Studying for linux test(help with shell understanding shell commands)

[Reily]

Currently going through our lab on the "sort" command. I'm understanding almost everything except the value listed after the -k switch.

for one question it asks: Sort the data first based on GID, then based on UID. The command it lists is:

sort -k 4n -k 3n (file name)

Then it asks: Sort the data based on shell value then username Value

sort -k 7 -k 1 (file name)

What does GID and UID stand for?

How am i supposed to know what number will sort by what value?

I honestly though the value after k just symbolized a colum to start sorting by. =\.

p.s -Sorry if this should be in a different forum it seemed fairly fitting to me.

[Conelton]

I believe that GID stands for Group Identification and UID stands for User Identification and that they are the names of the users and the user groups that they belong to (although I could be wrong). I'm not sure about the k argument to be honest. After a quick glance at the documentation, I agree with you, it looks like it just picks a column to sort the lines by. Perhaps the files are structured so that the 4th column is the GID and the 3rd is the UID and so on?

[Reily]

I believe that GID stands for Group Identification and UID stands for User Identification and that they are the names of the users and the user groups that they belong to (although I could be wrong). I'm not sure about the k argument to be honest. After a quick glance at the documentation, I agree with you, it looks like it just picks a column to sort the lines by. Perhaps the files are structured so that the 4th column is the GID and the 3rd is the UID and so on?

You are correct, however i was unable to find any documentation to suggest that until a friend said the teacher just explained it in class. I'm mostly just insanely stressed about this as the class is split into 3 tests worth 35/35/30% of our grade. This is test 2 of 3. I dislike how it is setup

[Reily]

New question.

command:

grep –r "Linux course at Mohawk College" . | grep –v ".txt"

what does the . before the pipe do for the command?

[lutzee]

The dot would indicate in the current directory

[MasterGeekMX]

i  will drop you a hint.

man [command name]

run that and you will get the manual of almost evry single command. It may sometimes be too technical, but helpfull nontheless.

 

EXAMPLE (Quite long, so I will hide it)

Spoiler

$ man grep

 

GREP(1)                                                                                                      User Commands                                                                                                      GREP(1)

NAME
       grep, egrep, fgrep - print lines that match patterns

SYNOPSIS
       grep [OPTION...] PATTERNS [FILE...]
       grep [OPTION...] -e PATTERNS ... [FILE...]
       grep [OPTION...] -f PATTERN_FILE ... [FILE...]

DESCRIPTION
       grep  searches for PATTERNS in each FILE.  PATTERNS is one or more patterns separated by newline characters, and grep prints each line that matches a pattern.  Typically PATTERNS should be quoted when grep is used in a shell
       command.

       A FILE of “-” stands for standard input.  If no FILE is given, recursive searches examine the working directory, and nonrecursive searches read standard input.

       In addition, the variant programs egrep and fgrep are the same as grep -E and grep -F, respectively.  These variants are deprecated, but are provided for backward compatibility.

OPTIONS
   Generic Program Information
       --help Output a usage message and exit.

       -V, --version
              Output the version number of grep and exit.

   Pattern Syntax
       -E, --extended-regexp
              Interpret PATTERNS as extended regular expressions (EREs, see below).

       -F, --fixed-strings
              Interpret PATTERNS as fixed strings, not regular expressions.

       -G, --basic-regexp
              Interpret PATTERNS as basic regular expressions (BREs, see below).  This is the default.

       -P, --perl-regexp
              Interpret PATTERNS as Perl-compatible regular expressions (PCREs).  This option is experimental when combined with the -z (--null-data) option, and grep -P may warn of unimplemented features.

   Matching Control
       -e PATTERNS, --regexp=PATTERNS
              Use PATTERNS as the patterns.  If this option is used multiple times or is combined with the -f (--file) option, search for all patterns given.  This option can be used to protect a pattern beginning with “-”.

       -f FILE, --file=FILE
              Obtain patterns from FILE, one per line.  If this option is used multiple times or is combined with the -e (--regexp) option, search for all patterns given.  The  empty  file  contains  zero  patterns,  and  therefore
              matches nothing.

