Bourne Shell Scripting/Appendix D: Cookbook
 Post a new Cookbook entry
|
| If you use the title box, then you do not need to put a title in the body. |
Branch on extensions
[edit | edit source]When writing a bash script which should do different things based on the extension of a file, the following pattern is helpful.
#filepath should be set to the name(with optional path) of the file in question
ext=${filepath##*.}
if [[ "$ext" == txt ]] ; then
#do something with text files
fi
(Source: slike.com Bash FAQ).
Rename several files
[edit | edit source]This recipe shows how to rename several files following a pattern.
In this example, the user has huge collection of screenshots. This user wants to rename the files using a Bourne-compatible shell. Here is an "ls" at the shell prompt to show you the filenames. The goal is to rename images like "snapshot1.png" to "nethack-kernigh-22oct2005-01.png".
$ ls
snapshot1.png snapshot25.png snapshot40.png snapshot56.png snapshot71.png
snapshot10.png snapshot26.png snapshot41.png snapshot57.png snapshot72.png
snapshot11.png snapshot27.png snapshot42.png snapshot58.png snapshot73.png
snapshot12.png snapshot28.png snapshot43.png snapshot59.png snapshot74.png
snapshot13.png snapshot29.png snapshot44.png snapshot6.png snapshot75.png
snapshot14.png snapshot3.png snapshot45.png snapshot60.png snapshot76.png
snapshot15.png snapshot30.png snapshot46.png snapshot61.png snapshot77.png
snapshot16.png snapshot31.png snapshot47.png snapshot62.png snapshot78.png
snapshot17.png snapshot32.png snapshot48.png snapshot63.png snapshot79.png
snapshot18.png snapshot33.png snapshot49.png snapshot64.png snapshot8.png
snapshot19.png snapshot34.png snapshot5.png snapshot65.png snapshot80.png
snapshot2.png snapshot35.png snapshot50.png snapshot66.png snapshot81.png
snapshot20.png snapshot36.png snapshot51.png snapshot67.png snapshot82.png
snapshot21.png snapshot37.png snapshot52.png snapshot68.png snapshot83.png
snapshot22.png snapshot38.png snapshot53.png snapshot69.png snapshot9.png
snapshot23.png snapshot39.png snapshot54.png snapshot7.png
snapshot24.png snapshot4.png snapshot55.png snapshot70.png
First, to add a "0" (zero) before snapshots 1 through 9, write a for loop (in effect, a short shell script).
- Use ? which is a filename pattern for a single character. Using it, I can match snapshots 1 through 9 but miss 10 through 83 by saying snapshot?.png.
- Use ${parameter#pattern} to substitute the value of parameter with the pattern removed from the beginning. This is to get rid of "snapshot" so I can put in "snapshot0".
- Before actually running the loop, insert an "echo" to test that the commands will be correct.
$ for i in snapshot?.png; do echo mv "$i" "snapshot0${i#snapshot}"; done
mv snapshot1.png snapshot01.png
mv snapshot2.png snapshot02.png
mv snapshot3.png snapshot03.png
mv snapshot4.png snapshot04.png
mv snapshot5.png snapshot05.png
mv snapshot6.png snapshot06.png
mv snapshot7.png snapshot07.png
mv snapshot8.png snapshot08.png
mv snapshot9.png snapshot09.png
That seems good, so run it by removing the "echo".
$ for i in snapshot?.png; do mv "$i" "snapshot0${i#snapshot}"; done
An ls confirms that this was effective.
Now change prefix "snapshot" to "nethack-kernigh-22oct2005-". Run a loop similar to the previous one:
$ for i in snapshot*.png; do \
> mv "$i" "nethack-kernigh-22oct2005-${i#snapshot}" \
> done
This saves the user from typing 83 "mv" commands.
Long command line options
[edit | edit source]The builtin getopts does not support long options so the external getopt is required. (On some systems, getopt also does not support long options, so the next example will not work.)
eval set -- $(getopt -l install-opts: "" "$@")
while true; do
case "$1" in
--install-opts)
INSTALL_OPTS=$2
shift 2
;;
--)
shift
break
;;
esac
done
echo $INSTALL_OPTS
The call to getopt quotes and reorders the command line arguments found in $@. set then makes replaces $@ with the output from getopt
Another example of getopt use can also be found in the Advanced Bash Script Guide
Process certain files through xargs
[edit | edit source]In this recipe, we want to process a large list of files, but we must run one command for each file. In this example, we want to convert the sampling rates of some sound files to 44100 hertz. The command is sox file.ogg -r 44100 conv/file.ogg, which converts file.ogg to a new file conv/file.ogg. We also want to skip files that are already 44100 hertz.
