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+<small>
+===== FHS =====
+To unify standard locations for files in Linux, a document
+[[https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard|Filesystem Hierarchy Standard]]
+has been created. Basing on traditional and commonly employed directory structures
+in Unix-like operating systems, the document recommends a converged directory structure,
+specifying intended use for the directories.
+Typically in Linux the system manual has an entry ''hier'' (and, in case of
+systemd-based distributions, also ''file-hierarchy'') that summarises the
+directory hierarchy used in the system.
+~~Exercise.#~~ On the basis of FHS / ''man hier'' refer the purpose of the following directories:
+\\       • ''/bin'', ''/usr/bin'' and ''/sbin'', ''/usr/sbin''
+\\       • ''/lib'' and ''/usr/lib''
+\\       • ''/usr/share''
+\\       • ''/etc''
+\\       • ''/opt''
+\\       • ''/root'' and ''/home''
+\\       • ''/tmp''
+\\       • ''/var''
+</small>
+===== Searching files =====
+==== locate ====
+The ''**[[https://en.wikipedia.org/wiki/Locate_(Unix)|locate]]**'' tool
+looks up words or wildcard expressions provided as arguments in a previously
+generated list of files.
+\\
+The list (or //base//) of files is usually updated daily - most distributions
+ship alongside ''locate'' a scheduled task that daily runs the command updating
+the database (''updatedb'').
+Contemporary implementations of ''locate'' verify, right before displaying the
+results, whether the matching file still exist and whether the user has
+sufficient permissions to access the file.
+\\
+<small>Unrestricted access to the complete list of files present in a system
+is considered a security vulnerability, hence the systems deploy various access
+restrictions to the list.</small>
+~~Exercise.#~~ Use ''locate --statistics'' to display file database statistic.
+<small>
+With ''--regex'' switch, the ''locate'' treats all arguments as regular
+expressions.
+\\
+Otherwise, each argument ''//arg//'' that contains no wildcards ((as understood
+by the [[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_13|shell]]
+– that is, ''*'' standing for any text, ''?'' standing for any character , and ''[…]'' standing for a range.))
+is actually converted to a wildcard expression ''*//arg//*'' before matching
+against the entries in the database.
+~~Exercise.#~~ Compare the results of the following commands:
+\\       • ''locate 'stdint'%%%%''
+\\       • ''locate '*stdint'%%%%''
+\\       • ''locate --regex '.*stdint'%%%%''
+\\       • ''locate --regex '.*stdint$'%%%%''
+</small>
+<small>
+''locate'' matches all arguments against full file path (or only against filename,
+if ''-b'' is given), and outputs a file name if at least one argument matches.
+To require a match against all arguments, one has to add the ''-A'' (''--all'')
+switch.
+~~Exercise.#~~ Compare the following commands:
+\\       • ''locate -A bin ps to pdf''   vs   ''locate bin ps to pdf''
+\\       • ''locate -b netpbm''   vs   ''locate netpbm''
+The option ''-0'' makes the ''locate'' separate the output file paths with
+a null byte ('\0') instead of a newline ('\n').
+</small>
+~~Exercise.#~~
+Locate files which name is exactly ''bin''.
+~~Exercise.#~~
+Locate files that contain ''pause'' in the filename and ''icons'' in the full path.
+<small>
+~~Exercise.#~~
+Locate files that contain a ''['' character in their names.
+</small>
+==== find ====
+The ''**[[https://en.wikipedia.org/wiki/Find_(Unix)|find]]**'' utility scouts
+recursively the directories to look for files matching the provided filters and
+executes the user-specified action (or just prints the paths of matching files
+if no action is provided).
+While the ''find'' utility is part of
+[[https://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html|POSIX standard]],
+most implementations cover more functionality than what the standard mandates.
+\\
+These materials summarise the implementation of ''find'' from [[https://www.gnu.org/software/findutils/|GNU findutils]]
+(which is commonly used in Linux).
+The ''find'' command has two noteworthy switches: \\
+''-L'' follows symlinks (default), \\
+''-H'' des not follows symlinks (and only then ''find'' may be used to look for symlinks).
+**''find'' has a fixed and counter-intuitive syntax – options come first, but
+then one must list paths to search, and only after all paths one may provide
+tests and actions.**
+Upon omitting list of paths, ''find'' searches in the current directory.
+Upon omitting actions, ''find'' prints the matching filenames to the standard output.
+**The order of the arguments matters**.\\
+''find -ls -name '*bash*'%%%%'' first lists each encountered file, and then checks if the filename contains 'bash',\\
+''find -name '*bash*' -ls'' first checks if the filename contains 'bash', and lists the file only if the test succeeded.
