Living on the Unix Command Line: The Importance of a Prompt

2014/12/06 13:05:19
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Over the last decade, I’ve continued to gravitate more and more toward Unix and the command line while many of my colleagues have gravitated toward GUI applications. This post is not about the pros and cons of GUI vs. CLI or how one is better than the other. Suffice it to say, I live with both, but spend a lot of time working in terminals.

Today we focus on the prompt. Many people leave the prompt set to the default for their distribution, which may be fine, but in my 20 years of Unix use, I’ve found some nice tweaks that make my life easier. I share them here because you may find them useful as well.

The default prompt for a Red Hat Enterprise Linux system looks like the following:

[smj@athena bin]$

This gives me an idea of which user I’m logged in as (smj), which server I’m logged in to (athena) and the current working directory (bin). Unfortunately, in the example above, my current working directory is actually /usr/local/bin. Based on the default prompt, I cannot tell if I’m in /usr/local/bin, /opt/bin, /usr/bin, /bin, or even /home/smj/bin. This presents a problem considering how often much of the Unix directory structure repeats itself.

Another issue I have is that I’m pretty sure I’m logged in to the bash shell, but can’t really be sure. I’ve had to endure many shells in my career, from sh to bash to csh to tcsh to ksh and some I can’t even remember because they appeared so infrequently.

So, to address these problems, I spent time trying to find a prompt that would provide enough information to be useful, while working across several platforms and shells.

My current bash prompt looks like so:

18:30:33 smj@athena:/usr/local/bin
bash $ -->

My current tcsh prompt looks like so:

18:40:01 sjone@procyon:/usr/local/bin
tcsh % -->

There is no color in either prompt. This avoids any issues between different terminal color schemes or terminal types. It works equally fine in the Linux console and hpterm.

You may be asking, why does he need all of this information in the prompt? Let’s review each part of my prompt.

Newline at start

Consider the following text from an open terminal with a prompt containing no information:

total 892
-r-sr-xr-x. 1 root bin 234404 Aug  3 18:35 cdcc
-r-xr-xr-x. 1 root bin  44568 Aug  3 18:35 dccif-test
-r-sr-xr-x. 1 root bin 627422 Aug  3 18:35 dccproc
#cat /etc/hosts   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
# e-2104-13
#netstat --inet
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0    116 athena.littleprojects.o:ssh        ESTABLISHED
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0      0 localhost:10024             localhost:58377             TIME_WAIT
tcp        0      0 athena.littleprojects.o:ssh      TIME_WAIT
tcp        0      0 localhost:10025             localhost:53230             TIME_WAIT
tcp        0      0 localhost:10025             localhost:56526             TIME_WAIT
tcp        0      0 localhost:10024             localhost:58375             TIME_WAIT
#gvim chicken.txt

Can you quickly glance out it and tell me which commands have been executed? I need some separator on the screen to indicate the individual command executions so I can tell what happened. Without a separator, it looks like one big garbled mess, so I opted for a newline between the prompt and the command executions.

Try it now?

total 892
-r-sr-xr-x. 1 root bin 234404 Aug  3 18:35 cdcc
-r-xr-xr-x. 1 root bin  44568 Aug  3 18:35 dccif-test
-r-sr-xr-x. 1 root bin 627422 Aug  3 18:35 dccproc

#cat /etc/hosts   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
# e-2104-13

#netstat --inet
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0    116 athena.littleprojects.o:ssh        ESTABLISHED
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0      0 athena.littleprojects.o:ssh        TIME_WAIT
tcp        0      0 localhost:10024             localhost:58377             TIME_WAIT
tcp        0      0 athena.littleprojects.o:ssh      TIME_WAIT
tcp        0      0 localhost:10025             localhost:53230             TIME_WAIT
tcp        0      0 localhost:10025             localhost:56526             TIME_WAIT
tcp        0      0 localhost:10024             localhost:58375             TIME_WAIT

#gvim chicken.txt

Now I can clearly see that the top is the output of some command that has scrolled off the screen, but the cat, netstat, and gvim commands came next. It’s not that I can’t figure out what commands were executed (or even look at the history), but that when I’m comparing commands and output to each other, I need to be able to quickly see which sections of output belong to which commands. For me it’s a legibility and time-saving measure.

