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CS155 Redirection

Redirection

What if we don’t want the output of a command to go to the screen but to a file? Or get the input to a command from a file instead of the keyboard?

Redirection allows us to change where a command gets input or sends output.

• > - redirect output to a new file
• >> - redirect output to an existing file and append to the end
• < - redirects the input to a file (instead of the keyboard)
• | - sends the output of one program to another program

Output redirection

% echo "hello there, children"
hello there, children
% echo "hello there, children" > chef
% ls
chef
% cat chef
hello there, children
% echo "hey chef" >> chef
% cat chef
hello there, children
hey chef
% date
Thu Nov 21 09:54:00 MST 2024
% date >chef
% cat chef
Thu Nov 21 09:54:00 MST 2024

• We can use echo to put text into a file.
• You will find this useful for your homework until we get to text editors.

More output redirection

% cp ~/monster .
% ls
monster
% ls >file_list
% ls
file_list  monster
% cat file_list
file_list  monster
% pwd >> file_list
% cat file_list
file_list  monster
/tmp/PmWiki.180251707

echo isn’t the only command whose output we can redirect.

Input redirection

% cp ~/monster .
% ls -l >tempfile
% cat <tempfile
total 4
-rw------- 1 cs155 class 401 Nov 21 09:54 monster
-rw------- 1 cs155 class   0 Nov 21 09:54 tempfile
% cat tempfile
total 4
-rw------- 1 cs155 class 401 Nov 21 09:54 monster
-rw------- 1 cs155 class   0 Nov 21 09:54 tempfile

Many commands accept input from a file or a command line argument.

Output redirection to a program

% ls -l > tempfile
% more tempfile
...
% rm tempfile
% ls -l | more
...
%

This is useful for combining operations without making temporary files

• Try ls -l | more for listing a directory with many files
• Try man cat | more to learn about the cat command

Summary

• To learn more about a command use: man or info
• To view files use: more or cat
• To modify the file system use: cp, mv, rm, mkdir, rmdir
• To send text somewhere use: echo
• To preserve text use: "
• To redirect the output of a command use: >, >>, |
• To redirect the input of a command use: <

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