Tips

These are short observations on Linux, it's use, operation, and configuration. Mostly, they are derived from my own experiences installing, configuring and using Linux systems, from embedded systems to desktops to servers.

Uninterruptable Sleep

Another question from usenet read:

I have read that no program can ignore SIGKILL and SIGSTOP. But, a task is in UNINTERRUPTIBLE state when it sleeps blocking all signals. Could someone help me understand the scenario? Isn't it blocking all signals (including SIGKILL and SIGSTOP)?

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The Difference between Softlinks and Hardlinks

Someone posted this question to one of the Linux newsgroups I follow:

Can someone give me a simple explanation of the difference between a soft link and a hard link? The documentation I've read mention these links but make no strong explanations of their meaning and how/when to use them. Thanks!


Here's how I explained the differences.



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Cleaning Up

With my desktop machine's last system upgrade, I changed how I managed my /tmp directory. Before the upgrade, my configuration mounted a tmpfs on /tmp at startup, but now, it mounts a separate ext2 partition for /tmp. Linux implements the tmpfs filesystemtype in swap, making the contents of the directory really temporary; files and directories in such a filesystem vanish on reboot. But, now with the ext2 filesystem, /tmp files and directories stick around.

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Quick and Dirty GUI Calendar

A while back, I needed a yearly calendar for my X desktop. Since, at the time, I ran a desktop that did not include an integrated Calendaring facility, I couldn't just click on something. So, I built a GUI perpetual calendar, in a shell script.

The script (which I called Xcal.sh) looked like
#!/bin/bash
WHEN=`date +%Y`
RC=0
while [ $RC -ne 103 ] 
do
  PREV=`expr $WHEN - 1`
  NEXT=`expr $WHEN + 1`

  cal $WHEN | \
  xmessage -buttons $PREV,$NEXT,Quit -file - -title "In the Year $WHEN"

  RC=$?
  case $RC in
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Validating Web-based logons with FTP

As part of a PHP web app I developed, I needed to authenticate the web client user with a remote system. Unfortunately, this system was a mainframe and setting up a web-enabled authentication product on it was somewhat timeconsuming and required a lot of administrivia. I wanted to avoid all that, so I had to come up with another way to authenticate web users remotely.

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Procmail Recipe - One Email Address, Multiple users

A while back, I switched ISPs and lost the multiple email addresses that the old ISP provided. With the new ISP, my wife and I were restricted to sharing a single public email address. With fetchmail, I could retrieve our email from my ISP's POP3 server and store it in our lan server for on-demand "offline" reading, but with one email address, either my wife and I had to read each other's mail, or one of us got no mail at all.

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