Here document
Here the document is redirection option to fill input stream of application by information native way:
application << delimiter
some useful text
delimiter
script body:
#!/usr/bin/env bash
grep perfect << EOF
PERFECT STORY
past perfect
is it Perfect?
EOF
The script running at console
script body in the yellow frame, output next
As we can see only the second string from 3 of the strings of text for the grep standard input stream delimited by “EOF” match the grep pattern: that is why it passed to grep standard output stream and be displayed in the console next.
Shell Scripting – Standard Input, Output and Error
Working on Linux applications we have several ways to get information from outside and to put it inside: command line args, environment variables, files. All of these sources are legal and good. But it has a finite size. Another way to establish communication is standard streams: input stream (stdin) used for getting data from outside of the app, output stream (stdout) to put data outside of the app, and error to put data outside of the app (stderr).
standard Linux application stream at sh
Each stream acts like a pipe: It has the same buffer to write and read the data. This buffer is available for reading from one application and available for writing from another one. On reading, the occupied buffer size will be reduced and it will be increased on writing. If the average rate of reading and writing is equal – then data passed over the stream can be any number of bytes long.
Table of Content
- Input/output streams operating in the example
- Error stream operating in the example
- Streams redirecting
- Discard the output
- Pipelined streams
- Here document
- Shell Scripting – Standard Input, Output and Error – FAQs