Shell Scripting: Expert Recipes for Linux, Bash, and More

touch

touch is a way to create a file, or update its timestamp, without actually writing anything to it. That may seem a bit useless at first, but it is actually quite useful. Some tools simply check for the existence of a file; when Solaris reboots and finds a file called /reconfigure, it will automatically do a reconfiguration reboot. Many init scripts create files in /var/lock/subsys to indicate that the service is running. An empty file is often all that is needed there, too. It can also be useful to freshen a file with a current timestamp; the make command modifies its behavior depending on how new the source and binary files are. The find command can be told to report only files newer than a supplied sample file.

touch can also be used to make things appear different than they are. This can be useful for backup utilities, which restore files with their original timestamps. It can also be used for other purposes, not all of which are honorable. If a systems administrator accidentally made the wrong change to a file, it would appear to be possible for him to cover his tracks by not changing the timestamp.

# ls -l /etc/hosts
-rw-r--r-- 1 root root 511 Feb 25 12:14 /etc/hosts
# grep atomic /etc/hosts
192.168.1.15    atomic
# stat -c "%y" /etc/hosts
2011-02-25 12:14:12.000000000 +0000
# timestamp='stat -c "%y" /etc/hosts'
# echo $timestamp
2011-02-25 12:14:12.000000000 +0000
# sed -i s/192.168.1.15/192.168.1.11/1 /etc/hosts
# ls -l /etc/hosts -rw-r--r-- 1 root root 511 Mar 31 16:32 /etc/hosts ...