Editing C shell (section)

==== I/O redirection ====

By default, when csh runs a command, the command inherits the csh's stdio file handles for [[stdin]], [[stdout]] and [[stderr]], which normally all point to the [[console window]] where the C shell is running.  The i/o redirection operators allow the command to use a file instead for input or output.

*<code>> <em>file</em></code> means stdout will be written to ''file'', overwriting it if it exists, and creating it if it doesn't.  Errors still come to the shell window.
*<code>>& <em>file</em></code> means both stdout and stderr will be written to ''file'', overwriting it if it exists, and creating it if it doesn't.
*<code>>> <em>file</em></code> means stdout will be appended at the end of ''file''.
*<code>>>& <em>file</em></code> means both stdout and stderr will be appended at the end of ''file''.
*<code>< <em>file</em></code> means stdin will be read from ''file''.
*<code><< <em>string</em></code> is a [[here document]].  Stdin will read the following lines up to the one that matches ''string''.

Redirecting stderr alone isn't possible without the aid of a sub-shell.

<syntaxhighlight lang="csh">
set filter = "$home"'/filter'
mkfifo "$filter"
cat "$filter" & ( ( ls /root/ || echo No access. ) > "$filter" ) >& /dev/null
</syntaxhighlight>

Systems supporting file descriptors as files may use the following workaround.

<syntaxhighlight lang="csh">
( ( ( echo ok ; '' ) > /dev/fd/0 ) >& /dev/null < /dev/fd/1 ) | ( echo "$<" bye )
</syntaxhighlight>