handy linux commands

When I started using Ubuntu in 2014 and I was new to using the command line, I kept a cheat sheet of stuff I would need but couldn't remember. At this point, most of the stuff below comes naturally to me - but I'm leaving it here anyway. It was of use to me, and so, who knows, it may be of use to some future newbies.

It mostly involves either software that come pre-installed with most Linux distros or is available from the usual repos.

renaming files

Make filenames lowercase for all files in working directory:
rename 'y/A-Z/a-z/' *
Capture strings in filenames and reuse them:
rename -n -e 's/(\d+)randomcrap(\d+)/$1randomcrap$2/' *
Remember to \escape symbols like ., # or !. To preview the result without renaming, add -n directly after rename.

Remove preceding zeroes from filenames:

rename 's/^0*//' *
Take all first lines from every .txt file in the working directory and put those lines in newfile.txt:
read -n1 *.txt > newfile.txt
Replace every occurence of the string 'foo' with the filename of the file it occurs in:
for f in *.txt
 id=$(basename "$file")
 sed -i -e "s/foo/$namex/g" $name
Remove any line in file.txt that does not contain the string 'foo':
sed -n '/foo/p' file.txt > newfile.txt

media conversion

Convert file to ogg audio, compression level 5:
ffmpeg -i in.* -acodec libvorbis -vn -q:a 5 out.ogg
Convert file to mp4 video, quality level 19:
ffmpeg -i in.* -c:v libx264 -crf 19 -strict experimental out.mp4
Change volume in video to audible levels:
ffmpeg -i in.* -vcodec copy -af "volume=100dB" out.*
Remove 10 seconds from start of file, output as ogg audio, compression level 5:
ffmpeg -i in.* -acodec libvorbis -ss 00:00:10 -vn -q:a 5 out.ogg
Convert video for Web use (webm):
ffmpeg -i in.* -vf scale=640:ih*640/iw -vcodec libvpx -b:v 0.4M -acodec libvorbis -qscale:a 5 out.webm
Convert video for Web use if original video height is an uneven number of pixels:
ffmpeg -i in.* -vf scale=640:1+ih*640/iw -vcodec libvpx -b:v 0.4M -acodec libvorbis -qscale:a 5 out.webm
Turn series of images into MP4 video (0001.tif, 0002.tif...)
ffmpeg -r 60 -f image2 -s 1004x1024 -i %04d.tiff -vcodec libx264 -crf 25 -pix_fmt yuv420p out.mp4
Reduce gamma of video:
ffmpeg -i in.* "eq=gamma=0.7" out.*

Extract subtitles from mkv video

You'll need to install MKVtoolnix. First, check which MKV track the subtitles are in:
mkvmerge -i input.mkv
Then extract the track:
mkvextract tracks input.mkv [trackID]:[output].srt
Example, if subtitles are in track 2:
mkvextract tracks movie.mkv

remove mkv metadata title

MKV files (films, series) are sometimes given very unhelpful titles. I have VLC set to display the metadata title and I hate not being able to see what episode of something I'm on - this takes quick care, editing files in place to delete the title tag. This removes the title from all mkv files in the working directory; it requires MKVtoolnix.
for f in *.mkv; do mkvpropedit -d title "$f"; done

split flac audio according to cue sheet

FLAC audio can come in a single file, which is then played according to a cue sheet, so a music player displays it as a set of tracks. Great, but I use a file manager that just sees a great big file. If you have the cue file and the FLAC file, install shnsplit and run this (change filenames of course) to split the file into individual FLAC files:
shnsplit -f yourfile.cue -t %n-%t -o flac yourfile.flac

combine vob dvd files into a single video file

This command combines DVD VOB files into a single VOB video file. Add or remove numbers from between the square brackets depending on how many files you have, and what they're called.
cat VIDEO_TS/VTS_01_[1234].VOB > vid.vob

some deja-dup/duplicity stuff

Restore specific file or directory in working directory:
deja-dup --restore FILE1 FILE2
Generate full list of all files in the current backup, write to file 'bakfiles':
duplicity list-current-files file:///path/to/backup/folder > bakfiles

documents & misc

Convert all .doc files in working directory to PDFs of the same filenames:
lowriter --convert-to pdf *.doc
Compress images in PDF files and output a new PDF file. This particular command compresses embedded images to 'ebook' levels; decent but not beautiful.
gs -sDEVICE=pdfwrite -dNOPAUSE -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dBATCH -sOutputFile=out.pdf in.pdf
The controls aren't terribly fine-grained, but it's good enough for most purposes. Other options for the -DPDFSETTINGS flag are /screen (shit), /printer (a bit better than ebook), /prepress (nice) and /default (the best).

List all installed fonts: