stdin stdout stderr

Stream Abbreviation Number
Standard input stdin 0
Standard output stdout 1
Standard error, or error stream stderr 2

Redirecting Standard Output and Standard Error

Redirection Operator Description Example
> Redirecting stdout, then overwrite ls -l /usr/bin > ls-output.txt
> ls-output.txt 清空文件
>> Redirecting stdout, then append ls -l /usr/bin >> ls-output.txt
2> Redirecting stderr, then overwrite ls -l /usr/bin 2> ls-error.txt
2>> Redirecting stderr, then append ls -l /usr/bin 2>> ls-error.txt
&> Redirecting both stdout and stderr, then overwrite ls -l /bin/usr &> ls-output.txt
&>> Redirecting both stdout and stderr, then append ls -l /bin/usr &>> ls-output.txt
2>&1 Same as &>, but the traditional way, which works with old versions of the shell ls -l /bin/usr > ls-output.txt 2>&1
>&2
1>&2		 Same as &>, but the traditional way, which works with old versions of the shell； ls -l /bin/usr > ls-output.txt >&2
echo "Invalid: $PARAM" >&2

/dev/null

  1.  $ ls -l /bin/usr 2> /dev/null

Redirecting Standard Input

Redirection Operator Description Example
< Redirecting stdin cat > file.txt , see also cat

Pipelines

| pipeline operator

  1. $ ls -l /usr/bin | less
  2. $ ls /bin /usr/bin | sort | less

Differences between redirection operator and pipeline operator

  • The redirection operator connects a command with a file
  • while the pipeline operator connects the output of one command with the input of a second command.

    Here Documents: << <<-

    Also called here script ```bash command << token text token

$ cat << EOF

some text some text more EOF some text some text more `` <<-`: the shell will ignore leading tab characters (but not spaces) in the here document.

Here String: <<<

A here string is like a here document, only shorter, consisting of a single string.

  1. $ file_info=$(grep "^$USER:" /etc/passwd)
  2. $ IFS=":" read user pw uid gid name home shell <<< "$file_info"