Time last command exited

At the beginning of the prompt is a timestamp.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

This timestamp serves as a “poor man’s time command” letting me know (roughly) just how long the previous command took to run. It also serves as a way to keep track of times if I have a meeting or other engagement coming up. I put it in front, so when I see the prompts in sequence I can compare them to one another.


Next is the username.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

I don’t have the same username for each account. I also want to know if I’m running as root or a regular user. Sometimes that forces me to notice that I’m root before I do something dangerous. With one terminal up, this is not a problem. When I’m switching between five, it becomes increasingly important to keep each straight.

Server hostname

And equally useful to the username is the hostname of the server to which I’m connecting.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

With the hostname I can tell different terminal sessions apart and ensure that I don’t make a mistake of typing the wrong command on the wrong server.

Full Path

As noted above, I like seeing the full path in my prompt. This way I can tell where I am on the server.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

I also have a newline inserted after the path because a long path can scroll pretty far across the screen and some terminals have an issue including both the command and a long prompt on the same line.

The whole username@hostname:path exists so I can easily copy and past this string into an SCP command for moving files between servers. This is easier than typing scp myfile username@hostname:path by hand, especially considering hitting <tab> doesn’t work to expand the path for the server you are copying to.

Shell Name

This was one of the last things I added. I use a lot of bash; a LOT OF bash, but occasionally I have accounts (thank you ODU) that are tcsh, or I need to use an ancient server running csh or sh. I wanted to ensure that I could readily identify the shell as I switch between systems, so I have the prompt state which shell is running.

18:30:33 smj@athena:/usr/local/bin
bash $ -->

This means that I actually have login scripts for each shell that attempt to create as close as possible an approximation of the bash prompt. For example, if I’m suffering through an actual Bourne Shell session, I only get this:
sh $ -->

For tcsh, I get this:

18:40:01 sjone@procyon:/usr/local/bin
tcsh % -->

The point is that the shell is identified on the line on which I’m typing commands. This way I will know about the shell so I can change my syntax for items, such as redirecting stderr to stdout, when using a shell other than bash.

I do not control all of the servers I log into. Even though I may prefer bash, administrators on other systems have everyone default to tcsh or other shells. I try to ensure that my login prompt reflects this difference, especially when I’m working across several machines.

"The Arrow"

I added the arrow (–>) with an additional space afterwards so I can separate the prompt from the command that has been run. I find this to be useful, much like the newline between prompts, when looking at the output from a series of commands. It allows me to separate the commands I run from their output.

No color

Finally, I work across multiple systems with different terminal emulators, some of which I cannot choose for myself. At work I use PuTTY or Attachmate Reflections. At home I prefer the Mac OS X terminal program, or < a href="">gnome-terminal, but I don’t always get to use my home computer for all of my projects. Sometimes, I don’t even get to configure the terminal how I would like!

Imagine that I had selected a color scheme using blue. Now imagine that same blue with a black background. Was that even legible? If cannot change the background to white or gray then I have to strain to read that text. Also, consider yellow on a white background. If I cannot reliably control my background colors, I really can’t select any meaningful foreground colors either.

Hence, I put not color into my prompt.

Setting up the prompt

To generate my prompt, I set the PS1 variable to the following value in my .bashrc file:

PS1='\n\t \u@\h:\w\nbash \$ --> '

The \n provides the newline, the \t provides the time, the \u provides the username, the \h the hostname, the \w the full path, and the rest is just text.

See the bash man page.

TCSH is not that much different:

set prompt = "\n%P %n@%m:%~\ntcsh \% --> "

Here, the %P provides the time, the %n provides the username, the %m the hostname, the %~ the full path (substituting ~ for my home directory), and the rest is just text.

Other shells do not have these fancy variables for telling time or providing other information, so for those shells I just set them with a text string providing as much meaningful information as I can get. I even have a custom Windows Command Prompt I use when I’m able to set it myself.


I hope you have found this article a useful resource. Though I realize not everyone is in the same situation I am, having to adapt to many differing environments, I hope that you can take away something useful from this discussion about the importance of a prompt on the command line.

For more information on how to set up your prompt effectively, Carla Schroder has a nice article discussing the use of colors and other features. For more in depth information on the bash prompt, check out the Giles Orr’s Bash Prompt HOWTO from the Linux documentation project. Understudy has a similar article to this which discusses more shells.

Whatever you do, take some pride in your prompt! It doesn’t just have to be that place where you give information, you can also get information.