       -i, --ignore-case
              Ignore case distinctions in patterns and input data, so that characters that differ only in case match each other.

       --no-ignore-case
              Do  not  ignore  case distinctions in patterns and input data.  This is the default.  This option is useful for passing to shell scripts that already use -i, to cancel its effects because the two options override each
              other.

       -v, --invert-match
              Invert the sense of matching, to select non-matching lines.

       -w, --word-regexp
              Select only those lines containing matches that form whole words.  The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character.   Similarly,  it
              must be either at the end of the line or followed by a non-word constituent character.  Word-constituent characters are letters, digits, and the underscore.  This option has no effect if -x is also specified.

       -x, --line-regexp
              Select only those matches that exactly match the whole line.  For a regular expression pattern, this is like parenthesizing the pattern and then surrounding it with ^ and $.

       -y     Obsolete synonym for -i.

   General Output Control
       -c, --count
              Suppress normal output; instead print a count of matching lines for each input file.  With the -v, --invert-match option (see below), count non-matching lines.

       --color[=WHEN], --colour[=WHEN]
              Surround  the  matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields and groups of context lines) with escape sequences to display them in color
              on the terminal.  The colors are defined by the environment variable GREP_COLORS.  The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority.  WHEN is never, always,  or
              auto.

       -L, --files-without-match
              Suppress normal output; instead print the name of each input file from which no output would normally have been printed.  The scanning will stop on the first match.

       -l, --files-with-matches
              Suppress normal output; instead print the name of each input file from which output would normally have been printed.  The scanning will stop on the first match.

       -m NUM, --max-count=NUM
              Stop  reading a file after NUM matching lines.  If the input is standard input from a regular file, and NUM matching lines are output, grep ensures that the standard input is positioned to just after the last matching
              line before exiting, regardless of the presence of trailing context lines.  This enables a calling process to resume a search.  When grep stops after NUM matching lines, it outputs any trailing  context  lines.   When
              the -c or --count option is also used, grep does not output a count greater than NUM.  When the -v or --invert-match option is also used, grep stops after outputting NUM non-matching lines.

       -o, --only-matching
              Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

       -q, --quiet, --silent
              Quiet; do not write anything to standard output.  Exit immediately with zero status if any match is found, even if an error was detected.  Also see the -s or --no-messages option.

       -s, --no-messages
              Suppress error messages about nonexistent or unreadable files.

   Output Line Prefix Control
       -b, --byte-offset
              Print the 0-based byte offset within the input file before each line of output.  If -o (--only-matching) is specified, print the offset of the matching part itself.

       -H, --with-filename
              Print the file name for each match.  This is the default when there is more than one file to search.

       -h, --no-filename
              Suppress the prefixing of file names on output.  This is the default when there is only one file (or only standard input) to search.

       --label=LABEL
              Display  input actually coming from standard input as input coming from file LABEL.  This can be useful for commands that transform a file's contents before searching, e.g., gzip -cd foo.gz | grep --label=foo -H 'some
              pattern'.  See also the -H option.

       -n, --line-number
              Prefix each line of output with the 1-based line number within its input file.

       -T, --initial-tab
              Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal.  This is useful with options that prefix their output to the actual content: -H,-n, and -b.  In
              order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to be printed in a minimum size field width.

       -u, --unix-byte-offsets
              Report  Unix-style  byte offsets.  This switch causes grep to report byte offsets as if the file were a Unix-style text file, i.e., with CR characters stripped off.  This will produce results identical to running grep
              on a Unix machine.  This option has no effect unless -b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.

       -Z, --null
              Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name.  For example, grep -lZ outputs a zero byte after each file name instead of the usual newline.  This option makes
              the  output  unambiguous,  even  in the presence of file names containing unusual characters like newlines.  This option can be used with commands like find -print0, perl -0, sort -z, and xargs -0 to process arbitrary
              file names, even those that contain newline characters.

   Context Line Control
       -A NUM, --after-context=NUM
              Print NUM lines of trailing context after matching lines.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching option, this has no effect and a warning
              is given.

