Small snippets worth sharing: A collection of tricks to solve common problems

These are tools and tricks I use regularly which are too small to give them full articles but too useful not to describe them.

PDF (drucken)

Calculate the CSP script-src hash for an inline script-tag

echo -n 'var inline = 1;' > /tmp/foobar
sha256sum /tmp/foobar | cut -d " " -f 1 | xxd -r -p | base64 

For background, see mdn: Content-Security-Policy/script-src.

Build Freenet with Java 8 on Guix

guix environment --ad-hoc icedtea:jdk -- \
  bash -c 'ssh kav freenet/run.sh stop; \
    ./gradlew --no-daemon clean build -x test && \
    for i in freenet.jar freenet.jar.new; do \
      scp build/libs/freenet.jar kav:freenet/$i; \
    done; \
    ssh kav freenet/run.sh start'

jump between versions within a filesystem tree

# start ~/path/to/source/branch-7.9/src/path/to/folder
cd $(echo $PWD | sed s/7.9/master/)
# now at ~/path/to/source/branch-master/src/path/to/folder

I use this regularly at work to avoid deep navigation. Typically via C-r 7.9/master — and for the reverse C-r master/7.9.

Optimize bash defaults: increase history size, update history instantly, share history

This is essential to re-use commands without typing them.

Add the following to ~/.bashrc:

# better bash history handling
# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
export HISTSIZE=100000
export HISTFILESIZE=1000000
# append to the history file, don't overwrite it
shopt -s histappend
# don't put duplicate lines or lines starting with space in the history.
# See bash(1) for more options
export HISTCONTROL=ignoredups:erasedups
# update the history with every command, not only at exit

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

# If set, the pattern "**" used in a pathname expansion context will
# match all files and zero or more directories and subdirectories.
shopt -s globstar

Activate readline and colors in the Guile REPL

To be enjoyable to use interactively, Guile requires readline and colors.

Just put the following in ~/.guile:

(cond ((false-if-exception (resolve-interface '(ice-9 readline)))
       (lambda (module)
         ;; Enable completion and input history at the REPL.
         ((module-ref module 'activate-readline))))
       (display "Consider installing the 'guile-readline' package for
convenient interactive line editing and input history.\n\n")))

      (unless (getenv "INSIDE_EMACS")
        (cond ((false-if-exception (resolve-interface '(ice-9 colorized)))
               (lambda (module)
                 ;; Enable completion and input history at the REPL.
                 ((module-ref module 'activate-colorized))))
               (display "Consider installing the 'guile-colorized' package
for a colorful Guile experience.\n\n"))))

optimize scanned image for homework in Gnome

With this trick you get a right-click menu in Nautilus (Gnome file manager) that optimizes a scanned file for sending as homework assignment result.

Save the following as ~/.local/share/nautilus/scripts/optimize-scan-for-homework and run chmod + ~/.local/share/nautilus/scripts/optimize-scan-for-homework.


# This script makes a scanned image suitable (=small enough) for
# sending as homework assignment by replacing almost-white pixes from
# scans by white pixels and then running pngquant

# thanks for the urldecoder goes to to https://stackoverflow.com/a/37840948
# license: cc by-sa (as this is stackoverflow)
urldecode() { : "${*//+/ }"; echo -e "${_//%/\\x}"; }
base="$(echo $(urldecode "$NAUTILUS_SCRIPT_CURRENT_URI") | cut -d / -f3-)"
while [ ! -z "$1" -a ! -e "$base/$1" ]; do shift; done

if [ -f "$filename" ] && [[ x"$(file -b --mime-type "$filename" | sed s,/.*,,)" = x'image' ]]; then
    if [ -f "${COLORFILE}" ]; then
       zenity --error --width 400 \
              --text "Temporary file ${COLORFILE} already exists, not overwriting" \
              --title "temp file exists: ${COLORFILE}";
        convert "${filename}" -contrast-stretch 1%x80% "${COLORFILE}"
        pngquant --skip-if-larger --strip --speed 1 "${COLORFILE}" || zenity --error --width 400 \
                 --text "running pngquant on ${COLORFILE} failed";
    zenity --error --width 400 \
                   --text "image optimization needs an image file, but \n${filename}\n is not an image file.\n\nIts mime type is $(file -b --mime-type "$filename")" \
                   --title "not an image: ${filename}"

Evaluate website logs with goaccess

uses goaccess.

cp logs/access_log_2021*.gz /tmp/
cd /tmp/
gunzip access_log_2021-0*
cat access_log* > aggregated_log.log
goaccess --all-static-files --ignore-crawlers -f aggregated_log.log

Now hit the number of the part you’re interested in, jump to next with tab.

Sort with s, expand with enter or space or o, scroll down with page down or CTRL-f and up with page up or CTRL-b.

Hit ? for more info.

Example, unique visitors per day, ordered by number of visitors:


To create a csv file:

goaccess --date-format='%d/%b/%Y' \
         --time-format='%H:%M:%S' \
         --log-format='%h %^[%d:%t %^] "%r" %s %b "%R" "%u"' \
         --max-items=99999999 --all-static-files --ignore-crawlers \
         -f /tmp/aggregated_log.log \
         -o /tmp/agg.csv

ArneBab 2020-05-18 Mo 00:00 - Impressum - GPLv3 or later (code), cc by-sa (rest)