redirection_and_async

Launching programs in background and async

&

• launches program in background in async, use fg command to bring back to foreground

`{command} &

nohup

• prevents SIGHUP to reach the program and write stdout to nohup.out file

nohup {command}

disown (bash, ksh, zsh)

• detach the process from job queue of user, it makes init as the parent of the process

Redirection

• Overwrite
  • {command} < {file} : sends input to stdin from file
  • {command} > {file} : overwrite output of stdout to file
• Append
  • {command} << {delimiter} : (here-document) reads input until it gets delimiter which marks ending of input
    • Syntax

      user:~$ {command} << {delimiter}
      here-document
      delimiter

    • Example

      user:~$ cat << stop
      > Hi World
      > How are you
      > stop
      Hi World
      How are you
      user:~$

  • {command} >> {file} : append output of stdout to file
• Merge
  • p >& q : Merges output from stream p with stream q
  • p <& q : Merges input from stream p with stream q
• {command1} | {command2} : (pipe) sends stdout of one command to stdin of another command
• Whenever you execute a program/command at the terminal, 3 files are always open viz., standard input, standard output, standard error

file descriptors (fd) and redirection:

• 0: stdin

 # reads stdin data from infile
{command} 0<infile

• 1: stdout

# writes stdout to outfile
{command} 1>outfile

• 2: stderr

# appends stderr to logfile
# >> is append, > is overwrite
{command} 2>>logfile

redirecting stderr (2) to stdout (1)

{command} 2>&1

write stdout nowhere

{command} > /dev/null
{command} 1>/dev/null

write stdout and stderr nowhere

{command} > /dev/null 2>&1

write stdout and stderr to file

{command} > file 2>&1