Commands by mkc (9)

  • This command is somewhat similar to 'nice', but constrains I/O usage rather than CPU usage. In particular, the '-c3' flag tells the OS to only allow the process to do I/O when nothing else is pending. This dramatically increases the responsiveness of the rest of the system if the process is doing heavy I/O. There's also a '-p' flag, to set the priority of an already-running process.


    6
    ionice -c3 find /
    mkc · 2009-02-19 17:23:12 3
  • The Linux /dev/full file simulates a "disk full" condition, and can be used to verify how a program handles this situation. In particular, several programming language implementations do not print error diagnostics (nor exit with error status) when I/O errors like this occur, unless the programmer has taken additional steps. That is, simple code in these languages does not fail safely. In addition to Perl, C, C++, Tcl, and Lua (for some functions) also appear not to fail safely. Show Sample Output


    9
    perl -e 'print 1, 2, 3' > /dev/full
    mkc · 2009-02-19 17:08:13 2
  • The backslash avoids any 'rm' alias that might be present and runs the 'rm' command in $PATH instead. In a misguided attempt to be more "friendly", some Linux distributions (or sites/etc.) alias 'rm' to 'rm -i'. Unfortunately, this trains users to expect that files won't actually be deleted until they okay it. This expectation will fail with catastrophic results when they use other distributions, move to other sites, etc., and doesn't really even work 100% even with the alias. It's too late to fix 'rm', but '\rm' should work everywhere (under bash).


    3
    \rm somefile
    mkc · 2009-02-19 16:55:54 0
  • This is the setup I'm using for my largest project. It gives 357 lines per page (per side), which makes it fairly easy to carry around a significant amount of code on a few sheets of paper. Try it. (I stick to the 80 column convention in my coding. For wider code, you'll have to adjust this.)


    1
    enscript -E -B -3 -r -s 0 --borders -fCourier4.8 --mark-wrapped-lines=arrow
    mkc · 2009-02-09 06:23:38 2
  • The date command does offset calculations nicely, handles concepts like "a month" as you'd expect, and is good for offsets of at least 100M years in either direction. Show Sample Output


    3
    date --date="1 fortnight ago"
    mkc · 2009-02-06 20:57:59 1
  • strace can be invaluable in trying to figure out what the heck some misbehaving program is doing. There are number of useful flags to limit and control its output, and to attach to already running programs. (See also 'ltrace'.) Show Sample Output


    5
    strace -f -s 512 -v ls -l
    mkc · 2009-02-06 02:45:33 1
  • This is priceless for discovering otherwise invisible characters in files. Like, for example, that stray Control-M at the end of the initial hash bang line in your script, which causes it to generate a mysterious error even though it looks fine. ('od' is the last word, of course, but for many purposes it's much harder to read.)


    0
    cat -A
    mkc · 2009-02-06 02:37:51 2
  • This is a quick and dirty way to generate a (non-floating-point) CPU-bound task to benchmark. Adjust "20" to higher or lower values, as needed. As a benchmark this is probably a little less bogus than bogomips, and it will run anywhere 'bc' does. Show Sample Output


    2
    echo '2^2^20' | time bc > /dev/null
    mkc · 2009-02-06 02:31:55 3
  • This command repeatedly gets the specified process' stack using pstack (which is an insanely clever and tiny wrapper for gdb) and displays it fullscreen. Since it updates every second, you rapidly get an idea of where your program is stuck or spending time. The 'tac' is used to make the output grow down, which makes it less jumpy. If the output is too big for your screen, you can always leave the 'tac' off to see the inner calls. (Or, better yet--get a bigger screen.) Caveats: Won't work with stripped binaries and probably not well with threads, but you don't want to strip your binaries or use threads anyway. Show Sample Output


    8
    watch -n 1 'pstack 12345 | tac'
    mkc · 2009-02-05 18:17:00 5

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list block devices
Shows all block devices in a tree with descruptions of what they are.

Convert seconds to [DD:][HH:]MM:SS
Converts any number of seconds into days, hours, minutes and seconds. sec2dhms() { declare -i SS="$1" D=$(( SS / 86400 )) H=$(( SS % 86400 / 3600 )) M=$(( SS % 3600 / 60 )) S=$(( SS % 60 )) [ "$D" -gt 0 ] && echo -n "${D}:" [ "$H" -gt 0 ] && printf "%02g:" "$H" printf "%02g:%02g\n" "$M" "$S" }

Block all IP addresses and domains that have attempted brute force SSH login to computer
Searches all log files (including archived bzip2 files) for invalid user and PAM authentication errors, both of which are indicative of brute force attempts at logging into computer. A list of all unique IP addresses and domain names is appended to hosts.deny. The command (and grep error messages) will work on Mac OS X 10.6, small adjustments may be needed for other OSs.

Blue Matrix
Same as original, but works in bash

Multi-thread any command
For instance: $ find . -type f -name '*.wav' -print0 |xargs -0 -P 3 -n 1 flac -V8 will encode all .wav files into FLAC in parallel. Explanation of xargs flags: -P [max-procs]: Max number of invocations to run at once. Set to 0 to run all at once [potentially dangerous re: excessive RAM usage]. -n [max-args]: Max number of arguments from the list to send to each invocation. -0: Stdin is a null-terminated list. I use xargs to build parallel-processing frameworks into my scripts like the one here: http://pastebin.com/1GvcifYa

Exclude grep from your grepped output of ps (alias included in description)
Surround the first letter of what you are grepping with square brackets and you won't have to spawn a second instance of grep -v. You could also use an alias like this (albeit with sed): alias psgrep='ps aux | grep $(echo $1 | sed "s/^\(.\)/[\1]/g")'

Set Time Zone in Ubuntu
Reconfigures time zone in Ubuntu, which I cannot figure out how to do through the GUI. Worked like a charm to set my time zone to CEST from EDT.

Scan Subnet for IP and MAC addresses

a function to find the fastest DNS server
http://public-dns.info gives a list of online dns servers. you need to change the country in url (br in this url) with your country code. this command need some time to ping all IP in list.

Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.


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