Using launchd to run scheduled jobs

2011/08/29 14:25:22
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My new server’s predecessor ran Linux. I had a few scheduled jobs, like the ones that back up this blog, its database, and so on. On Linux the tool used for scheduling jobs is cron. Lo and behold, Apple has deprecated cron on their systems in lieu of an Apple-grown tool named launched. Launchd’s job is to replace cron, init, the rc scripts, the file alteration monitor, and a whole host of other normal Unix utilities in favor of one huge behemoth process that starts at boot. I’ve got mixed feelings about this concept, as it goes against the Unix philosophy of “doing one thing and doing it well”, but I chose OSX and have tried to learn their way of doing things.

I found several articles describing how one creates the plist files used by launchd. These XML files contain the information on the process you wish to run, including when to run it, how to run it, and who is allowed to run it. Here I will detail how I used it to merely run a scheduled job every night at midnight.

To start with, I have a simple script that backs up the database on the server:

# if we don't have a backup directory, make it
if [ ! -e $BACKUPDIR ]; then
  mkdir -p $BACKUPDIR
# run the remote database dump command
ssh $REMHOST "mysqldump --verbose --user=root --password='XXXXXXXXXXXX' --all-databases" &gt; $OUTPUTFILE
# if the zipped output file already exists, move it before zipping the new one
if [ -f $OUTPUTFILE.bz2 ]; then
  mv $OUTPUTFILE.bz2 old-$OUTPUTFILE.bz2
chown smj:staff $OUTPUTFILE.bz2

This script is creatively entitled backup-database. I want it to run every night so I have an exact copy of this website’s database in case the web server goes down and I have to reinstall everything.

Under cron, I would run crontab -e and then put the following line into the editor that is brought up:

0 * * * *    $HOME/bin/backup-database

Apple made this simple step a lot more complicated than, in my opinion, it needed to be; but they gave me a tool that is a lot more powerful that mere cron.

To use launchd, I needed to make a file in the directory /Library/LaunchDaemons named org.littleprojects.backup.lawrence.db.plist that contains the following XML code:

<!--?xml version="1.0" encoding="UTF-8"?-->

I’m a little annoyed at how Apple handled the whole key-value syntax, but I’ll let that be. The file reads as follows: create a job named org.littleprojects.backup.lawrence.db that will run the program /Users/smj/bin/backup-database at the 0th hour and 0th minute of ever day (midnight). I’m not sure if this is any less difficult to read than the cron syntax, but it is more difficult to write to be sure.

Once I created the file, I then needed to load it into launchd, which could be done by running the following commands:

sudo launchctl load org.littleprojects.backup.lawrence.db.plist

If you do this you, do not need to reboot as others claim. This is Unix and reboots should only be necessary for major operating system changes. Rebooting in order to create a scheduled job is not only ridiculous, but time-wasting and leads to error-prone behavior like making many untested scheduled jobs at once in order to save on reboots. I always try to find a way around reboots to avoid error-prone behaviors!

So, I did the same thing for other scripts I wanted to run at certain times. I understand why apple decided to use launchd, I just wish they would have made it easier to configure. Lingon exists in the app store for the express purpose of helping you create these plist XML files, but it will not load them for you and instead recommends you reboot after each edit, which I noted my distaste for above.

In the future I will be exploring how to best use launchd for other purposes, like restarting services, and scheduling scripts to run when other system events occur.


Randomly crashing Mac Mini Server running Lion [Solution]

2011/08/18 19:08:24
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I’d been looking for a home server solution that offered me more disk space, but that ran cool and quieter than my server-closet behemoth. I wanted the server to also be able to execute scripts and scheduled jobs. I researched several options, including the Dell Zino, but none seemed to capture my attention like the Mac Mini Server.

The day it arrived, I set it up and spent hours pouring over Apple’s documentation. I got file sharing, SSH, WebDAV, and other services set up easily, but realized that I had to download Server Admin Tools separately in order to get access to DHCP and DNS. Some admins speculate that this is a separate download to prevent would be admins from accidentally turning on DHCP in a network that already had it. Screen sharing worked flawlessly. I had it using my 46″ television as a monitor, so watching online video and iTunes was awesome!

The next day I went back to my new toy, and… it had locked up. I was not pleased. Murmuring curses and threats of sending the server back didn’t bring the server back from its limbo. This was UNIX! UNIX was not supposed to do this! I felt betrayed by a company I’ve been growing in support for. Why did they do this to me?