First, we need the sampling rates of our files. One way is to use the file command:
$ file *.ogg
audio_on.ogg: Ogg data, Vorbis audio, mono, 44100 Hz, ~80000 bps
beep_1.ogg: Ogg data, Vorbis audio, stereo, 44100 Hz, ~193603 bps
cannon_1.ogg: Ogg data, Vorbis audio, mono, 48000 Hz, ~96000 bps
...
(The files in this example are from Secret Maryo Chronicles.) We can use grep -v to filter out all lines that contain '44100 Hz':
$ file *.ogg | grep -v '44100 Hz'
cannon_1.ogg: Ogg data, Vorbis audio, mono, 48000 Hz, ~96000 bps
...
jump_small.ogg: Ogg data, Vorbis audio, mono, 8000 Hz, ~22400 bps
live_up.ogg: Ogg data, Vorbis audio, mono, 22050 Hz, ~40222 bps
...
We finished with "grep" and "file", so now we want to remove the other info and leave only the filenames to pass to "sox". We use the text utility cut. The option -d: divides each line into fields at the colon; -f1 selects the first field.
$ file *.ogg | grep -v '44100 Hz' | cut -d: -f1
cannon_1.ogg
...
jump_small.ogg
live_up.ogg
...
We can use another pipe to supply the filenames on the standard input, but "sox" expects them as arguments. We use xargs, which will run a command repeatedly using arguments from the standard input. The -n1 option specifies one argument per command. For example, we can run echo sox repeatedly:
$ file *.ogg | grep -v '44100 Hz' | cut -d: -f1 | xargs -n1 echo sox
sox cannon_1.ogg
...
sox itembox_set.ogg
sox jump_small.ogg
...
However, these commands are wrong. The full command for cannon_1.ogg, for example, is sox cannon_1.ogg -r 44100 conv/cannon_1.ogg. "xargs" will insert incoming data into placeholders indicated by "{}". We use this strategy in our pipeline. If we have doubt, then first we can build a test pipeline with "echo":
$ file *.ogg | grep -v '44100 Hz' | cut -d: -f1 | \
> xargs -i 'echo sox {} -r 44100 conv/{}'
sox cannon_1.ogg -r 44100 conv/cannon_1.ogg
...
sox itembox_set.ogg -r 44100 conv/itembox_set.ogg
sox jump_small.ogg -r 44100 conv/jump_small.ogg
...
It worked, so let us remove the "echo" and run the "sox" commands:
$ mkdir conv
$ file *.ogg | grep -v '44100 Hz' | cut -d: -f1 | \
> xargs -i 'sox {} -r 44100 conv/{}'
After a wait, the converted files appear in the conv subdirectory. The above three lines alone did the entire conversion.
Simple playlist frontend for GStreamer
[edit | edit source]If you have GStreamer, the command gst-launch filesrc location=filename ! decodebin ! audioconvert ! esdsink will play a sound or music file of any format for which you have a GStreamer plugin. This script will play through a list of files, optionally looping through them. (Replace "esdsink" with your favorite sink.)
#!/bin/sh
loop=false
if test x"$1" == x-l; then
loop=true
shift
fi
while true; do
for i in "$@"; do
if test -f "$i"; then
echo "${0##*/}: playing $i" > /dev/stderr
gst-launch filesrc location="$i" ! decodebin ! audioconvert ! esdsink
else
echo "${0##*/}: not a file: $i" > /dev/stderr
fi
done
if $loop; then true; else break; fi
done
This script demonstrates some common Bourne shell tactics:
- "loop" is a boolean variable. It works because its values "true" and "false" are both Unix commands (and sometimes shell builtins), thus you can use them as conditions in if and while statements.
- The shell builtin "shift" removes $1 from the argument list, thus shifting $2 to $1, $3 to $2, and so forward. This script uses it to process an "-l" option.
- The substitution ${0##*/} gives everything in $0 after the last slash, thus "playlist", not "/home/musicfan/bin/playlist".