lk/system/bash_tricks.md
2022-11-28 23:50:53 +01:00

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---
title: "bash_tricks"
tags: [ "Documentation", "System" ]
---
# Track Live Changes
See changes in a file as it changes:
> tail -f *somefile*
See changes in a directory, as it changes:
> watch -d ls *directory*
# Automatic mp3 Tagging
> !/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 ``a*b.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 `a*b.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 "Alice" 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