PORTAGE_sharedMagicStreams - SamuelLarkin/LizzyConversion GitHub Wiki

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Magic Streams

Almost all programs in the PORTAGE shared suite use what's known as a ''magic stream'', which transparently handles any of the following in place of a file name argument:

• standard input or output (specified as "-")
• a pipe
• a compressed (.gz, .z, .bz2 or .lzma) file

For example, suppose some program portageii_pgm reads an input file and writes an output file. Normally it gets run like this:

portageii_pgm -in infile -out outfile

Now suppose the files are compressed. With magic stream, any of the following could be used:

portageii_pgm -in infile.gz -out outfile.gz

gzip infile.gz | portageii_pgm -in - -out - | gzip > outfile.gz

portageii_pgm -in "zcat infile.gz|" -out outfile.gz

And what if you want to pipe two or more inputs to a PORTAGE shared program? You can specify as many pipes as there are file arguments:

head `yourFile.txt` | portageii_pgm \
 -in1 i1.gz                       \
 -in2 "head -4 i2.txt|"           \
 -in3 i3.txt                      \
 -in4  -                          \
 -out1 o1.gz                      \
 -out2 "| head -4 > outfile.txt"  \
 -out3 o3.txt                     \
 -out4 -                          \
 -out5 "/dev/null"

Here in4 will contain the first lines of yourFile.txt and out4 is going to be displayed on stdout. NOTE that only one "-" as input can work correctly since every magic stream opened with "-" is attached to cin (stdin). Similarly, multiple output streams opened with "-" will yield mixed output on cout (stdout).

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