetype6 libtiff4

How to get the process start date and time

2008-09-21T21:37:00.000-07:00

How can we determine when a running process was started?

The venerable ps command deserves first consideration.

Most Linux command-line users are familiar with either the standard UNIX notation or the BSD notation when it comes to specifying ps options.

If ps -ef is what you use, that is the UNIX notation.

$ ps -ef
UID     PID  PPID C STIME TTY      TIME CMD
root      1    0  0 Sep20 ?    00:00:03 init [3]
  
peter  1218    1  0 Sep20 ?    00:21:35 /usr/lib/iceweasel/firefox-bin -a firefox
  
peter  4901    1  1 16:34 ?    00:01:12 /usr/bin/emacs-snapshot-gtk

The STIME column displays the start time or date. From the above, we can tell that process 4901 (emacs) began execution at 16:34 (4:34 pm). But on what day, today?

From the ps man page: 'Only the year will be displayed if the process was not started the same year ps was invoked, or "mmmdd" if it was not started the same day, or "HH:MM" otherwise.'

So, emacs was started at 16:34 TODAY.

What is the start time for the other process, the firefox process with pid 1218?

The STIME for process 1218 reads Sep20 (which was yesterday). But what time yesterday?

The default ps -ef only tells you the start date but NOT the time if a process was NOT started on the same day.

If the BSD notation is more familiar to you, you will find that ps aux yields similar results.

$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY STAT START   TIME COMMAND
root         1  0.0  0.1   1576   540 ?   Ss   Sep20   0:03 init[3]
  
peter     1218  0.5  8.9 201252 45456 ?   Sl   Sep20  21:35 /usr/lib/iceweasel/firefox-bin -a firefox

The Start column in the above output only reveals the start date if the process is older than the current day.

There are (at least) 2 ways to determine the exact start time if the process was started before the current day.

Solution 1
Specify elapsed time in the ps output format.

$ ps -eo pid,cmd,etime
  PID CMD                             ELAPSED
 1218 /usr/lib/iceweasel/firefox-bin -  2-16:04:45

The above ps command specifies 3 fields to be included in the output: the process pid, the command, and the elapsed time, respectively.

etime is the elapsed time since the process was started, in the form dd-hh:mm:ss. dd is the number of days; hh, the number of hours; mm, the number of minutes; ss, the number of seconds.

The firefox command started execution 2 days, 16 hours, 4 minutes and 45 seconds ago. To find the exact time, you need to do some simple math.

If you prefer the BSD notation, issue this command:

$ ps axo pid,cmd,etime
  PID CMD                             ELAPSED
  
 1218 /usr/lib/iceweasel/firefox-bin -  2-16:04:57

Solution 2
Get the process pid and read off the timestamp in the corresponding subdirectory in /proc.

First, get the process pid using the ps command (ps -ef or ps aux)

Then, use the ls command to display the creation timestamp of the directory.

$ ls -ld /proc/1218
dr-xr-xr-x 5 peter peter 0 Sep 20 16:14 /proc/1218

You can tell from the timestamp that the process 1218 began executing on Sept 20, 16:14.

If you can think of another clever way to get the start time and date, please let us know.

How to find and delete all hard links to a file

2008-09-02T20:00:00.001-07:00

Deleting a file is deceptively simple. You can simply use the rm command like this.

$ rm file1

However, if the file has one or more hard links to it, life gets more interesting. You need to seek and destroy all hard links to the file.

A hard link is essentially another name for a file. Hard links to file1 can be created as follows:

$ ln file1 file2
$ ln file1 tmp/file3

file2 and file3 become another name of file1.

How do I know if some file has a hard link to it?

Do a long listing of the file like this.

$ ls -l file1
-rw-r--r-- 3 peter peter 0 2008-09-01 16:15 file1

Note that the hard link count has the value 3 (this is the number to the left of the file owner). It means that the physical file has 3 names: file1 and two others.

If you only rm file1, the other 2 names still exist, and users can still access the physical file using these 2 names.

To find out all hard links to file1, use this command.

