watch
Sadly, the watch command is a newer discovery for me. This useful ncurses-based utility allows you to monitor activity generated by the output of other commands over time. Here's a quick example:
watch 'ls -la /tmp'
This will display the contents of the /tmp directory, refreshing it every 2 seconds. The refresh time is configurable via a parameter, i.e.:
watch -n 10 'ls -la /tmp'
The -n parameter takes input in seconds, thus the above command-line would generate output every 10 seconds. Using watch to monitor a directory for changes is useful in many scenarios. At times, I've found myself using this to monitor processes writing files. However, here's an example that I use even more frequently:
watch "netstat -na |egrep '(SYN_SENT|ESTABLISHED|TIME_WAIT|FIN_WAIT_1|FIN_WAIT_2)'"
This effectively allows you to monitor the many states of open network sockets as they happen. While there are other tools that can do this (i.e. ntop), but the upside is that watch is included out-of-the-box in many Linux distros. Since we're only manipulating command-line parameters, the level of customization is very high. If you throw in an additional grep statement before grabbing the socket state, you can effectively filter on other criteria, such as IP addresses.
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| Figure 1: monitoring network sockets using 'watch'. |
It's worth noting that you'll not need to include all of these socket states in most situations. If you're not having issues with connectivity on either end, you're likely to only observe ESTABLISHED and TIME_WAIT. For obvious reasons, I have found SYN_SENT useful for noting issues with remote hosts. The others are included for completeness.
whatis
Put quite simply, the whatis utility displays short summaries of man pages on the command-line.
$ whatis whatis
whatis (1) - display manual page descriptions
cal
As the name indicates, cal displays all or part of a calendar. While not an earth-shaking game-changer, the cal utility can be quite useful. Here are a few examples:
Display a full calendar for the year 2012: cal 2012
Display calendar for November 1955: cal 11 1955
Display calendar for the 8th month of the current year: cal -m 8
This can also be achieved via: cal 8 2013
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| Figure 2: Demonstrating the 'cal' command. |
Perhaps it's my nature in being a "CLI guy", but I much prefer this to clicking through any of the wonky date & time widgets. I also find this to be much faster than most GUI, especially when you're in need of a date many months or years from today. What day of the week is the wife's birthday next year? cal N 2014 - done!
lsblk
List block devices (aka lsblk) is part of the util-linux software package. While it's a relatively new utility to me, it certainly is useful. The default output of lsblk looks somewhat similar to df
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0 7:0 0 666.1M 1 loop /rofs
sda 8:0 0 465.8G 0 disk
├─sda1 8:1 0 39.2M 0 part
├─sda2 8:2 0 14.8G 0 part
└─sda3 8:3 0 450.9G 0 part
sr0 11:0 1 695.3M 0 rom /cdrom
$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/cow 1932460 395084 1537376 21% /
udev 1924792 4 1924788 1% /dev
tmpfs 772988 876 772112 1% /run
/dev/sr0 711980 711980 0 100% /cdrom
/dev/loop0 682240 682240 0 100% /rofs
tmpfs 1932460 156 1932304 1% /tmp
none 5120 4 5116 1% /run/lock
none 1932460 180 1932280 1% /run/shm
While the output of each utility is similar, there are some real differences. First, lsblk includes a very clear partition hierarchy in the NAME column. I find this to be very easy to read as well as convenient. Next, we have the MAJ and MIN columns, which refer to Linux device major and minor numbers. The RM and RO columns, which stand for 'removable' and 'read-only' respectively, are things I wish df had been telling me all along. The TYPE column is self explanatory, but is another example of output that I find to be useful.
While the default output of lsblk is great, it's also very customizable. The -o switch allows you to specify a comma-separated list of which columns are present in the output. Here's an example:
$ lsblk -o NAME,MODEL
NAME MODEL
loop0
sda WDC WD5000AAKX-7
├─sda1
├─sda2
└─sda3
sr0 DVD+-RW GH70N
This is definitely a convenient way to determine where physical disks are mapped. I no longer have to dig through dmesg output! Well... not for this reason, anyway.
If you're like me, you may have many physical systems for various reasons. Do you have a built-in multi-card reader on your PC? Do you remember which device maps to which type of reader? I don't.
$ lsblk -a -o NAME,MODEL
NAME MODEL
ram0
ram1
ram2
[ ... SNIP ... ]
loop7
sda WDC WD5000AAKX-7
├─sda1
├─sda2
└─sda3
sr0 DVD+-RW GH70N
sdb SD/MMC
sdc Compact Flash
sdd SM/xD Picture
sde MS/MS-Pro
Now I recall! The -a switch shows all devices. Note that the output has been trimmed. There are a lot of other columns available, including STATE, OWNER, GROUP, and MODE. Check lsblk --help for many more options.
That's all for now. I hope you find this information to be useful in your day-to-day CLI adventures.
