Creating empty files with touch

You have been using touch since lesson 1 to create files. That is its most common use today, but a side effect of its real job. Every file carries a modification time — the moment its contents last changed, the date ls -l shows — and touch sets that time to *now*. If the file does not exist, it has to be created first so there is something to stamp. Hence the empty file.

~% cd lab/l12
l12% touch todo.txt
l12% touch a.log b.log c.log
l12% ls -l
total 0
-rw-r--r--    1 root     root             0 Oct  4 19:02 a.log
-rw-r--r--    1 root     root             0 Oct  4 19:02 b.log
-rw-r--r--    1 root     root             0 Oct  4 19:02 c.log
-rw-r--r--    1 root     root             0 Oct  4 19:02 todo.txt

Size 0, today's date. Several names make several files. Existing files are not emptied or changed in any way — only their timestamp moves — so touch is always safe to run on something that already exists. That is a real difference from editors and from > redirection (lesson 22), both of which can destroy contents.

Why anyone would update a timestamp

Build tools such as make decide what to recompile by comparing dates: if a source is newer than its output, rebuild. Touching a file forces that. Backup and sync tools pick files by age. find -newer and find -mtime (lesson 36) select files by date, and ls -t sorts by it. Marking "I looked at this today" or staging a realistic test scenario are everyday reasons to touch a file that already exists.

Choosing the time: -d, -t, -r

l12% touch -d 2019-03-10 stale.txt
l12% ls -l stale.txt
-rw-r--r--    1 root     root             0 Mar 10  2019 stale.txt
l12% touch stale.txt
l12% ls -l stale.txt
-rw-r--r--    1 root     root             0 Oct  4 19:03 stale.txt

Notice how ls -l shows the year for the 2019 file and the clock time for today's. That is ls saving space: entries older than about six months get the year instead of the time. The data behind both is the same full timestamp; ls -l --full-time shows it completely.