Using Redirection & Pipes in WSL
Using Redirection & Pipes in WSL
In WSL, many commands can read their input either from a file or from a previous command in a pipeline of commands. There are four that control the flow of data between commands or files.
The bar character, "|", between two commands says send the output to the next command.
The less than character, "<" says get the input for a command from the file following the <
The greater than character, ">" says send your output to a file.
Two greater than characters, ">>" says add your output to the end of a file.
Some additional things to note:
If there's no ">" or ">>" the output of the last command goes to the screen.
Error and status messages are normally sent to the screen.
In documentation etc., the input, output and error/status message streams are named STDIN, STDOUT and STDERR.
If you have command1 | command2 then STDOUT for command1 becomes STDIN for command2.
There's an example of piping in the instructions to enumerate the markers in a file. Let's look at it in detail:
cat filename.db | sed -e "s/ .*$//" | sort | uniq -c > sfm-list.txt
There's 4 commands in the pipeline:
cat gets a filename(s) on the command line and sends the contents to STDOUT.
cat filename.db | sed -e "s/ .*$//"
is the same as:
sed -e "s/ .*$//" < filename.db
sed -e does some regular expression commands on STDIN and then passes the result to STDOUT
This pipe does the sed command s/from/to/
s/from/to/ takes text that matches from and changes it into to.
the to can be empty, in which case from is deleted.
The effect here is to delete everything following the first space, i.e., everything but the leading SFM.
That converts each line into just its SFM.
sort does what it says; it sorts STDIN and then passes the result to STDOUT
this puts all the same SFMs together.
uniq reads STDIN and ignores repeats. then passes the result to STDOUT
The -c option adds a count of how many times a line was repeated.
The > sfm-list.txt at the end store the sorted, counted list of SFM marker into sfm-list.txt