       -B NUM, --before-context=NUM
              Print NUM lines of leading context before matching lines.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching option, this has no effect and a warning
              is given.

       -C NUM, -NUM, --context=NUM
              Print NUM lines of output context.  Places a line containing a group separator (--) between contiguous groups of matches.  With the -o or --only-matching option, this has no effect and a warning is given.

   File and Directory Selection
       -a, --text
              Process a binary file as if it were text; this is equivalent to the --binary-files=text option.

       --binary-files=TYPE
              If a file's data or metadata indicate that the file contains binary data, assume that the file is of type TYPE.  Non-text bytes indicate binary data; these are either output bytes that are improperly encoded  for  the
              current locale, or null input bytes when the -z option is not given.

              By  default,  TYPE  is binary, and grep suppresses output after null input binary data is discovered, and suppresses output lines that contain improperly encoded data.  When some output is suppressed, grep follows any
              output with a one-line message saying that a binary file matches.

              If TYPE is without-match, when grep discovers null input binary data it assumes that the rest of the file does not match; this is equivalent to the -I option.

              If TYPE is text, grep processes a binary file as if it were text; this is equivalent to the -a option.

              When type is binary, grep may treat non-text bytes as line terminators even without the -z option.  This means choosing binary versus text can affect whether a pattern matches a file.  For example, when type is binary
              the pattern q$ might match q immediately followed by a null byte, even though this is not matched when type is text.  Conversely, when type is binary the pattern . (period) might not match a null byte.

              Warning:  The  -a  option might output binary garbage, which can have nasty side effects if the output is a terminal and if the terminal driver interprets some of it as commands.  On the other hand, when reading files
              whose text encodings are unknown, it can be helpful to use -a or to set LC_ALL='C' in the environment, in order to find more matches even if the matches are unsafe for direct display.

       -D ACTION, --devices=ACTION
              If an input file is a device, FIFO or socket, use ACTION to process it.  By default, ACTION is read, which means that devices are read just as if they were ordinary files.  If ACTION  is  skip,  devices  are  silently
              skipped.

       -d ACTION, --directories=ACTION
              If an input file is a directory, use ACTION to process it.  By default, ACTION is read, i.e., read directories just as if they were ordinary files.  If ACTION is skip, silently skip directories.  If ACTION is recurse,
              read all files under each directory, recursively, following symbolic links only if they are on the command line.  This is equivalent to the -r option.

       --exclude=GLOB
              Skip any command-line file with a name suffix that matches the pattern GLOB, using wildcard matching; a name suffix is either the whole name, or a trailing part that starts with a non-slash character immediately after
              a  slash  (/)  in the name.  When searching recursively, skip any subfile whose base name matches GLOB; the base name is the part after the last slash.  A pattern can use *, ?, and [...] as wildcards, and \ to quote a
              wildcard or backslash character literally.

       --exclude-from=FILE
              Skip files whose base name matches any of the file-name globs read from FILE (using wildcard matching as described under --exclude).

       --exclude-dir=GLOB
              Skip any command-line directory with a name suffix that matches the pattern GLOB.  When searching recursively, skip any subdirectory whose base name matches GLOB.  Ignore any redundant trailing slashes in GLOB.

       -I     Process a binary file as if it did not contain matching data; this is equivalent to the --binary-files=without-match option.

       --include=GLOB
              Search only files whose base name matches GLOB (using wildcard matching as described under --exclude).

       -r, --recursive
              Read all files under each directory, recursively, following symbolic links only if they are on the command line.  Note that if no file operand is given, grep searches the working directory.  This is equivalent to  the
              -d recurse option.

       -R, --dereference-recursive
              Read all files under each directory, recursively.  Follow all symbolic links, unlike -r.

   Other Options
       --line-buffered
              Use line buffering on output.  This can cause a performance penalty.

