Redirecting output: > and >>

When ls prints a listing it is not writing to the screen on purpose. It writes to a channel called standard output (stdout), and the shell connected that channel to your terminal before the command started. Redirection is the act of connecting it somewhere else. The command does not know or care: ls behaves identically whether its output ends up on screen or in a file.

> creates or overwrites

command > file runs the command with stdout pointing at file. If the file does not exist it is created; if it exists its content is replaced, with no confirmation. The shell does this before running the command, which has two consequences: the file is created even if the command prints nothing or fails, and if you redirect ls into a file inside the directory being listed, the new file shows up in the listing.

>> appends

command >> file also creates the file if needed, but when it exists the output is added after the current content. This is what you want for logs, notes you build up over time, or collecting results from several commands into one place. The difference between > and >> is a single character and it is the difference between keeping and losing data, so make a habit of pausing before you press Enter on a >.

~% mkdir demo; cd demo
demo% echo first > notes.txt
demo% cat notes.txt
first
demo% echo second >> notes.txt
demo% cat notes.txt
first
second
demo% echo third > notes.txt
demo% cat notes.txt
third
demo% ls > listing.txt
demo% cat listing.txt
listing.txt
notes.txt

Useful habits

• > file on its own, with no command, creates an empty file or truncates an existing one to zero bytes. It is a quick way to clear a log.
• The shell's noclobber option (set -C) makes > refuse to overwrite existing files; >| forces it. It works in this VM's ash shell too.
• Spaces around > are optional: echo hi >f, echo hi > f and echo hi>f are the same.
• Redirection only captures standard output. Error messages still reach the screen; the next lesson deals with them.