I think I’ve solved the problem. Nothing useful was in the logs, but I came across a forum post where someone had suggested that there were bugs [1][2] in Lion’s display drivers that caused some Macs to lock up when returning from sleep.

I went into System Preferences, then chose Energy Saver. I slid the slider next to Display Sleep all the way to the right, choosing Never. I have not had a lockup since.

I can’t recommend this for everyone, because they don’t have a display that is either off, or using another input, like my TV, but it might work in a pinch if you don’t mind manually turning your display off. Also, I’m a little surprised that it’s affecting my brand new Mac Mini Server, because the articles I’ve found refer to older Macs.

Update: This may have been fixed by the 10.7.1 patch. I haven’t tested it yet.

Testing RAM on a Macbook Pro

2011/02/03 23:46:02
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My Macbook Pro came with 4GB of RAM in 2 2GB modules.  In December, I ordered 8 GB (2 4GB modules) of RAM from Crucial so I could max out the RAM in the machine, as I intend to keep it for a while.

Early in January I started to see applications crash.  VMWare Fusion became almost unusable.  I had never seen this behavior from my Mac before, so figured the new RAM had something to do with it, but wanted to verify my assumption before blindly blaming Crucial for bad RAM.  At that point I had to learn how to best test RAM on a Mac.

On a PC, I would have booted a CD running memtest86, but none of my Internet research indicated that was possible for the Mac.

Instead, I had to install memtest and boot the machine into single user mode in order to run memtest with the least amount of interference from running programs. I wanted to document what I had done in a blog post so I could remember next time.

Note: This article involves configuring your Mac in a way that could make it difficult for you to boot it normally if you don’t know what you’re doing. If you don’t feel comfortable following these instructions, take your Mac to a technician who not only feels comfortable fixing your computer, but is also competent to do so.

To tell the Mac to boot into verbose 64-bit single user mode, go to Applications/Utilities/Terminal, and type:

sudo nvram boot-args="-s arch=x86_64 -v"

You may be asked for the password you type to make changes to the system, type the password and hit enter.

Reboot your Mac.

Your Mac will boot to a prompt.  You may be surprised at all of the text that goes by.   These are kernel messages that are normally hidden by the nifty Apple logo.

At the prompt type:

memtest all

This will attempt to test as much RAM as it can.  Remember that some of your RAM is being taken up by minimal amount of software that booted your machine into single user mode (the kernel and bash), so you won’t be able to test everything, but if the flaw was in the RAM being used by the software running at this point, the machine would have failed to boot.

If your RAM is good, the test will take hours and you will see something like:

Memtest version 4.22 (64-bit)
Copyright (C) 2004 Charles Cazabon
Copyright (C) 2004-2008 Tony Scaminaci (Macintosh port)
Licensed under the GNU General Public License version 2 only
Mac OS X 10.6.6 (10J567) running in single user mode
Memory Page Size:  4096
System has 2 Intel core(s) with SSE
Requested memory: 7555MB (7922552832 bytes)
Available memory: 7555MB (7922552832 bytes)
NOTE:  Memory request is too large, reducing to acceptable value...
Allocated memory:  7343MB (7700721344 bytes) at local address 0x00000000101000000
Attempting memory lock... locked successfully
Partitioning memory into 2 comparison buffers...
Buffer A: 3671MB (3850360672 bytes) starts at local address 0x0000000101000000
Buffer B: 3671 MB (3850360672 bytes) starts at local address 0x00000001e67fd760
Running 1 test sequence... (CTRL-C to quit)
Test sequence 1 of 1:
Running tests on full 7343MB region...
  Stuck Address       : ok
  Linear PRN            : ok
Running comparison tests using 3671MB buffer...
  Random Value        : ok
  Compare XOR         : ok
  Compare SUB         : ok
  Compare MUL         : ok
  Compare DIV          : ok
  Compare OR           : ok
  Compare AND         : ok
  Sequential Increment:ok
  Solid Bits               :ok
  Block Sequential      :ok
  Checkerboard         :ok
  Bit Spread             :ok
  Bit Flip                  :ok
  Walking Ones         :ok
  Walking Zeroes       :ok
All tests passed!  Execution time:  7068 seconds.