       -U, --binary
              Treat  the file(s) as binary.  By default, under MS-DOS and MS-Windows, grep guesses whether a file is text or binary as described for the --binary-files option.  If grep decides the file is a text file, it strips the
              CR characters from the original file contents (to make regular expressions with ^ and $ work correctly).  Specifying -U overrules this guesswork, causing all files to be read  and  passed  to  the  matching  mechanism
              verbatim; if the file is a text file with CR/LF pairs at the end of each line, this will cause some regular expressions to fail.  This option has no effect on platforms other than MS-DOS and MS-Windows.

       -z, --null-data
              Treat  input  and  output data as sequences of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline.  Like the -Z or --null option, this option can be used with commands like sort -z to
              process arbitrary file names.

REGULAR EXPRESSIONS
       A regular expression is a pattern that describes a set of strings.  Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.

       grep understands three different versions of regular expression syntax: “basic” (BRE), “extended” (ERE) and “perl” (PCRE).  In GNU grep there is no difference in available functionality between basic and  extended  syntaxes.
       In other implementations, basic regular expressions are less powerful.  The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.  Perl-compatible
       regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but work only if PCRE is available in the system.

       The fundamental building blocks are the regular expressions that match a single character.  Most characters, including all letters and digits, are regular expressions that match themselves.  Any meta-character  with  special
       meaning may be quoted by preceding it with a backslash.

       The period . matches any single character.  It is unspecified whether it matches an encoding error.

   Character Classes and Bracket Expressions
       A  bracket  expression  is  a  list  of  characters  enclosed by [ and ].  It matches any single character in that list.  If the first character of the list is the caret ^ then it matches any character not in the list; it is
       unspecified whether it matches an encoding error.  For example, the regular expression [0123456789] matches any single digit.

       Within a bracket expression, a range expression consists of two characters separated by a hyphen.  It matches any single character that sorts between the two characters, inclusive, using the locale's collating  sequence  and
       character set.  For example, in the default C locale, [a-d] is equivalent to [abcd].  Many locales sort characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent
       to [aBbCcDd], for example.  To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value C.

       Finally, certain named classes of characters are predefined within bracket expressions, as follows.  Their names are self explanatory, and they are [:alnum:], [:alpha:], [:blank:], [:cntrl:], [:digit:], [:graph:], [:lower:],
       [:print:],  [:punct:],  [:space:],  [:upper:],  and [:xdigit:].  For example, [[:alnum:]] means the character class of numbers and letters in the current locale.  In the C locale and ASCII character set encoding, this is the
       same as [0-9A-Za-z].  (Note that the brackets in these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket expression.)  Most meta-characters lose their special
       meaning inside bracket expressions.  To include a literal ] place it first in the list.  Similarly, to include a literal ^ place it anywhere but first.  Finally, to include a literal - place it last.

   Anchoring
       The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line.

   The Backslash Character and Special Expressions
       The  symbols \< and \> respectively match the empty string at the beginning and end of a word.  The symbol \b matches the empty string at the edge of a word, and \B matches the empty string provided it's not at the edge of a
       word.  The symbol \w is a synonym for [_[:alnum:]] and \W is a synonym for [^_[:alnum:]].

   Repetition
       A regular expression may be followed by one of several repetition operators:
       ?      The preceding item is optional and matched at most once.
       *      The preceding item will be matched zero or more times.
       +      The preceding item will be matched one or more times.
       {n}    The preceding item is matched exactly n times.
       {n,}   The preceding item is matched n or more times.
       {,m}   The preceding item is matched at most m times.  This is a GNU extension.
       {n,m}  The preceding item is matched at least n times, but not more than m times.

   Concatenation
       Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated expressions.

   Alternation
       Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either alternate expression.

   Precedence
       Repetition takes precedence over concatenation, which in turn takes precedence over alternation.  A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression.

   Back-references and Subexpressions
       The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.

   Basic vs Extended Regular Expressions
       In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

EXIT STATUS
       Normally the exit status is 0 if a line is selected, 1 if no lines were selected, and 2 if an error occurred.  However, if the -q or --quiet or --silent is used and a line is selected, the exit status is 0 even if  an  error
       occurred.

ENVIRONMENT
       The behavior of grep is affected by the following environment variables.

