linux:basics
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linux:basics [2019/04/12 14:09] mstraub [Filtering & Selecting] |
linux:basics [2019/09/03 10:57] mstraub [Compressing files] |
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| This page covers basic usage of / working with Linux. | This page covers basic usage of / working with Linux. | ||
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| + | A great resource are the man pages but also the [[https://www.gnu.org/software/coreutils/manual/html_node/index.html|official documentation of the GNU Coreutils]] | ||
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| ==== Compressing files ==== | ==== Compressing files ==== | ||
| - | For compressig files the zip formats gzip (.gz) and bzip2 (.bz2) are commonly used - especially the former. | + | For compressing files the zip formats gzip (.gz), bzip2 (.bz2) and xz (.xz, which uses LZMA compression) are commonly used. |
| + | |||
| + | Gzip is the most commonly used one, but according to [[https://www.rootusers.com/gzip-vs-bzip2-vs-xz-performance-comparison|this in-depth comparison]] .xz is the superior format - it is fast to decompress, achieves high compression rates and also has reasonable compression times until level 2 or 3. | ||
| + | |||
| + | The following commands (un)compress a single file and then remove the old (un)compressed file. | ||
| <code bash> | <code bash> | ||
| gzip my.txt # creates the gzipped file my.txt.gz and remove my.txt | gzip my.txt # creates the gzipped file my.txt.gz and remove my.txt | ||
| - | gzip -d my.txt.gz # recreates my.txt and removes my.txt.gz; same as gunzip | + | gunzip my.txt.gz # recreates my.txt and removes my.txt.gz; same as gzip -d |
| - | bzip2 my.txt # similar to gzip, but using bzip2 compression | + | |
| - | bzip2 -d my.txt.gz # similar to gunzip; same as bunzip2 | + | |
| </code> | </code> | ||
| - | All these commands (un)zip a single file and then remove the old (un)zipped file. | + | The commands ''bzip2'' + ''bunzip2'' and ''xz'' + ''unxz'' work in the same fashion. |
| + | For all zip formats there is a ''cat''-style alias which allows for the creation of an uncompressed data stream: | ||
| <code bash> | <code bash> | ||
| - | gunzip #unzip, same as gzip -d | + | zcat # stream uncompressed gzip file to stdout, same as gzip -cd |
| - | zcat #unzip to stdout, same as gzip -cd | + | bzcat |
| + | xzcat # same as xz --decompress --stdout | ||
| </code> | </code> | ||
| - | The programs ''gzip'' and ''bzip2'' have two "aliases": ''gunzip'' / ''bunzip2'' and ''zcat'' / ''bzcat''. These extra commands provide a short way to typically used functions. | + | |
| ==== Compressed Archives ==== | ==== Compressed Archives ==== | ||
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| ''date'' is a versatile tool for formatting dates / times and setting the system time ([[linux:installation#date_timezone|see also]]). | ''date'' is a versatile tool for formatting dates / times and setting the system time ([[linux:installation#date_timezone|see also]]). | ||
| - | Get the current system time | + | Print the current time in different formats |
| <code bash> | <code bash> | ||
| - | date # print current system time (human readable) | + | date # default - a human readable format |
| + | date --rfc-3339=date # only the date - similar to ISO 8601 | ||
| + | date --rfc-3339=seconds # date and time - similar to ISO 8601 | ||
| + | date +%Y-%m-%dT%H%M # custom format useful for scripts, e.g. 2019-08-14T1129 | ||
| </code> | </code> | ||
| + | |||
| + | Convert unix timestamps | ||
| + | <code bash> | ||
| + | date -d @1278923870 # print the unix timestamp in a human readable format | ||
| + | </code> | ||
| + | |||
| + | |||
| + | |||
| ====== Working with Text ====== | ====== Working with Text ====== | ||
| Line 785: | Line 802: | ||
| </code> | </code> | ||
| - | Two good trick to create readable regexes is to (1) escape the whole regex with single quotes and (2) use a different separator than ''/'' when appropriate. See for yourself in this example of replacing all backslashes in a line with slashes. | + | Two good tricks to create readable regexes are to (1) escape the whole regex with single quotes and (2) use a different separator than ''/'' when appropriate. See for yourself in this example of replacing all backslashes in a line with slashes. |
| <code bash> | <code bash> | ||
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| <code bash> | <code bash> | ||
| sed -n 1~2p file.txt #only print odd lines (=print line 1 and then print each line at step 2) | sed -n 1~2p file.txt #only print odd lines (=print line 1 and then print each line at step 2) | ||
| + | </code> | ||
| + | |||
| + | By default ''sed'' regular expressions are limited, e.g. do not support matching groups. Use the switch ''-E'' to activate extended regular expressions and enjoy matching groups: | ||
| + | |||
| + | <code bash> | ||
| + | sed -E 's#(.*),(.*)#\1;\2#g' #replace a single comma with a semicolon | ||
| + | </code> | ||
| + | |||
| + | However, even with extended expressions ''sed'' does not support non-greedy expressions as it is possible with PCRE (perl compatible regular expressions). A good resort is to simply use ''perl'' itself: | ||
| + | |||
| + | <code bash> | ||
| + | perl -pe 's/.*thevalue="(.*?)".*/\1/g' file.txt | ||
| </code> | </code> | ||
linux/basics.txt · Last modified: 2020/12/21 09:51 by mstraub
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