Read and Use System Documentation

Linux commands offer a plethora of command-line switches, making it challenging to remember every option. As you use a command regularly, you naturally learn the available options. However, during your initial attempts, it’s common to forget them after only a few tries. That’s why Linux offers several ways to access built-in help manuals and documentation directly from the command line.

For example, when using the long listing format with the ls command, you might not recall if the correct option is -P or something else. Instead of guessing, you can instantly refer to the help text by typing:

$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

Mandatory arguments to long options are mandatory for short options too.
  -a, --all               do not ignore entries starting with .
  -A, --almost-all        do not list implied . and ..
  -B, --ignore-backups    do not list implied entries ending with ~
  -I, --ignore=PATTERN    do not list implied entries matching shell PATTERN
  -k, --kibibytes         default to 1024-byte blocks for disk usage
  -l                     use a long listing format
  -c                     with -lt: sort by, and show, ctime (time of last
                         modification of file status information);

Scrolling through the output, you can quickly locate the specific option you need (for example, the -L flag). Notice how all the command-line options are neatly sorted alphabetically and come with concise descriptions.

Viewing Help and Listing Directories

Below is an example workflow where you use the help option and then list the directory contents:

$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

Mandatory arguments to long options are mandatory for short options too.
  -a, --all              do not ignore entries starting with .
  -A, --almost-all       do not list implied . and ..
  -B, --ignore-backups   do not list implied entries ending with ~
  -I, --ignore=PATTERN   do not list implied entries matching shell PATTERN
  -k, --kibibytes        default to 1024-byte blocks for disk usage
  -l                    use a long listing format
  -c                     with -lt: sort by, and show, ctime (time of last
                        modification of file status information);
                        with -l: show ctime and sort by name;
                        otherwise: sort by ctime, newest first

$ ls -l
bin/    libexec/    sbin/
lib/    local/      share/

Using the --help flag is especially useful when you momentarily forget an option—even for commands featuring numerous switches.

Detailed Command Help with journalctl

For more complex commands, you may need a comprehensive explanation. Take journalctl for example, a command used to read system logs. Entering journalctl --help will provide detailed information:

$ journalctl --help
journalctl [OPTIONS...] [MATCHES...]

Query the journal.

Options:
  --system                Show the system journal
  --user                  Show the user journal for the current user
  -M --machine=CONTAINER  Operate on local container
  -S --since=DATE         Show entries not older than the specified date
  -U --until=DATE         Show entries not newer than the specified date
  -C --cursor=CURSOR      Show entries starting at the specified cursor
      --after-cursor=CURSOR  Show entries after the specified cursor
      --show-cursor       Print the cursor after all the entries
  -b --boot[=ID]         Show current boot or the specified boot
      --list-boots       Show terse information about recorded boots

When you run this command, the output is presented in a pager, allowing you to scroll through the text with the arrow keys or page up/page down, and by pressing Q, you can exit the viewer.

$ journalctl --help
journalctl [OPTIONS...] [MATCHES...]
Query the journal.
Options:
  --system              Show the system journal
  --user                Show the user journal for the current user
  -M --machine=CONTAINER Operate on local container
  -S --since=DATE       Show entries not older than the specified date
  -U --until=DATE       Show entries not newer than the specified date
  -c --cursor=CURSOR    Show entries starting at the specified cursor
  --after-cursor=CURSOR Show entries after the specified cursor
  --show-cursor         Print the cursor after all the entries
  -b --boot[=ID]       Show current boot or the specified boot
  --list-boots          Show terse information about recorded boots

Accessing Manual Pages (man)

Every significant command in Linux comes with its own manual ("man") page. To access a command's manual, type:

$ man journalctl

This displays a brief description, a synopsis outlining the command syntax, and a detailed explanation of how the command works. For instance:

$ man journalctl

JOURNALCTL(1)

NAME
    journalctl - Query the systemd journal

SYNOPSIS
    journalctl [OPTIONS...] [MATCHES...]

DESCRIPTION
    journalctl may be used to query the contents of the systemd(1) journal as written by systemd-journald.service(8).

    If called without parameters, it will show the full contents of the journal, starting with the oldest entry collected.

    If one or more match arguments are passed, the output is filtered accordingly. A match is in the format "FIELD=VALUE", e.g. "_SYSTEMD_UNIT=httpd.service", referring to structured journal entry components. See systemd.journal-fields(7) for a list of well-known fields. When multiple matches cover different fields, the log entries are filtered by all; if two matches apply to the same field, they function as alternatives (logical OR). The character "+" can also be used as a separator to combine matches in a disjunction.

Many manual pages also conclude with practical examples. For example, the journalctl man page provides examples such as:

man journalctl

EXAMPLES
Without arguments, all collected logs are shown unfiltered:

journalctl

With one specified match, only the log entries matching the expression are shown:

journalctl _SYSTEMD_UNIT=avahi-daemon.service

When matching two different fields, only entries that satisfy both conditions are displayed simultaneously:

journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097

If two matches refer to the same field, entries matching any of the specified values are shown:

journalctl _SYSTEMD_UNIT=avahi-daemon.service _SYSTEMD_UNIT=dbus.service

Using the separator "+", you can combine expressions with a logical OR. For example, the command below displays all messages from the Avahi service process with PID 28097 along with all messages from the D-Bus service regardless of its process:

journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097 + _SYSTEMD_UNIT=dbus.service

Distinguishing Similar Man Pages

Sometimes, two man pages may share the same command name. For example, there is both a command and a programming function for printf. The manual categorizes these pages into sections. You can observe this by checking the man page for man itself:

To view the manual page for the printf command itself, specify section 1:

$ man 1 printf

For the printf function (used in programming), specify section 3:

$ man 3 printf

During online exams, the Linux Foundation allows the use of both man and the --help option, so remember to utilize them if you forget a command option.