       The  locale  for  category  LC_foo is specified by examining the three environment variables LC_ALL, LC_foo, LANG, in that order.  The first of these variables that is set specifies the locale.  For example, if LC_ALL is not
       set, but LC_MESSAGES is set to pt_BR, then the Brazilian Portuguese locale is used for the LC_MESSAGES category.  The C locale is used if none of these environment variables are set, if the locale catalog is  not  installed,
       or if grep was not compiled with national language support (NLS).  The shell command locale -a lists locales that are currently available.

       GREP_OPTIONS
              This  variable  specifies default options to be placed in front of any explicit options.  As this causes problems when writing portable scripts, this feature will be removed in a future release of grep, and grep warns
              if it is used.  Please use an alias or script instead.

       GREP_COLOR
              This variable specifies the color used to highlight matched (non-empty) text.  It is deprecated in favor of GREP_COLORS, but still supported.  The mt, ms, and mc capabilities of GREP_COLORS have priority over it.   It
              can  only  specify  the  color  used  to highlight the matching non-empty text in any matching line (a selected line when the -v command-line option is omitted, or a context line when -v is specified).  The default is
              01;31, which means a bold red foreground text on the terminal's default background.

       GREP_COLORS
              Specifies the colors and other attributes used to highlight various parts of the output.  Its value is a colon-separated list of capabilities that defaults to ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36 with the
              rv and ne boolean capabilities omitted (i.e., false).  Supported capabilities are as follows.

              sl=    SGR  substring  for  whole selected lines (i.e., matching lines when the -v command-line option is omitted, or non-matching lines when -v is specified).  If however the boolean rv capability and the -v command-
                     line option are both specified, it applies to context matching lines instead.  The default is empty (i.e., the terminal's default color pair).

              cx=    SGR substring for whole context lines (i.e., non-matching lines when the -v command-line option is omitted, or matching lines when -v is specified).  If however the boolean rv capability and the -v command-line
                     option are both specified, it applies to selected non-matching lines instead.  The default is empty (i.e., the terminal's default color pair).

              rv     Boolean value that reverses (swaps) the meanings of the sl= and cx= capabilities when the -v command-line option is specified.  The default is false (i.e., the capability is omitted).

              mt=01;31
                     SGR  substring for matching non-empty text in any matching line (i.e., a selected line when the -v command-line option is omitted, or a context line when -v is specified).  Setting this is equivalent to setting
                     both ms= and mc= at once to the same value.  The default is a bold red text foreground over the current line background.

              ms=01;31
                     SGR substring for matching non-empty text in a selected line.  (This is only used when the -v command-line option is omitted.)  The effect of the sl= (or cx= if rv) capability remains active when this kicks in.
                     The default is a bold red text foreground over the current line background.

              mc=01;31
                     SGR  substring  for  matching non-empty text in a context line.  (This is only used when the -v command-line option is specified.)  The effect of the cx= (or sl= if rv) capability remains active when this kicks
                     in.  The default is a bold red text foreground over the current line background.

              fn=35  SGR substring for file names prefixing any content line.  The default is a magenta text foreground over the terminal's default background.

              ln=32  SGR substring for line numbers prefixing any content line.  The default is a green text foreground over the terminal's default background.

              bn=32  SGR substring for byte offsets prefixing any content line.  The default is a green text foreground over the terminal's default background.

              se=36  SGR substring for separators that are inserted between selected line fields (:), between context line fields, (-), and between groups of adjacent lines when nonzero context is specified (--).  The default is  a
                     cyan text foreground over the terminal's default background.

              ne     Boolean  value  that  prevents  clearing to the end of line using Erase in Line (EL) to Right (\33[K) each time a colorized item ends.  This is needed on terminals on which EL is not supported.  It is otherwise
                     useful on terminals for which the back_color_erase (bce) boolean terminfo capability does not apply, when the chosen highlight colors do not affect the background, or when EL is too  slow  or  causes  too  much
                     flicker.  The default is false (i.e., the capability is omitted).

              Note that boolean capabilities have no =... part.  They are omitted (i.e., false) by default and become true when specified.

