This post shows how to redirect command output or static text to a file.

The redirection operators > and >>

Write a string to a file, replacing its contents:

$ echo "hello" > foo.txt

Use >> to append instead:

$ echo "hello" >> foo.txt

Append command output into a file:

$ ls >> foo.txt

Use cat to check the result:

$ cat foo.txt

A real-world example is adding fish to your available shells:

$ command -v fish | sudo tee -a /etc/shells

The tee command

You can also pipe into the tee command:

$ command -v fish | tee foo.txt

Use the -a flag to append:

$ command -v fish | tee -a foo.txt

The paste command

This command outputs filenames separated by commas:

$ ls -1 | paste -sd "," -
foo.txt,bar.txt,baz.txt

It breaks down to:

-1

(The numeric digit one.) Force output to be one entry per line. This is the default when output is not to a terminal.

-s

Concatenate all of the lines of each separate input file in command line order. The newline character of every line except the last line in each input file is replaced with the tab character, unless otherwise specified by the -d option.

-d

Use one or more of the provided characters to replace the newline characters instead of the default tab.

-

The hyphen at the end means standard input.

To keep one filename per line, skip paste:

$ ls -1 > foo.txt

If you want to limit the results, globs can be used in normal fashion. This lists only jpg files:

$ ls -1 *.jpg | paste -sd "," - > jpegs.txt

Or jpgs and pngs:

$ ls -1 *.{jpg,png} | paste -sd "," - > bitmaps.txt

Make a CSV

A primitive receipt management system. Name your receipts in this format:

yyyy-mm-dd,recipient,price,item,reference,.ext

An example file might look like this:

2015-01-01,DigitalOcean,10,server,165496-21,.pdf

Then print them out into a file:

$ ls -1 *.pdf > receipts-2015.csv

Then open it in a spreadsheet application.