A general editing session with TECO might look like this:
- A user starts TECO from the command line.
- TECO prints a command prompt (usually an asterisk (*), unless the TECO_PROMPT environment variable is defined), indicating that it is ready to accept one or more commands.
- The user then enters a single command, or a sequence of multiple commands called a command string.
- TECO executes these commands and when done, prints its prompt again.
- Rinse and repeat, until a command is entered that causes TECO to exit.
Such steps could be used in order to read an input file, search for some desired text, make changes to it, and write out the results to an output file.
Editing sessions make use of an edit buffer that is used by TECO to read in text from files, insert and delete text, search for or replace text, and write out text to files. This buffer is initially 64K bytes, but is capable of growing to 1MB. The current contents of an edit buffer is referred to as a page. Each page is usually delimited by a <FF>, and is divided into lines, which are delimited by a line terminator such as <LF>, <VT>, or <FF>. Lines in turn are comprised of single 8-bit ASCII characters. TECO provides commands to manipulate pages, lines, and characters.
TECO has classically functioned as a pipeline editor, in which text once written to a file could not be further modified without ending the current editing session, and starting a new one to read the file again. However, TECO-64 provides optional virtual memory support (as did TECO-32) that makes it possible to page backward through a file.
When editing text, TECO makes use of a buffer pointer called dot that specifies the current position. This is used to determine how commands that move the pointer or affect the text are executed. dot is a movable position indicator which is never located directly on a character, but is always either between characters, before the first character in the buffer, or after the last character in the buffer.
The general format for TECO commands is as follows:
[[ m ,] n ] [ : [ : ]] [ @ ] command [ q ] [ text1 [ <delim> ][ text2 [ <delim> ]]]
The fields in brackets are either optional, or are dependent on the command being executed. These are for illustration only; none of TECO's commands use all of these fields.
| Field | Description | Examples |
|---|---|---|
| m | First numeric argument. | 10,20K |
| , | Separator between m and n arguments. | 0,8192ET |
| n | Second (or only) numeric argument. | 42R |
| : | Colon modifier. Affects the behavior of the following command, often in order to return a value indicating success or failure of the command rather than aborting execution and issuing an error. | :ERinput.c` |
| :: | Double colon modifier. Affects the behavior of the following command, but differently than :. Often used for "anchored" searches. | ::Sbaz` |
| @ | At signed modifier. Specifies an alternative delimiter will be used for text arguments that follow the command, using a matched pair of characters. | @^A/hello/ @FC/baz/foo/ |
| command | The TECO command to be executed. | V, ET, === |
| q | The name of the Q-register that the command will use. | 1XA |
| text1 | The first text argument for the command. See description below. | Ifoobaz` |
| text2 | The second text argument for the command. See description below. | FSfoo`baz` |
| <delim> | The command delimiter. Only required for commands that allow text arguments when there is no at sign modifier. | EWoutput.c` |
Note that if a command allows both the colon (including double colon) and at sign modifiers, they may be specified in either order.
Numeric arguments are used by many commands to specify such things as a repeat count, a position within the edit buffer, or bits to set or clear in flag variables. Some commands limit their numeric arguments to positive values, while others also allow zero or negative arguments.
Most commands that accept n arguments allow for a default if n is not explicitly stated, as shown in the examples below.
| Command | Default Value | Default Action |
|---|---|---|
| C | 1 | Advance 1 character |
| R | 1 | Back over 1 character |
| L | 1 | Advance 1 line |
| J | 0 | Jump to start of buffer |
| V | 1 | View 1 line |
| D | 1 | Delete 1 character |
| K | 1 | Kill 1 line |
| S, N, etc. | 1 | Search for first occurrence |
| % | 1 | Increment Q-register by 1 |
| X | 1 | Extract one line |
If an m argument is also allowed, omitting it will likely result in different behavior than if specified. See the specific command for details.
TECO can be built for either 32- or 64-bit support. If it has been build for the former, then numeric arguments may range from -2^31 to 2^32-1. If it has been built for the latter, then numeric arguments may range from -2^63 to 2^32-1.
Colon modifiers change the action of the following command, as illustrated in the examples below.
| Example Command(s) | Description |
|---|---|
| :ER, :S, :FN | Do not abort the command if it failed, but return a value of 0 to flag that an error occurred, and return -1 if it succeeded. This can be used by subsequent commands to take corrective action or print an informative message if needed. |
| :Gq | Type the result on the terminal instead of copying it to the text buffer. |
| := | Print a numeric value, but do not print a <CR><LF> after it. |
Many TECO commands allow or require a text argument, which is a character string that follows the command and is usually terminated by a <delim> character. This provides additional information to the command being executed. The string may not include a <delim>, since this would terminate the string prematurely, but it may include any other character.
Commands that allow a single text argument are normally delimited as follows:
| Command | Description |
|---|---|
| command<delim> | General command, where <delim> is ESCape or an alternative delimiter such as accent grave. |
| ^Ahello, world!<CTRL/A> | The type-out command may start with a CTRL/A or literal ^A, but it must be terminated by a CTRL/A. |
| !tag! | Any tag or comment must be start and end with an exclamation mark. |
| Example | Description |
|---|---|
| Sabc` | Search for the string "abc" |
| ^UAhello, world!` | Insert the text "hello, world!" into Q-register A |
| OBEGIN` | Branch to the tag specified by the string "BEGIN" |
Depending on the command, it may also be possible to use a "null" text argument by terminating the command with a <delim>. The effect of doing so is dependent on the specific command, but using the example of the S command above, if that was followed by an " S' " command, that would cause TECO to search for the next occurrence of "abc".
Some TECO commands allow or require two text arguments. Each argument must be followed by a <delim> character, as shown below:
FSabc`def`
At sign modifiers are used to allow an alternate form of delimiting text arguments that follow a command, by specifying that they will instead be delimited by a pair of matching characters:
@FS/foo/baz/
@^A+hello, world!+
If both text arguments are used, then only one delimiter is required between them.
Also, when an at sign modifier is used, whitespace is allowed between the command and the first delimiter:
@S /abc/
Delimiters used with an at sign modifier may be any graphic ASCII character (ASCII 33 through 126, inclusive). Control characters and spaces are not allowed as delimiters. This is a change from earlier TECOs that would allow any character as a delimiter.
If the E1&4 flag bit is set, then paired braces may additionally be used to delimit text arguments. This also has the advantage of allowing whitespace, not only before the first delimiter, but between the text arguments.
@S{foo}
@FN{foo}{baz}
@FS {foo} {baz}
If text arguments are delimited using this form, then commands such as the following are no longer valid:
@^A{hello{
@^A}goodbye}
TECO has many special numeric values and flags which are accessible through TECO commands. Some of these values, such as the text buffer pointer, reflect the state of TECO. Others control TECO’s behavior in various ways.