              See  the  Select  Graphic  Rendition (SGR) section in the documentation of the text terminal that is used for permitted values and their meaning as character attributes.  These substring values are integers in decimal
              representation and can be concatenated with semicolons.  grep takes care of assembling the result into a complete SGR sequence (\33[...m).  Common values to concatenate include 1 for  bold,  4  for  underline,  5  for
              blink,  7  for  inverse,  39  for  default  foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for
              default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

       LC_ALL, LC_COLLATE, LANG
              These variables specify the locale for the LC_COLLATE category, which determines the collating sequence used to interpret range expressions like [a-z].

       LC_ALL, LC_CTYPE, LANG
              These variables specify the locale for the LC_CTYPE category, which determines the type of characters, e.g., which characters are whitespace.  This category also determines the character  encoding,  that  is,  whether
              text is encoded in UTF-8, ASCII, or some other encoding.  In the C or POSIX locale, all characters are encoded as a single byte and every byte is a valid character.

       LC_ALL, LC_MESSAGES, LANG
              These variables specify the locale for the LC_MESSAGES category, which determines the language that grep uses for messages.  The default C locale uses American English messages.

       POSIXLY_CORRECT
              If set, grep behaves as POSIX requires; otherwise, grep behaves more like other GNU programs.  POSIX requires that options that follow file names must be treated as file names; by default, such options are permuted to
              the front of the operand list and are treated as options.  Also, POSIX requires that unrecognized options be diagnosed as “illegal”, but since they are not really against the law the default is  to  diagnose  them  as
              “invalid”.  POSIXLY_CORRECT also disables _N_GNU_nonoption_argv_flags_, described below.

       _N_GNU_nonoption_argv_flags_
              (Here  N  is  grep's  numeric  process ID.)  If the ith character of this environment variable's value is 1, do not consider the ith operand of grep to be an option, even if it appears to be one.  A shell can put this
              variable in the environment for each command it runs, specifying which operands are the results of file name wildcard expansion and therefore should not be treated as options.  This behavior is available only with the
              GNU C library, and only when POSIXLY_CORRECT is not set.

NOTES
       This man page is maintained only fitfully; the full documentation is often more up-to-date.

COPYRIGHT
       Copyright 1998-2000, 2002, 2005-2020 Free Software Foundation, Inc.

       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

BUGS
   Reporting Bugs
       Email bug reports to the bug-reporting address 〈bug-grep@gnu.org〉.  An email archive 〈https://lists.gnu.org/mailman/listinfo/bug-grep〉 and a bug tracker 〈https://debbugs.gnu.org/cgi/pkgreport.cgi?package=grep〉 are available.

   Known Bugs
       Large repetition counts in the {n,m} construct may cause grep to use lots of memory.  In addition, certain other obscure regular expressions require exponential time and space, and may cause grep to run out of memory.

       Back-references are very slow, and may require exponential time.

EXAMPLE
       The  following  example outputs the location and contents of any line containing “f” and ending in “.c”, within all files in the current directory whose names contain “g” and end in “.h”.  The -n option outputs line numbers,
       the -- argument treats expansions of “*g*.h” starting with “-” as file names not options, and the empty file /dev/null causes file names to be output even if only one file name happens to be of the form “*g*.h”.

         $ grep -n -- 'f.*\.c$' *g*.h /dev/null
         argmatch.h:1:/* definitions and prototypes for argmatch.c

       The only line that matches is line 1 of argmatch.h.  Note that the regular expression syntax used in the pattern differs from the globbing syntax that the shell uses to match file names.

SEE ALSO
   Regular Manual Pages
       awk(1), cmp(1), diff(1), find(1), perl(1), sed(1), sort(1), xargs(1), read(2), pcre(3), pcresyntax(3), pcrepattern(3), terminfo(5), glob(7), regex(7).

   Full Documentation
       A complete manual 〈https://www.gnu.org/software/grep/manual/〉 is available.  If the info and grep programs are properly installed at your site, the command

              info grep

       should give you access to the complete manual.

GNU grep 3.4                                                                                                   2019-12-29                                                                                                       GREP(1)