$ find /home -xdev -samefile file1
/home/peter/file1
/home/peter/tmp/file3
/home/peter/file2

Note that we specified /home as the search starting point. You should replace /home with the mount point for the filesystem in which your file1 was created. (This is likely to be either /home or / for files in your home directory.)

The reason for using the filesystem's mount point is that hard links are restricted to reside in the same filesystem as the physical file. This means that file2 and file3 must be in the same filesystem as file1. Therefore, to find ALL hard links to a file, you must start search at the mount point of the filesystem.

The -xdev option instructs find NOT to traverse directories in other filesystems.

To find and delete all hard links to a file:

$ find /home -xdev -samefile file1 | xargs rm

An alternative (and slightly more complicated) method is by using inodes. An inode is a data structure assigned to a physical file that contains all its meta-information except the file's name(s).

All hard links to a physical file share the same inode, and have the same inode id number. Therefore, if you know the inode number of a file, you can find all hard links to the file by searching for files with the same inode number.

To display a file's inode number, use ls -i:

$ ls -li file1
2655341 -rw-r--r-- 3 peter peter 0 2008-09-02 19:09 file1

The first column in the above listing shows the file's inode number.

Now that you know the inode number of file1, you can use the find command to search for all files that have the same inode number:

$ find /home -xdev -inum 2655341
/home/peter/file1
/home/peter/tmp/file3
/home/peter/file2

To delete all hard links to the physical file:

$ find /home -xdev -inum 2655341 | xargs rm

How I repaired a corrupted grub menu.lst config file

2008-08-17T17:34:00.000-07:00

Imagine the shock when I discovered my Debian Etch machine would not boot after I ran a "routine" apt-get upgrade. The upgrade involved quite a number of packages, including the kernel image.

GRUB, the default boot loader, came to an abrupt stop with the error message
Error 15: File Not found.

The culprit was the file /[01;31mvmlinuz-[00m2.6.18-6-k7.

The weird-looking file name suggested strongly that the GRUB config file, /boot/grub/menu.lst, got corrupted by the upgrade.

At this point, I had the following options:

Use a rescue CD/DVD like Knoppix to boot into the system, correct the menu.lst file, and reboot.

While in GRUB, repair the corrupted pre-set GRUB commands, boot up the system, then correct the menu.lst file, and reboot.

I chose the second option. Below was my experience, followed by some suggestions based on the hard lessons I learned.

I rebooted the system. At the GRUB menu, I selected the corrupted OS entry, and typed e to edit this entry's associated pre-set boot commands. (Note that the OS entries and their associated commands are taken from the menu.lst file.)

The boot commands associated with the OS entry were:

root  (hd0,0)
kernel  /[01;31mvmlinuz-[00m2.6.18-6-k7 root=/dev/mapper/tiger-root ro 
savedefault

That did not look right because of the funny looking characters in the kernel command and the fact that the boot commands from before were root, kernel, and initrd, but NOT savedefault.

ROOT

The root command specifies and mounts GRUB's root drive and partition where the boot directory is located (/boot). This is usually (hd0,0) which means the first partition of the first hard disk. Note, GRUB's notation for numbering drives and partitions starts from 0, not 1.

If you are not sure what to set the root drive, enter the GRUB batch command mode by pressing b, and enter the following find command.

grub> find /grub/stage1
(hd0,0)

The find command searches for the file named grub/stage1 and displays the root drive and partition which contains the file. Note that if the drive does not have a partition designated for /boot, you need to prepend /boot to the command argument (find /boot/grub/stage1)

So far so good: I did not have to modify the root drive.

KERNEL

The kernel command specifies and loads the Linux kernel image. The default image file name was corrupted. To correct, I selected the kernel command, and pressed e to edit the line.

