|Department of Engineering|
|University of Cambridge > Engineering Department > computing help|
dateand the date will be shown on screen, but type
date > outand the output goes into the file called out.
hostname >> outwill append hostname's output to the file out. stderr (where error message go) can also be redirected. Suppose you try to list a non-existent file (blah, say)
ls blahYou will get an error message `blah not found'. There's a file on all unix systems called /dev/null, which consumes all it's given. If you try
ls blah > /dev/nullyou still wouldn't get rid of the error message because, not surprisingly, the message is being sent to stderr not stdout, but
ls blah 2>/dev/nullredirecting channel 2, stderr, will do the trick.
You can also redirect output into another process's input. This is
done using the `|' character (often found above the
date | wc'
and the output from date will be `piped' straight into a program that
counts the number of lines, words and characters in its input.
cat is a program that given a filename prints the file on-screen, so if you have a text-file called text you can do
cat text | wc or
wc < text instead of the usual
wc text to count
the contents of the file.
Standard output can also be `captured' and put in a variable. Typing
d=$(date)will set the variable d to be whatever string the command date produces. The use of
$()to do this supercedes the use of backquotes (i.e.
d=`date`) in older shells.
$(<datafile)is a faster way of doing
$(cat datafile)to put the contents of a file into a variable.
If you want to print lots of lines of text you could use many echo statements. Alternatively you could use a here document
cat<<END Here is a line of text and here is another line. ENDThe cat command uses as its input the lines between the END words (any word could be used, but the words have to be the same).
If you want to read from a file a line at a time (or want to get input from the keyboard) use read. You can use one of the methods below - the second may be much the faster.
echo Reading /etc/motd using cat cat /etc/motd | while read line do echo "$line" done echo Reading /etc/motd using exec redirection exec 0</etc/motd while read line do echo "$line" done