Editing CONFIG.SYS (section)

== Issues ==
[[File:PC-MOS-386 boot screen.jpg|thumb|[[PC-MOS]] detecting a missing {{code|CONFIG.SYS}} on startup]] 
The system can still boot if this file is missing or corrupted. However, this file, along with [[AUTOEXEC.BAT]], is essential for the complete bootup process to occur with the DOS operating system. These files contain information that is used to customize the operating system for personal use. They also contain the requirements of different software application packages. A DOS system would require troubleshooting if either of these files became damaged or corrupted.

If CONFIG.SYS does not contain a <kbd>SHELL</kbd> directive (or the file is corrupt or missing), DOS typically searches for COMMAND.COM in the root directory of the boot drive.<ref name="Paul_2004_YESCHAR"/> If this is not found, versions of DOS before 6.0 will not start up. MS-DOS 6.0/PC&nbsp;DOS 6.1 and Novell DOS 7<!-- not sure about PalmDOS right now --> and higher will instead display a prompt to enter the path and filename of a command processor. This recovery prompt is also displayed when the primary command processor is aborted due to faults or if it is exited deliberately.<ref name="Paul_1997_NWDOSTIP"/> (In the case of COMMAND.COM, the internal <code>EXIT</code> command is disabled only when the shell was started with <code>/P</code>.) This also provides limited means to replace the shell at [[runtime (program lifecycle phase)|runtime]] without having to reboot the system.
Since the MS-DOS 7.0 and higher COMMAND.COM executable is incompatible with DR-DOS,<ref name="Paul_2004_COMMAND"/> but typically resides in the root of drive C: in dual-boot scenarios with DR-DOS, DR-DOS 7.02 and higher no longer allow to bypass <kbd>SHELL</kbd> directives in ({{keypress|Ctrl}}+){{keypress|F5}}/{{keypress|F7}}/{{keypress|F8}} "skip"/"trace"/"step" modes.<ref name="Paul_1997_OD-A3"/><ref name="Paul_2004_YESCHAR"/><ref name="Paul_2004_COMMAND"/> (Some later issues added ({{keypress|Ctrl}}+){{keypress|F6}} to reinvoke the former {{keypress|F5}} "skip" behaviour in order to allow recovery from problems with invalid <kbd>SHELL</kbd> arguments as well.<ref name="Paul_2004_YESCHAR"/>) Also, if no <kbd>SHELL</kbd> directive could be found when skipping CONFIG.SYS processing via ({{keypress|Ctrl}}+){{keypress|F5}} (and also with ({{keypress|Ctrl}}+){{keypress|F7}}/{{keypress|F8}}, when the default file extension has been changed with <code>SYS /DR:ext</code>),<ref name="Paul_2017_DR"/> the user is prompted to enter a valid shell file name ''before'' trying to load COMMAND.COM from the root.<ref name="Paul_1997_OD-A3"/><ref name="Paul_2004_COMMAND"/> Pressing {{keypress|ENTER}} without specifying a file will assume the former default.<ref name="Paul_1997_OD-A3"/>

Depending on the version, the size of the CONFIG.SYS file is limited to a few kilobytes under MS-DOS/PC&nbsp;DOS (up to 64&nbsp;KB in most recent versions), whereas the file's size is unlimited<!-- only limited by the file system --> under DR-DOS.<ref name="Paul_1997_NWDOSTIP"/><ref name="Paul_2004_YESCHAR"/> This is because the former operating systems (since DOS 3.0<ref name="Ernst_1987_DRIVER"/>) will compile the file into some tokenized in-memory representation<ref name="Ernst_1987_DRIVER"/> before they sort and regroup the directives to be processed in a specific order (with device drivers always being loaded before TSRs), whereas DR-DOS interprets the file and executes most directives line-by-line, thereby giving full control over the load order of drivers and TSRs via <kbd>DEVICE</kbd> and <kbd>INSTALL</kbd> (for example to solve load order conflicts or to load a program debugger before a device driver to be debugged)<ref name="Paul_1997_OD-A3"/><ref name="Paul_2004_YESCHAR"/> and allowing to adapt the user interaction and change the flow through the file based on conditions like processor types installed, any type of keys pressed, load or input errors occurring, or return codes given by loaded software.<ref name="Paul_1997_NWDOSTIP"/><ref name="Paul_1997_OD-A3"/> This becomes particularly useful since <kbd>INSTALL</kbd> can also be used to run non-resident software under DR-DOS, so that temporary external programs can be integrated into the CONFIG.SYS control flow.<ref name="Paul_1997_NWDOSTIP"/><ref name="4DOS_8.00_HELP"/><ref name="Paul_1997_OD-A3"/>

In MS-DOS/PC&nbsp;DOS 2.0 through 4.01, the length of the <kbd>SHELL</kbd> line was limited to 31 characters, whereas up to 128 characters are possible in later versions.<ref name="Paul_1997_NWDOSTIP"/><ref name="4DOS_8.00_HELP"/> DR-DOS even accepts up to 255 characters.<ref name="Paul_1997_NWDOSTIP"/><ref name="4DOS_8.00_HELP"/> CONFIG.SYS directives do not accept [[long filename]]s.