AkiVaMu Just tiny things come to mind...

Linux shell redirection

05 Feb 2017
Linux

Even though I googled and used this many time, I always forgot about all these: &2>1, 1>2&, 1>&2. So I hope that writing it down myself can help to remember their rule.

Linux file descriptor

  • 0: stdin
  • 1: stdout
  • 2: stderr

Redirect to a file name

Redirect output to a file named 1

echo test>1 # test
echo test >1 # test
echo test > 1 # test

Redirect stdout/stderr to a file named 1

# redirect stdout
echo test 1>1 # test
echo test 1> 1 # test
echo test 1 > 1 # test 1

# redirect stderr
echo test 2>1 # test

Redirect both stdout and stderr to a file named 1

echo test &>1 # redirect stdout + stderr to file named `1`

Redirect to a file descriptor

echo test 2>&1 # redirect stderr to stdout
echo test 2> &1 # shell error (space character before &1)

echo test >&2 # redirect stdout to stderr
echo test 1>&2 # redirect stdout to stderr

stderr and Pipes

This won’t work

$ grep hosts /etc | grep 'Is a directory' > result.txt
grep: /etc: Is a directory
$ cat result.txt

If we want to pipe the stderr, have to redirect stderr to stdout first

$ grep hosts /etc 2>&1 | grep 'Is a directory' > result.txt
$ cat result.txt
grep: /etc: Is a directory

Reference

http://www.jstorimer.com/blogs/workingwithcode/7766119-when-to-use-stderr-instead-of-stdout
http://www.tldp.org/LDP/abs/html/io-redirection.html

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