+=== Tests ===
+|Basic tests:||
+^ ''find …'' ^ matches: ^
+|'' -name //pattern//'' \\ ''-iname //pattern//''| file name to the //pattern// \\ (''-i'' performs case-insensitive match)|
+|'' -path //pattern//'' \\ ''-ipath //pattern//''| file path to the  //pattern// \\ (''-i'' performs case-insensitive match)|
+|''-type {f|d|l|p|s|c|b}''|file type – ''f'' for ordinary file, ''d'' for directory, ''l'' for symlink, etc.|
+|''-user //name//'' \\ ''-group //name//''|file owner / group|
+|  \\ ''-perm  //mode//'' \\ ''-perm -//mode//'' \\ ''-perm  /%%%%//mode//'' \\  |permissions: \\ '' '' same as provided \\ ''-'' all provided bits set \\ ''/'' at least one of the provided bits set \\ mode can be either octal (''22'') or symbolic (''go+w'') |
+|''-size //s//''|size; ''512c'' stands for 512B; ''512M'' stands for 512MB; \\ ''-512c'' stands for at most 512B, ''+512c'' stands for at least 512B \\ Warning: the default unit, denoted with ''b'', are dist blocks, not bytes|
+|''-atime //d//'' / ''-amin //min//'' \\ ''-ctime //d//'' / ''-cmin //min//'' \\ ''-mtime //d//'' / ''-mmin //min//'' |times: access, change, modification in //d//ays or //min//utes|
+The tests can be:
+  * joined with ''-a'' (logical //and//, used when no operator is specified) and ''-o'' (logical //or//) \\ eg., ''find -name //baz// -o -name //bar//''
+  * grouped in parentheses – ''('' and '')'', but beware – parentheses are treated specially by bash, hence one must escape them as ''\('' and ''\)'' \\ eg., ''find -size -1M \( -name //baz// -o -name //bar// \)''
+  * negated by ''!'' \\ eg., ''find ! -type f ! -type d''
+<small>
+Other important tests include:
+\\
+''-mindepth //n//'' and ''-maxdepth //n//'' specify how deep find will scot the directories.
+\\
+''-xdev'' forbids entering directories where other filesystems are mounted. \\
+That is: if one looks for a file in ''/'', then ''find'' will not scout the ''/mnt/cdrom'' directory if a compact disc has been mounted there.
+</small>
+<small>
+=== Actions ===
+|Selected actions:||
+|''-print''| the default action - prints filenames separated by a newline |
+|''-print0''| prints filenames separated by a null byte |
+|''-delete''| removes matching files |
+|''-exec … {} … ;''| executes provided program, for each matching file separately (see below)|
+|''-exec … {} +''| executes provided program, for all matching files at once (see below)|
+<html><div></html>
+If ''find -exec program arg1 arg2 \{\} \;'' finds the files: ''one'', ''two'' i ''three'', then it executes:\\
+<html><div style="display:inline-block;line-height: 1em"></html>
+''program arg1 arg2 one''\\
+''program arg1 arg2 two''\\
+''program arg1 arg2 three''
+<html></div></div></html>
+If ''find -exec program arg1 arg2 \{\} +'' finds the files: ''one'', ''two'' i ''three'', then it executes:\\
+''program arg1 arg2 one two three''
+In ''-exec'', each occurrence of the ''{}'' expression gets replaced by the filename. \\
+Warning: ''{}'' (and '';'') are treated specially by the shell, hence they must be
+escaped (that is, ''\{\}'') or enclosed in quotation marks (''"{}"'').
+\\
+In the ''-exec … {} +'' syntax, the argument ''{}'' must appear once, as the last argument.
+</small>
+=== Exercises ===
+~~Exercise.#~~ Issue, from your home directory, the commands ''find'' (with no
+arguments), ''find .config/..'' oraz ''find ~''. \\ How do the results differ?
+~~Exercise.#~~ Find all files in the ''/srv/'' and ''/var/lib/zypp/'' directories
+using one command.
+~~Exercise.#~~ Find, in your home directory, files which names end with ''.xml''.
+~~Exercise.#~~ Find empty files in your home directory
+<small>
+~~Exercise.#~~ Find and display files in ''/usr/include'' that are not greater
+than 32 bytes. \\
+Warning: for ''find'' the unit ''b'' or no unit denotes disk blocks.
+Bytes are denoted ''c''.
+</small>
+~~Exercise.#~~ Find all files outside your home directory.
+~~Exercise.#~~ Find files with no read permission for the group in your home
+directory.
+<small>
+~~Exercise.#~~
+Execute the command: ''find \( -type d -ls \) -o \( -print \)''.
+When the action ''-ls'' is executed, and when the action ''-print'' is executed?
+What and why would happen of one would remove the parentheses?
+</small>
+~~META:
+language = en
+~~

Jan Kończak

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