Classic Shell Scripting

Updating Modification Times with touch

We have used the touch command a few times to create empty files. For a previously nonexistent file, here are equivalent ways of doing the same thing:

cat /dev/null  > some-file               Copy empty file to some-file
printf ""      > some-file               Print empty string to some-file
cat /dev/null >> some-file               Append empty file to some-file
printf ""     >> some-file               Append empty string to some-file
touch            some-file               Update timestamp of some-file

However, if the file exists already, the first two truncate the file to a zero size, whereas the last three effectively do nothing more than update its last-modification time. Clearly, the safe way to do that job is with touch, because typing > when you meant >> would inadvertently destroy the file contents.

touch is sometimes used in shell scripts to create empty files: their existence and possibly their timestamps, but not their contents, are significant. A common example is a lock file to indicate that a program is already running, and that a second instance should not be started. Another use is to record a file timestamp for later comparison with other files.

By default, or with the -m option, touch changes a file's last-modification time, but you can use the -a option to change the last-access time instead. The time used defaults to the current time, but you can override that with the -t option, which takes a following argument of the form [[CC]YY]MMDDhhmm[.SS], where the century, year within the century, and seconds are optional, ...