What should the file name be? Different Linux distributions name the kernel image file differently. Don't fret if you can't remember its name. The GRUB command line offers file name completion. So just enter kernel / and then hit tab.
grub> kernel /
Possible files are: System.map-2.6.18-6-k7 config-2.6.18-5-k7 config-2.6.18-6-k7 initrd.img-2.6.18-5-k7 vmlinuz-2.6.18-5-k7 grub initrd.img-2.6.18-6-k7 System.map-2.6.18-5-k7 vmlinuz-2.6.18-6-k7

For Debian, Ubuntu, Fedora, and Mandriva, the kernel image file is named vmlinuz followed by the kernel release number and the machine architecture (e.g., vmlinuz-2.6.18-6-k7). From the options returned by the file name completion feature, choose the kernel image with the latest release number.

The rest of the kernel parameters looked OK, and required no change.

grub> kernel /vmlinuz-2.6.18-6-k7 root=/dev/mapper/tiger-root ro

INITRD

Next, I had to replace the savedefault command with the initrd command.

initrd specifies the ramdisk image file. The RAM disk is used for loading modules required to access the root filesystem.

I first selected the savedefault command and pressed e to edit the line. Again, you could use the Tab key to help you complete the filename for the RAM disk file.

grub> initrd   /initrd.img-2.6.18-6-k7

Boot & Edit menu.lst

After I made the above changes, I went back to the GRUB main menu, and pressed b to boot.

This time, the machine booted up successfully, and everything worked just fine for me.

I was not done however. Unless I changed the source of the problem (the corrupted menu.lst), the machine would come up with the same boot error in the next reboot. So, as root, I opened the file /boot/grub/menu.lst and edited the commands.

Before I could modify the corrupted commands, I needed to first locate the corresponding OS entry in the file. Each OS entry occupies a separate section in the file, beginning with its own title line. So, I scrolled down the file until I reached the target title line.

title Debian GNU/Linux, kernel [01;31m-[00m2.6.18-6-k7
root (hd0,0)
kernel /[01;31mvmlinuz-[00m2.6.18-6-k7 root=/dev/mapper/tiger-root ro 
savedefault

After correcting the commands, save the file, and reboot the machine.

Lessons Learned

The GRUB config file (menu.lst) can get corrupted, and when it does, it spells real trouble.

The commands to boot the OS are not something anyone tends to remember. So, it makes sense to have a print out of the menu.lst file or a backup copy.

What I do is backup the menu.lst file in the same directory as menu.lst (say, call it menu.lst.bak).

The advantage of saving it in the same directory (as opposed to somewhere over the network) is that if the menu.lst file ever gets corrupted again, you can still display the backup copy at the grub command prompt. During the GRUB boot up process, you can display the backup file by simply entering the following at the GRUB command prompt.

 grub> cat /grub/menu.lst.bak

The above cat command displays the backup menu.lst file. Armed with the knowledge of the correct commands to use, you can then edit the commands as shown in this article.

How to show apt log history

2008-08-09T11:06:00.001-07:00

Users of Debian-based distributions (myself included) often brag about Debian's supposedly superior package management tool-set. This tool-set includes a choice of several excellent package managers such as dpkg, apt, synaptic, and aptitude. The various tools are all first class at what they are designed to do.

In my opinion, one major feature gap is a command-line interface for viewing the apt change log. After a recent routine Etch package upgrade, I discovered that the grub menu.lst file got corrupted. So, I wanted to check the recent apt activities to find out which packages were the possible suspects.

A google search revealed that viewing change history is not that simple. Aptitude writes change log info to /var/log/aptitude. Synaptic users can get the same info through its graphical user interface. But is there a standard change log file that the most common Debian package managers write to? The second question is whether there exists a command-line tool for accessing it.

It turns out that such a log exists, and it is /var/log/dpkg.log. This is a single log file that records all the apt activities, such as installs or upgrades, for the various package managers (dpkg, apt-get, synaptic, aptitude).

