< stdin

Bash shell uses stdin to take input. In input redirection. A file is made input to the command and this redirection is done with the help of ‘<‘ sign.

Sample syntax command of ca < commmand :

# cat < <filename>

Like example of ca < command :

# cat < file.txt

See your terminal after run the command, command “cat < file.txt” has taken ‘file.txt’ as input and displayed its content.

<< here document

The here document sometimes also called here-is-document, is a way in which you can enter input until a certain sequence(usually EOF) is type. The EOF (End Of File) can be type or can be calle by pressing (ctrl + d) keys.

Any word can be use in place of ‘EOF’ like we have used ‘last’.

Sample syntax of cat < <EOF> :

# cat < <EOF> <filename>

Like example of cat << <EOF> :

# cat <<EOF> file.txt

See your terminal after run the command, in first example, ‘file.txt’ is end when we typed ‘EOF’ and ‘last‘ in second example.

<<< here string

The here string is used to directly pass strings to a command.

Like example of <<< :

# base64 <<< format.txt

See your terminal, in this example we have used base64 which we’ll study later. Command “base64 <<< format.txt” has decoded file ‘format.txt’ and then by using command ‘base64 -d’ we got back our file ‘format.txt’.

Linux Output Redirection

Output redirection is use to put output of one command into a file or into another command.

stdout

The stdout is redirect with a ‘>’ greater than sign. When shell meets the ‘>’ sign, it will clear the file as you already know.

Like example of > :

# echo Hello everyone. > afile.txt

See your terminal after run the command, greater than sign ‘>’ redirects the command ‘echo’ output into a file ‘afile.txt’.

Output File Is Erased

In output redirection, during scanning of a command line, shell will encounter through ‘>’ sign and will clear the file.

Like example of file ereased :

# zcho Welcome > afile.txt

See your terminal after run command, command “zcho Welcome > afile.txt” is wrong but still file ‘afile.txt’ is clear.

noclobber

We can prevent file deletion while using ‘>’ sign with the help of noclobber option.

Sample syntax of noclobber :

# set -o noclobber    (To prevent overwrite)  
# set +o noclobber    (To overwrite)  

Like example of noclobber :

# echo Learn Linux. > newfile.txt

See your terminal after run the command, command “set -o noclobber” prevents file from getting overwrite.

set +o noclobber command allows you to overwrite the existing file.