41.4.1 TAB Is Just Another Character to UNIX

Typing TAB sends a single TAB character to the UNIX system. If you're editing a file, the editor probably puts that single TAB character into the file. Later, when you use cat (25.2), pr (43.7), lp (43.2), and so on, they read each TAB and send out that single character to your terminal, printer, or whatever. The TAB is usually interpreted by the hardware device itself. Before that, it's a single character like any other. (But see the stty -tabs command below.)

If your terminal has a setup mode, enter setup mode and look at the tabstop settings. They're probably set at columns 9, 17, 25, and so on. When your terminal receives a TAB from the UNIX system, the terminal moves the cursor to the terminal's next tabstop.

For example, your terminal might have different tabstops - maybe 11, 21, 31, and so on. Let's say that you're catting a file. The programmer who made the file had her terminal tabstops set at the default 8-column intervals. When she typed it, she used TABs (shown as in this example) to indent the lines. Her terminal showed the file this way:

% cat prog
while read line; do
       set $line
       for word in line; do
              case "$1" in
...

If your terminal has tabstops set at 12 characters, the same file would look like this:

% cat prog
while read line; do
           set $line
           for word in line; do
                      case "$1" in
...

Neither the UNIX kernel nor the cat program did anything different to the prog file. The terminal interprets the TABs.

If you want to display or edit a file that has different tabstops than your terminal, what can you do?

• expand	Use a UNIX utility that "expands" (converts) TABs into spaces. On BSD systems, expand does the job. The terminal never sees the TABs in this case, so its tabstop settings are never used:

  % expand prog
  while read line; do
          set $line
          for word in line; do
                  case "$1" in
  ...

  On System V, use pr with its -t -e options:

  % pr -t -e prog
  while read line; do
          set $line
          for word in line; do
                  case "$1" in
  ...

  So, no matter what your terminal tabstops are, the file will look normal.

  If you want to use other tabstops, use an option. For instance, get 4-character tabstops with expand -4 or pr -t -e4.

• Tell your text editor to use different tabstops. The editor will probably "expand" the TABs into spaces before it sends them to your terminal. For instance, in vi, type:

  :set tabstop=4

  to edit programs with 4-character tabstops. The prog file from above would look like:

  % vi prog
  while read line; do
     set $line
     for word in line; do
        case "$1" in
  ...

  although it has TAB characters in exactly the same places. If you have a text editor handy, try changing its tabstops while you display a file.

If you make a file with non-standard tabstops, it's a good idea to put a comment in the file so people will know. Or, before you save the file for the last time, expand the TABs into spaces. With vi on a system that has expand, for example, this command would convert TABs to spaces at 4-column tabstops:

:%!	:%!expand -4

The opposite of expand is, naturally enough, unexpand (24.6). It converts spaces back to TABs at 8-column tabstops.

41.4.2 Telling UNIX to Expand TABs

I said above that the UNIX kernel and device drivers don't usually expand TABs into spaces. Sometimes, if you're having a lot of trouble with funny-looking tabstop settings, you might ask the device driver to expand the TABs into spaces. The command is:

% stty -tabs

Now in most cases, UNIX will not send TABs to your terminal. It's better to fix your terminal's tabstops, if you can.