Regarding a command-line tool for accessing apt history, there used to be a package named apt-history for viewing apt-get activities. Its web site suggests that work in the project has been discontinued due to the fact that dpkg now does logging. It went on to recommend the use of a simple bash function (also named apt-history) to access the log file (/var/log/dpkg.log)

Below is the bash function from that web site.

function apt-history(){
      case "$1" in
        install)
              cat /var/log/dpkg.log | grep 'install '
              ;;
        upgrade|remove)
              cat /var/log/dpkg.log | grep $1
              ;;
        rollback)
              cat /var/log/dpkg.log | grep upgrade | \
                  grep "$2" -A10000000 | \
                  grep "$3" -B10000000 | \
                  awk '{print $4"="$5}'
              ;;
        *)
              cat /var/log/dpkg.log
              ;;
      esac
}

I've tried it, and it works.

To set it up, insert the above code into /root/.bashrc.

To run apt-history, you need to become root (for example, sudo -i). Entering apt-history with no parameter will simply dump the change log file. To select what activities you want to see, you can enter one of install, upgrade, remove, rollback as a single parameter to apt-history.

$ sudo -i
Password:
# apt-history install
2008-08-01 21:37:00 install googlizer  0.3-3
2008-08-09 08:20:54 install agrep  4.17-3
2008-08-09 08:28:26 install abuse-frabs  2.10-7
2008-08-09 08:28:27 install abuse  1:0.7.0-5
#

Learn more about a command when no man info page is available

2008-08-04T13:55:00.000-07:00

To read the documentation about a command, the first thing we do is to read its manual (man) page.

$ man cd
No manual entry for cd

But what if no man page is available for that command?

This could be due to a number of different reasons. For example, the Linux system was originally installed without any man pages at all. I worked with a Linux-based mobile gateway device that has 0 man pages pre-installed. This is to save disk space on a 1 GB Solid State Drive (SSD). In most typical desktops or servers, man pages are pre-installed.

A man page can be missing for a particular command because the administrator, for whatever reason, did not install the man page for that command. If that is the case, google is your best friend. The challenge here is to narrow down the search to get to the man page quickly.

There is yet another reason why there is no man page for a command. If it is a bash shell built-in command, it will NOT have its own man page. Documentation is still available but it is a bit harder to get to it. It turns out that information about bash built-in commands can be found inside the bash man page. You simply enter man bash, and search for the SHELL BUILTIN COMMANDS section (using the command /^SHELL BUILTIN). Then scroll down until you reach the built-in command you are looking for.

$ man bash

How do you know if something is a bash built-in command in the first place?

Use the type command (another bash built-in).

$ type cd
cd is a shell builtin

Note that not all Linux distributions behave the same way when you man a bash built-in command. Debian Etch, the distro I use at home, reports no man page when you man cd. Some distributions, like the Red Hat-based Centos, will display the bash man page. In this case, you are one step ahead of the rest.

Pondus: A personal weight management software

2008-07-30T20:23:00.000-07:00

A week after my return from a New England & Canada cruise, I found myself busy searching for a personal weight management software.

Why weight management? See the chocolate dessert buffet pictures taken on board the Holland America Maasdam cruise ship.

I came across pondus, a python program that is free and open-sourced.

Pondus has a rather modest feature set: the ability to enter one's weight over time, and have it plotted in a chart. It does not have too many bells and whistles, but it is very simple to use.

Pondus is packaged with certain Debian releases (namely, Lenny and Sid). So a simple apt-get install pondus will put it on your system. Because I run Debian Etch, I need to install it from source files.

I downloaded the source tar ball as well as the install instructions from here. The instructions are straight-forward, and I installed pondus without a glitch.

To run pondus, just type pondus in the command line. When you run pondus for the first time, it is not too terribly exciting .... because you have not entered any weight data.

Before you enter your weight, you should customize the unit of measure that pondus will use(lbs versus kgs). Click on the Tool icon (the one with the screwdriver and wrench) to bring up Preferences. For me, I prefer pounds (that is the unit my scale uses).

Now, let's enter some weight data, and draw a pretty chart.

To enter a weight, click on the Plus icon.

To plot the data, click on the Chart icon.

Pondus has some additional useful features such as data import and export in CSV format, as well as the ability to save the chart in png or svg (2-dimensional vector graphics) format.

