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README.md

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README

Demonstrate and work through some stdout and stderr redirection

Reasons

In the shell it's necessary to watch the output from both pipes. Some tools are quite good with seperating stderr from stdout whereas others just pipe all error messages to stdout.

NOTES:

  • The difference between > and | is that one redirects to a file and one to a process. Examples are here

TODO:

  • find out if people use a 3rd FD for warnings and trace..
  • piping just stderr to stdin git fetch --dry-run 2>| tee
  • prefixing name of pipe to each line....

File Descriptors

# show what tty my stdin, stdout and stderr are connected to.
lsof +f g -ap $$ -d 0,1,2

Quick helper

It can be good to try different tools with a variety of paramaters to understand how the tool behaves regarding error logging.

# filter stdoout to show only error stream
cat ./no-file 1> /dev/null     
# filter stderr to show only out stream 
cat ./no-file 2> /dev/null

Redirecting to files

mkdir ./out

# generates stdout and stderr
./generate.sh   

# filter off stdout 
./generate.sh > ./out/stdout.txt   

# filter off stderr
./generate.sh 2> ./out/stderr.txt    

#filter both 
./generate.sh > ./out/stdout2.txt 2> ./out/stderr2.txt

Redirecting to processes

# prepend each stream with tag
./generate.sh 2> >(awk '{print "stderr:" $0}') > >(awk '{print "stdout:" $0}')

Merging pipes (works in both bash and zsh)

# merge stderr into stdout
./generate.sh &> ./out/stdout_combined.txt 

# merge stderr into stdout
./generate.sh &> >(awk '{print "stdout:" $0}')

Resources

  • redirection_tutorial here
  • redirection on bash hackers here
  • Input/Output Redirection in the Shell here
  • Bash One-Liners Explained, Part III: All about redirections here