Jan Kończak

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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/9799919799/utilities/V3_chap02.html#tag_19_14|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/9799919799/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 (<small>notice that with this switch ''find'' won't match any files with ''-type l''</small>), \\
''-H'' does not follow symlinks when scouting provided paths (default behaviour).

**''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 disk 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 scout 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/..'' and ''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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