An interesting feature is that you can set time-sensitive goals (aka, Weight Planner). For example, you can set a goal such as "weigh 180 lbs on August 15". This feature is disabled by default. To enable it, bring up Preferences as above, and click Use Weight Planner checkbox to select it, and choose whether to include the weight plan in your plots. Note that this feature is not for everyone (do I really need yet another remainder of my shortcomings?)

Pondus is adequate for tracking my body weight. I'm still searching for software that can also track my blood pressure. Won't it be perfect if a single software can manage both weight and blood pressure?

How to do reverse DNS lookup

2008-07-28T06:00:00.000-07:00

Most people can better remember domain names, e.g., www.gnu.org, than their corresponding IP addresses, 199.232.41.10. (In this example, www.gnu.org is the home of the Free Software Foundation.) We delegate the responsibility to machines, aka, the DNS servers, to resolve the domain names for us.

Sometimes, we do need to manually lookup the IP address of a domain name. You may already be familiar with the nslookup command which is now deprecated. We use the dig command to make DNS queries.

 $ dig +noall +answer www.gnu.org
www.gnu.org.            67      IN      CNAME   gnu.org.
gnu.org.                67      IN      A       199.232.41.10

The IP address is displayed in the A record, and is 199.232.41.10.

The +noall, +answer combination basically tells dig to only report the answer of the DNS query and skip the rest of the output.

You can also use the dig command with the -x option to do a reverse DNS lookup. A reverse DNS lookup means you want to look up the domain and host name of an IP address.

 $ dig +noall +answer -x 199.232.41.10
10.41.232.199.in-addr.arpa. 36000 IN    CNAME   rev-c41-10.gnu.org.
rev-c41-10.gnu.org.       300     IN      PTR     www.gnu.org.

The PTR record is the one that contains the domain host name. The domain name is, as you expect, www.gnu.org.

Note that PTR records are not required for IP addresses. If a PTR record is not defined for an IP address, you cannot do a remote DNS lookup.

How to count number of files in a directory

2008-07-18T13:32:00.001-07:00

Now that I am back from vacation, I had to take care of some chores, like uploading the pictures taken with my digital camera.

I stepped out during the long upload process (400+ pictures). When I returned, it was already done. To just make sure all pictures are now on the server, I wanted to count the number of files in the targetdir directory.

$ ls -1 targetdir | wc -l
454

The above command reads ls dash one piped to wc dash letter l.

Note that if you had used ls dash letter l instead, the count is one greater than the actual number of files. This is because ls dash letter l outputs an extra total line:

$ ls -1 targetdir 
total 529436
-rw-r--r-- 1 peter peter  1510976 Jul 13  2008 DSCN1001.jpg
....

The above method will count symbolic links as well as subdirectories in targetdir (but not recursively into subdirectories).

If you want to exclude subdirectories, you need a heavier duty tool than ls.

$ find targetdir -type f  -maxdepth 1 | wc -l

-type f ensures that the find command only returns regular files for counting (no subdirectories).

By default, the find command traverses into subdirectories for searching. -maxdepth 1 prevents find from traversing into subdirectories. If you do want to count files in the subdirectories, just remove -maxdepth 1 from the command line.

Note that the find command does NOT classify a symbolic link as a regular file. Therefore, the above find -type f command does not return symbolic links. As a result, the final count excludes all symbolic links.

To include symbolic links, add the -follow option to find.

$ find targetdir -type f -follow  -maxdepth 1 | wc -l

Dual pane Linux file managers: mc and emelfm

2008-06-21T16:26:00.000-07:00

I have been computing all my life without using a dual pane file manager until one day I decided that having one will greatly enhance my qualify of life. What I had in mind was something that will make my life easier in the copying or moving of files from 1 directory to another.

By dual pane (or twin-pane) file manager, I mean a file manager that displays two directories side by side (one active, one passive). The term is confusing because often a dual pane file manager has a third pane that lets you enter commands to execute on the active directory.

Staying true to my preference for Command Line Interface (CLI), I first tried Midnight Commander (mc). As advertised, mc is a file manager that is loaded with features. What I am looking for though is modest: a file manager with 2 directory panes that will satisfy my very basic day-to-day file management needs.

