IO Redirection in Bash

Basics

1>filename
# Redirect stdout to file "filename."

1>>filename
# Redirect and append stdout to file "filename."

2>filename
# Redirect stderr to file "filename."

2>>filename
# Redirect and append stderr to file "filename."

&>filename
# Redirect both stdout and stderr to file "filename."
# This operator is now functional, as of Bash 4, final release.


2>&1
# Redirects stderr to stdout.
# Error messages get sent to same place as standard output.
  >>filename 2>&1
      bad_command >>filename 2>&1
      # Appends both stdout and stderr to the file "filename" ...
  2>&1 | [command(s)]
      bad_command 2>&1 | awk '{print $5}'   # found
      # Sends stderr through a pipe.
      # |& was added to Bash 4 as an abbreviation for 2>&1 |.

i>&j
# Redirects file descriptor i to j.
# All output of file pointed to by i gets sent to file pointed to by j.

>&j
# Redirects, by default, file descriptor 1 (stdout) to j.
# All stdout gets sent to file pointed to by j.

Complete

COMMAND_OUTPUT >

Redirect stdout to a file.

Creates the file if not present, otherwise overwrites it.

ls -lR > dir-tree.list

Creates a file containing a listing of the directory tree.

: > filename

The > truncates file "filename" to zero length.

If file not present, creates zero-length file (same effect as 'touch').

The : serves as a dummy placeholder, producing no output.

filename

The > truncates file "filename" to zero length.

If file not present, creates zero-length file (same effect as 'touch').

(Same result as ": >", above, but this does not work with some shells.)

COMMAND_OUTPUT >>

Redirect stdout to a file.

Creates the file if not present, otherwise appends to it.

Single-line redirection commands (affect only the line they are on):

--------------------------------------------------------------------

1>filename

Redirect stdout to file "filename."

1>>filename

Redirect and append stdout to file "filename."

2>filename

Redirect stderr to file "filename."

2>>filename

Redirect and append stderr to file "filename."

&>filename

Redirect both stdout and stderr to file "filename."

This operator is now functional, as of Bash 4, final release.

M>N

"M" is a file descriptor, which defaults to 1, if not explicitly set.

"N" is a filename.

File descriptor "M" is redirect to file "N."

M>&N

"M" is a file descriptor, which defaults to 1, if not set.

"N" is another file descriptor.

#==============================================================================

Redirecting stdout, one line at a time.

LOGFILE=script.log

echo "This statement is sent to the log file, "$LOGFILE"." 1>$LOGFILE echo "This statement is appended to "$LOGFILE"." 1>>$LOGFILE echo "This statement is also appended to "$LOGFILE"." 1>>$LOGFILE echo "This statement is echoed to stdout, and will not appear in "$LOGFILE"."

These redirection commands automatically "reset" after each line.

Redirecting stderr, one line at a time.

ERRORFILE=script.errors

bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE. bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE. bad_command3 # Error message echoed to stderr, #+ and does not appear in $ERRORFILE.

These redirection commands also automatically "reset" after each line.

#=======================================================================

2>&1

Redirects stderr to stdout.

Error messages get sent to same place as standard output.

filename 2>&1 bad_command >>filename 2>&1 # Appends both stdout and stderr to the file "filename" ... 2>&1 | [command(s)] bad_command 2>&1 | awk '{print $5}' # found # Sends stderr through a pipe. # |& was added to Bash 4 as an abbreviation for 2>&1 |.

i>&j

Redirects file descriptor i to j.

All output of file pointed to by i gets sent to file pointed to by j.

&j

Redirects, by default, file descriptor 1 (stdout) to j.

All stdout gets sent to file pointed to by j.

0< FILENAME < FILENAME

Accept input from a file.

Companion command to ">", and often used in combination with it.

grep search-word <filename

[j]<>filename

Open file "filename" for reading and writing,

#+ and assign file descriptor "j" to it.

If "filename" does not exist, create it.

If file descriptor "j" is not specified, default to fd 0, stdin.

An application of this is writing at a specified place in a file.

echo 1234567890 > File # Write string to "File". exec 3<> File # Open "File" and assign fd 3 to it. read -n 4 <&3 # Read only 4 characters. echo -n . >&3 # Write a decimal point there. exec 3>&- # Close fd 3. cat File # ==> 1234.67890

Random access, by golly.

|

Pipe.

General purpose process and command chaining tool.

Similar to ">", but more general in effect.

Useful for chaining commands, scripts, files, and programs together.

cat *.txt | sort | uniq > result-file

Sorts the output of all the .txt files and deletes duplicate lines,

finally saves results to "result-file".

keywords

log redirection

Bash Commands Kafka Commands

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