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[dev-chommik]

fun fact about TCP

There's one less-than-expected thing about TCP: even if you provide a small buffer to recv() and the other side sends exactly the same amount of data in the send() call, there's no guarantee that the data is received in one call.

Take for example Python:

# sock is a socket.socket(), already connected data = socket.recv(4) length = struct.unpack('=l', data)[0] # this might fail!

There are two ways to mitigate it:

really portable way: receive in a loop, until all data are received -- but beware of recv() returning b'' - this means there's an error in the communication

Linux/Unix way: use MSG_WAITALL flag to force recv() to block until whole buffer is filled

[dev-chommik]

convert glob-style pattern to regular expression in python

Python's fnmatch library makes it easy to convert human-readable glob pattern to re.match()-compatible regexps:

% python3 >>> import fnmatch >>> fnmatch.translate('dupa.[ch]pp.*') 'dupa\\.[ch]pp\\..*\\Z(?ms)' >>>

[dev-chommik]

split string in python

Python splits string via str.split() and str.rsplit() as well, the latter of which starts splitting from the end of the string. This can be useful when using the second parameter, maxsplit.

For example:

>>> "string with multiple spaces".split(" ", maxsplit=1) ['string', 'with multiple spaces']

This parameter can be used e.g. to split command and arguments apart.

[dev-chommik]

find DNS servers in nmcli

NetworkManager usually puts "127.0.1.1" in /etc/resolv.conf. To get currently used DNS servers, type:

% nmcli d show | grep DNS IP4.DNS[1]: 10.1.2.3 IP4.DNS[2]: 10.2.3.4

[dev-chommik]

quickly open links in Terminator

Instead of using Open link from context menu, you can click the link while holding Control key.

[dev-chommik]

search backwards in less

In addition to searching forwards (/pattern), you can search backwards: ?pattern

[dev-chommik]

print strace summary

To summarise syscalls by time or count, use strace -c. To include child processed, add -f switch. For example:

λ strace -cf git status % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 100.00 0.000008 0 247 24 lstat 0.00 0.000000 0 48 read 0.00 0.000000 0 3 write ... list goes on ...

[dev-chommik]

vim: run command without exiting Insert

In Vim, you can exist Insert mode to enter one command using Ctrl-O, for example: (in insert) Ctrl-O, type :w, then ↵ Enter, then back to writing.

[dev-chommik]

vim: increment/decrement number under cursor

To quickly in-/decrement number under cursor in Vim, use (in Normal):

to increment: Ctrl-A

to decrement: Ctrl-X

[dev-chommik]

shorter for-loops in zsh

When running a for-loop in zsh, you can skip do-done part, leaving only command, for example:

% for day in Mon Tue Wed Thu; date -d "next $day" Mon Apr 25 00:00:00 CEST 2016 Tue Apr 26 00:00:00 CEST 2016 Wed Apr 27 00:00:00 CEST 2016 Thu Apr 28 00:00:00 CEST 2016

Caveat: this doesn't work on multiple commands in one loop — for day in Mon Tue Wed Thu; echo $day; date -d "next $day" won't work as expected

[dev-chommik]

vim: switch case

In Normal mode, use ~ to switch case of letter under cursor. Similar commands are:

gU – to switch to upper-case

gu – to switch to lower-case

For example: gUU converts whole line to upper-case and guiw to convert word surrounder by cursor to lower-case.

[dev-chommik]

list mounts by fs type

You can list mountpoints by file system type using mount -tTYPE. Example:

# mount -ttmpfs tmpfs on /dev/shm type tmpfs (rw) tmpfs on /run type tmpfs (rw,nosuid,nodev,mode=755)

[dev-chommik]

install rpm by url

For those Debian-inclined, this may be surprising. You can install RPMs by providing an URL to either yum or rpm -i:

rpm -i https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm yum install https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm

[dev-chommik]

automatically set prompt colour based on hostname

When logging on multiple machines with synchronized ~/.bashrc / ~/.zshrc you probably don't want to set PS1 on each machine separately. Solution based on hostname will be handy.

To derive user@hostname (or any other host-identifying command) into uniformly-distributed colours, use:

_host_colour=$(echo $(whoami)@$(hostname -f) | sum | awk "{print \$1 % 256 }")

GNU tool sum generates simple, short checksum which are then shortened down to 256 (the default Xterm colour count). Now add the variable to your PS1:

# Bash export PS1="\[\033[38;5;\${_host_colour}m\]\u@\h\[\033[0m\] % " # Zsh (requires module colors) export PS1="%F{\${_host_colour}}%n@%m%f %% "

Protip: If you want to change colour per-host, just set variable _host_colour. You can do it even after PS1 was set.

[dev-chommik]

bash traps

Bash traps override rather than stack. Example:

λ trap 'echo A' SIGINT λ trap 'echo B' SIGINT λ trap 'echo C' SIGINT λ kill -SIGINT $$ C

[dev-chommik]

search in aptitude

You can use regular expressions in aptitude search. Example:

^node$

will search for the whole package name (in this case: npm)

[dev-chommik]

variable names in $(( expr ))

You can use variable names without $ in bash:

a=3 echo $(( a - 1 ))

Note on compatibility: in Bash, you have to prefix special variables like $?, unlike Zsh.