lk/system/bash_tricks.md

title	tags
bash_tricks	Documentation System

Automatic mp3 Tagging

/u/OneTurnMore on Reddit:

!/usr/bin/env bash IFS=$'\n' for f in $(find . -type f); do > id3tool $f -t $(basename $f | cut -d- -f3 | sed 's/.[^,]*$//') done

One can also use

sed s/,[^.]*$//

... in order to avoid multiple full stops messing up syntax.

We can use \. as a literal full stop.

[^ ] means anything other than the containing character, so [^\.] would mean `anything other than a full stop'.

* in sed means this character is repeated any number of times, including 0, so files with no . character would still be processed fine.

$ means `end of the line'.

Apparently sed uses `regex, not globing'.

Regular expressions (``regex'') looks for patterns and is used with find and grep. It interprets *' as a wildcard, ?' as a single-character wildcard, and [12,1,2] as characters matching one of a set (in this case, `12 or 1 or 2 but not 21'].

If the shell is set to find file ``ab.txt'' then it will pass this first to regex, and hit items like aab.txt' and abb.txt'. If it finds nothing, it'll then use globbing, and interpret `ab.txt' literally.

Automatic Renaming

There are a bunch of files:

• Column CV.aux
• Column CV.log
• Column CV.out
• Column CV.pdf
• Column CV.tex
• tccv.cls

Goal: swap the word Column'' for Malin'' in all files.

IFS=$'\n'

for f in $(find . -name "Col*"); do

mv "$f" $(echo "$f" | sed s/Column/Malin/)

done

IFS is the field separator. This is required to denote the different files as marked by a new line, and not the spaces.

Arguments and Input

The rm' program takes arguments, but not stdin' from a keyboard, and therefore programs cannot pipe results into rm.

That said, we can sometimes pipe into rm with xargs rm' to turn the stdin into an argument. For example, if we have a list of files called list.txt' then we could use cat as so:

cat list.txt | xargs rm

... However, this wouldn't work if spaces were included, as rm would take everything literally.

Numbers

Add number to variables with:

• let "var=var+1"

• let "var+=1"

• let "var++"

• ((++var))

• ((var=var+1))

• ((var+=1))

• var=$(expr $var + 1)

((n--)) works identically.

Finding Duplicate Files

find . -type f -exec md5sum '{}' ';' | sort | uniq --all-repeated=separate -w 15 > all-files.txt

... add blank line to top of first file.

awk '/^$/{getline;print;}' all-files.txt > uniq.txt

diff all-files.txt uniq.txt | grep '/' | cut -d '.' -f 2,3,4,5 | sed 's#/##' | sed 's/ /\ /g' | xargs rm

Output random characters.

cat /dev/urandom | tr -cd [:alnum:] | dd bs=1 count=200 status=none && echo