1. almost all the return data is text type.
2. pipeline collect different commands into complicated one.
3. data stream
1. STDIN -0
2. STDOUT -1
3. STDERR -2
4. redirect
1. > STDIN->file(override)
2. >> STDIN->file(add)
3. 2> STDERR->file
4. 2>&1 STDERR->STDIN
5. < file->STDIN
5. pipeline
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