I found myself customizing the look and feel of mc in the early adoption stage. The nostalgic white text on blue background had to go.

You can run mc with the -b option to force black and white.
$ mc -b

If you don't want to go that far, you can customize the color used by editing the init file (~/.mc/ini).

I inserted the following lines into the ~/.mc/ini file.

[Colors]
base_color=lightgray,green:normal=green,default:selected=white,gray:marked=yellow,default:markselect=yellow,gray:directory=blue,default:executable=brightgreen,default:link=cyan,default:device=brightmagenta,default:special=lightgray,default:errors=red,default:reverse=green,default:gauge=green,default:input=white,gray:dnormal=green,gray:dfocus=brightgreen,gray:dhotnormal=cyan,gray:dhotfocus=brightcyan,gray:menu=green,default:menuhot=cyan,default:menusel=green,gray:menuhotsel=cyan,default:helpnormal=cyan,default:editnormal=green,default:editbold=blue,default:editmarked=gray,blue:stalelink=red,default

I also rearranged the panes so that they are placed horizontally (instead of vertically). You can interactively customize the placement using the mc Options menu (when you are already in mc). Alternatively, you can edit the ~/.mc/ini file, and make sure that the following horizontal_split line appears in the [Midnight-Commander] section:

[Midnight-Commander]
....
horizontal_split=1

My mc window looked like this after the above customization.

Another major annoyance is that if you run mc in a Gnome window, you cannot use the F10 key to quit mc. It turns out that F10 is normally the key to access the menu-bar in Gnome. To quit mc, you use your mouse to click the Quit soft-key.

If it bothers you as much as it bothered me, you might want to disable the standard F10 menu-bar shortcut key for Gnome. To do that:
$ gconftool-2 --set /apps/gnome-terminal/global/use_menu_accelerators --type bool false

After giving it a good run for about 3 weeks, I gave up on mc. The determining factor was how mc handles the Tab key.

Like other dual pane file managers, mc has a third pane that acts as the command line input window. However, the type of commands that will run in that window seems very limited. I tried commands such as date, pwd, ls, but they all showed no output. The most annoying feature to me is the Tab key. I am used to tapping the Tab key in a bash window for command completion. If you hit Tab while you are typing away in the mc's command line window, the effect is that mc switches the active focus to the other pane.

I won't be surprised if someone tells me there is a way to customize the handling of the Tab key. But, at that point in time, I concluded that I will look for some other file manager that requires less adjustment and customization.

I first became aware of emelfm because it is packaged with the Damn Small Linux (DSL) distribution. I decided to take a look at it because if it is included in DSL, it should be simple enough to satisfy my modest needs.

emelfm is a GUI-based double pane file manager. It has a clean, simple, and intuitive UI. It does the basic file copying and moving. It also has some nice additional features such as the ability to set bookmarks and apply filters to filenames.

One minor annoyance I find is when I try to copy a file in the active window to the same location. Essentially, what I want to do is copy and paste a file (to a new name) in the same directory in the active pane. I cannot use the Copy button to do it because it will only copy the file to the other pane. Instead, I need to enter the cp command explicitly in emelfm's command line window to do the copy.

[P. Evans pointed out that emelfm has a Clone plugin that copies a file to the same directory. Click Configure and then Buttons. Add a button and select the Clone plugin.]

Using the command line window in emelfm turns out to be a pleasant experience (as compared to mc). Hitting the Tab key will only switch the active pane if your mouse cursor is not in the command line window. If the cursor is in the command line window, hitting the Tab key does what I expect: command and filename completion. The commands that did not work in mc all worked well in emelfm (date, pwd, ls).

I have been happily using emelfm since then. There are other GUI-based dual pane file managers out there: e.g., Krusader, Double Commander. I have not tried them. However, I do most file management on the CLI, and only occasionally use a GUI-based file manager to do file copying. As such, emelfm satisfies my requirement for a GUI-based file manager.