If your RAM is bad, you will likely get a response really fast, and you will see something like:

*** Address Test Failed *** One or more DIMM address lines are non-functional.


FAILURE: possible bad address line at offset 0x06b3a4c8.


FAILURE: 0xbea1ce76 != 0xbea1ce7e at offset 0x06b3a4c8.


*** Memory Test Failed *** Please check transcript for details.

If you get a message like:

Attempting memory lock... ERROR: Memory lock failed - reason unknown.

WARNING: Testing with unlocked memory may be slower and less reliable

hit CTRL-C and make sure you booted into 64-bit mode by typing the following:

uname -m

If the command doesn’t return x86_64 then you are not running in 64-bit mode and your Mac can’t reach the RAM above 4GB. Please reference the above command for booting your Mac into single user mode, as it includes the flags to set it to 64-bit mode, then reboot.

If the command above does return x86_64, then you are running in 64-bit mode and this message is probably a preview of test failures to come.

If you have more than one module of RAM, then take all of them out, and place one in the machine, boot, and run the test. After that one finishes test, note the results, turn the Mac off, take the RAM module out, and put the next one in. This way you can figure out which module is bad.

When done, make sure you have good RAM in the machine, boot it again, and type:

sudo nvram boot-args=""

This will give you your “normal” Apple logo screen on boot.

I hope you will find this article as useful as I will the next time we need to test RAM on a Mac.

Today, Crucial was nice enough to send me some new RAM after I sent them the bad module.  So far, it appears to be passing memtest.  Cross your fingers for me!  :)

A memo on Open Source

2010/06/10 03:34:13
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In light of the arrival of the iPhone 4, I’ve been confronted with an age-old question:  what is the current state of open source?

Apple is ruling the future of the phone market with iPhone.  Android is the up-and-coming competitor.  Google is betting on their Linux phone, and betting hard.  Google’s goal is to make money with their ads.  I figured it would work, until I saw that Apple had iAds, something far more media-rich than Google Ads.

Apple is creating the tablet market with the iPad.  Some manufacturers have been working on Android tablets, but they will be late to the game.

Apple is moving into the mobile gaming market with the iPhone/iPod/iPad.  Android hasn’t really started to achieve the level of choice available in the Apple App Store.

Microsoft is continuing to hold onto its desktop OS market.  OS X is making inroads into this market because of Microsoft’s failure to get wide adoption of Windows Vista.  It remains to be seen if Windows 7 can recoup those losses.  Desktop Linux failed on many fronts for many reasons.  OS X and Windows have something Linux lacked, a consistent interface for applications.  Ubuntu is the most promising Desktop Linux available for the average user, but it can’t overcome the inconsistency across the UIs of the thousands of applications it supports.

Microsoft is continuing the hold onto its business server market.  This is the market of file sharing and directory services.  Neither Apple nor the Open Source world have been able to offer an alternative in these areas that has the level of adoption as Windows Servers.

Linux seems to be best suited for appliances, like wireless routers, but it remains to be seen how many hardware manufacturers see it as beneficial to continue to use Linux rather than implementing their own OS and utilities.

Linux seems to be the platform of choice for hosting application servers, like JBoss.  Unfortunately for Linux, most (all?) of these application servers can also be easily run on Windows or OS X.

Linux seems to be the platform of choice for web servers.  This is largely because Linux is inexpensive and IIS is not as feature-rich as Apache.  Apache can be easily run on Windows or OS X.

So, where does Open Source fit into this new world order?

There are several options for the Open Source developer (not in any particular order):

  1. Write applications in Java, Scala, or some other language that is platform independent, in hopes that it will be available on the largest number of platforms.  This will not help you on iOS, where you are stuck with Apple’s API and Apple’s implementation of Objective-C.
  2. Continue to develop applications for the LAMP platform.  As most of the these apps only need the AMP without the Linux, get used to the idea that folks might run it on Windows or OS X.
  3. Write some libraries that can be incorporated into iOS apps.  This may violate Apple’s terms of use, so be careful.
  4. Continue to write desktop applications that only run on Linux.
  5. Android.  Google is actually achieving some consistency for apps on its Linux platform, but not to the degree that Apple’s draconian tactics have achieved.
  6. Make something NEW.  Actually innovate in a way that forces the Apples and the Microsofts of the world to fear, and, eventually copy, the idea/concept/software.  This is an area where open source once shined.