How to give input to command from a file

How to give input to command from a file

< 

A < symbol connects the command’s STDIN to the contents of an existing file.

Example

$ mail -s "Mail test" johndoe < /tmp/mymessage

Read from text file and send as mail

How to store command output in text file

>

Replaces the file’s existing contents

Example

echo "This is a test message." > /tmp/mymessage

To redirect both STDOUT and STDERR to the same place, use the >& symbol.

To redirect STDERR only, use 2>.

Example

$ find / -name core > /tmp/corefiles 2> /dev/null

This command line sends matching paths to /tmp/corefiles, discards errors, and

sends nothing to the terminal window.

How to append to contents of a file

>> 

Appends to the file

echo "This is a test message." >> /tmp/mymessage

How to execute command conditionally on success of command.

$ lpr /tmp/t2 && rm /tmp/t2

To execute a second command only if its precursor completes successfully, you

can separate the commands with an && symbol

cp --preserve --recursive /etc/* /spare/backup \

|| echo "Did NOT make backup"

Conversely, the || symbol executes the following command only if the preceding

command fails (produces a nonzero exit status).