$ man man
$ man 1 printf
$ man 3 printf

Delving into a manual page might take some time, but it is invaluable in understanding the inner workings of a command.

Using apropos to Find Commands

What if you can’t remember the name of the command you need? The apropos command searches the man page database for keywords in their short descriptions.

For instance, to search for man pages related to directories, you might enter:

$ apropos director
directory        directories
$ apropos director
director: nothing appropriate

If you encounter an error, it might be because the mandb database hasn’t been created or updated. You can generate or update it manually with:

$ sudo mandb

After updating the database, running:

$ apropos director

will list relevant entries, such as mkdir. Keep in mind that apropos displays entries from all sections—including system calls from section 2—which may be overly advanced for everyday use. Since common commands are usually located in sections 1 and 8, you can filter the results using the -s option:

$ apropos -s 1,8 director
ls (1)                    - list directory contents
ls (1p)                   - list directory contents
mcd (1)                   - change MSDOS directory
mdeltree (1)              - recursively delete an MSDOS directory and its contents
mdir (1)                  - display an MSDOS directory
mdu (1)                   - display the amount of space occupied by an MSDOS direc...
mkdir (1)                 - make directories
mkdir (1p)                - make directories
mkdir (2)                 - create a directory
mkdir (3p)                - make a directory relative to directory file descriptor
mkdirat (2)               - create a directory

This filtering can help you quickly locate the specific command you need.

Command Auto-Completion

Another useful feature in Linux is command auto-completion. When you type a command like systemctl and press TAB, the terminal automatically completes the command. For example:

$ systemctl

Many commands provide auto-completion suggestions for additional arguments. For instance, after entering systemctl followed by a space, pressing TAB twice might display options such as:

$ systemctl 
add-requires           emergency           isolate              poweroff            show
add-wants             enable              is-system-running     preset              show-environment
cancel                exit                kexec                reboot              start
cat                   force-reload        kill                 reenable            status
condreload            get-default         link                 reload             stop
condrestart           halt                help                 reload-or-restart   suspend
condstop              list-dependencies   list-jobs           rescue              switch-root

Note that auto-completion may not list every single option. For example, if you type:

$ systemctl list-dependencies

and then press TAB, the command might auto-complete the remaining text.

Auto-completion isn’t limited to commands—it also applies to file and directory names. For example:

$ ls /usr/
bin/
lib/
libexec/
local/
sbin/
share/

For long filenames like wordpress_archive.tgz, you might only have to type the first few characters (e.g., wor) and press TAB to complete the name.

Hands-On Practice with man and --help

While help pages and manuals are invaluable, the initial learning curve might be steep. A good exercise is to choose a command you’re less familiar with and learn it exclusively by using man and the --help option. This method is especially useful during exams when theoretical questions may probe knowledge you may have forgotten. Proficiency in quick look-ups using manual pages can save you significant time.

Consider additional command options often found in manual pages, such as those for grep:

-I        equivalent to --binary-files=without-match
-d, --directories=ACTION      how to handle directories;
                                ACTION is 'read', 'recurse', or 'skip'
-D, --devices=ACTION          how to handle devices, FIFOs and sockets;
                                ACTION is 'read' or 'skip'
-r, --recursive                recurse like --directories=recurse
-R, --dereference-recursive    likewise, but follow all symlinks
--include=GLOB                search only files that match GLOB (a file pattern)
--exclude=GLOB                skip files that match GLOB
--exclude-from=FILE           skip files that match any file pattern from FILE
--exclude-dir=GLOB            skip directories that match GLOB
-L, --files-without-match      print only names of FILEs with no selected lines
-l, --files-with-matches       print only names of FILEs with selected lines
-c, --count                    print only a count of selected lines per FILE
-T, --initial-tab              make tabs line up (if needed)
-Z, --null                     print 0 byte after FILE name

Context control:
-B, --before-context=NUM       print NUM lines of leading context
-A, --after-context=NUM        print NUM lines of trailing context
-C, --context=NUM              print NUM lines of output context
-NUM                           print NUM lines between matches with context
--group-separator=SEP         print SEP on line between matches with context
--no-group-separator          do not print separator for matches with context
--color=[WHEN],               use markers to highlight the matching strings;
    --colour=[WHEN]           WHEN is 'always', 'never', or 'auto'
-U, --binary                   do not strip CR characters at EOL (MSDOS/Windows)

When FILE is '-', read standard input. With no FILE, read '.' if recursive, '!' otherwise. With fewer than two FILEs, assume -h.
Exit status is 0 if any line is selected, 1 otherwise; if any error occurs and -q is not given, the exit status is 2.

Report bugs to: [email protected].
GNU grep home page: <https://www.gnu.org/software/grep/>
General help using GNU software: <https://www.gnu.org/gethelp/>
aaron@kodekould:~$

By practicing with these manual pages, you’ll develop the skills to quickly look up details and gain a deep understanding of options—an essential ability for effective Linux system administration and for successfully passing exams.