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.

    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

    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).

    \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.)

    enscript -E -B -3 -r -s 0 --borders -fCourier4.8 --mark-wrapped-lines=arrow
    mkc · 2009-02-09 06:23:38 1
  • 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

    date --date="1 fortnight ago"
    mkc · 2009-02-06 20:57:59 0
  • 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

    strace -f -s 512 -v ls -l
    mkc · 2009-02-06 02:45:33 0
  • 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.)

    cat -A
    mkc · 2009-02-06 02:37:51 0
  • 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

    echo '2^2^20' | time bc > /dev/null
    mkc · 2009-02-06 02:31:55 1
  • 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

    watch -n 1 'pstack 12345 | tac'
    mkc · 2009-02-05 18:17:00 3

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easily trace all Nginx processes
Nginx (and other webservers like Apache) can be awkward to trace. They run as root, then switch to another user once they're ready to serve web pages. They also have a "master" process and multiple worker processes. The given command finds the process IDs of all Nginx processes, joins them together with a comma, then traces all of them at once with "sudo strace." System trace output can be overwhelming, so we only capture "networking" output. TIP: to kill this complex strace, do "sudo killall strace". Compare with a similar command:

Show a config file without comments

Extracting a range of pages from a PDF, using GhostScript
In this example we extract pages 14-17

Exclude .svn, .git and other VCS junk for a pristine tarball
~$ tar --version tar (GNU tar) 1.20

Use color grep by default
Alias the grep command to show colored results by default.

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" }

Find usb device in realtime
Using this command you can track a moment when usb device was attached.

pngcrush all .png files in the directory
Find all pngs in directory structure and pngcrush them, none destructive. You can just remove the "{}.crush" part if you want destructive.

Anti Syn Ddos
Ddos syn attack

Clean up poorly named TV shows.
Replace 'SHOWNAME' with the name of the TV show. Add -n to test the command without renaming files. Check the 'sample output'.

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