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+====== Bash shell ======
+Guide on the usage of the most common commands of the bash shell
+<html>
+<iframe width="560" height="315" src="https://www.youtube.com/embed/5Dg3w5m7F_Q" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
+</html>
+===== Generic Linux Command Syntax =====
+<code bash>
+command [-options] [arguments]
+</code>
+  * If a command is to long to stay on a single row, it may be continued on the next row using the character "''\''" as last character of the row
+  * More than one command can be executed on the same line, separating the commands with ";"
+<code bash>
+command1; command2;...
+</code>
+  * In this case commands will be executed sequentially
+===== Help =====
+All linux commands are documented, and their documentation can be accessed using:
+  * ''man <command>'': gives an extensive description of command
+  * ''apropos <command>'': search a keyword in the manual page descriptions
+  * ''whatis <command>'': gives a short description of command
+  * ''command -h'' or ''command -help'': gives a description of command options
+===== The Unix file system =====
+The Unix file system is a hierarchical file system organized in directory:
+  * ''/'' is the symbol that represents the root of the file system
+  * ''.'' (dot) is the actual directory
+  * ''..'' (dot dot) is the father directory
+  * ''~'' is the home directory, for example ''/home/username''
+A file that starts with "''.''" (dot) is an //hidden file//.
+Files within a directory can be accessed using:
+  * an //absolute path//: ''/dir1/dir2/file''
+  * a //relative path//: ''subdir1/subdir2/file''
+===== Files Commands =====
+<code bash>
+cp [-fir] src1 src2 ... dest
+</code>
+  * copy one or more files, possibly in a directory
+<code bash>
+rm [-fir] file1 file2 ...
+</code>
+  * delete ''file1'', ''file2'', ...
+  * files can be directories if the ''-r'' option is used. In this case the entire directory content will be recursively deleted
+<code bash>
+mv [-fi] file1 file2 ... dest
+</code>
+  * move one or more file, possibly in a directory
+File commands options are:
+  * ''-f'' force, no confirmation is required
+  * ''-i'': interactive, confirmation is required for each file
+  * ''-r'': recursive, the file command works recursively for each subdirectory
+===== Directories Commands =====
+<code bash>
+cd dir
+</code>
+  * goes into the ''dir'' directory
+  * ''cd ..'': goes into the father directory
+  * ''cd'' (alone): goes into the home directory
+<code bash>
+pwd
+</code>
+  * prints the name of the current directory
+<code bash>
+mkdir dir
+</code>
+  * makes the ''dir'' directory
+<code bash>
+rmdir dir
+</code>
+  * removes the ''dir'' directory
+  * ''dir'' must be empty
+===== Link Commands =====
+A //Symbolic link// is a file that contains a reference to another file or directory
+  * **Hard link**: more than one file name point to the same file
+  * **Soft link**: is a special file that contains the file that must be pointed
+<code bash>
+ln src alias
+</code>
+  * makes an //hard link// between ''src'' and ''alias''
+  * //hard links// may not normally point to directories and they can not link paths on different volumes, for this reason usually //soft link// are preferred
+<code bash>
+ln -s src alias
+</code>
+  * makes a symbolic //soft link// between ''src'' and ''alias''
+===== Files and Directories protection =====
+In Linux, every file or directory has special protections that allow differential access to the different users of the system
+The system users are grouped into three sets:
+  * **u**: //user//, the owner of the file
+  * **g**: //group// users
+  * **o**: //other// users
+  * **a**: //all// users
+Files protections are:
+  * **r**: //read// permission
+  * **w**: //write// permission
+  * **x**: //execution// permission
+Directories protections are:
+  * **x**: directory crossing
+  * **r**: files list
+  * **w**: files creation and/or deletion
+===== Files and Directories protection Commands =====
+Change the group of the files:
+<code bash>
+chgrp [-R] group file1 file2 ...
+</code>
+Change the owner (and potentially the group) of the files:
+<code bash>
+chown [-R] user[:group] file1 file2 ...
+</code>
+Change the protection of the files:
+<code bash>
+chmod [-R] protection file1 file2 ...
+</code>
+  * protection is the sum of: 4 for ''r'', 2 for ''w'' and 1 for ''x'' respectively for user, group and others
+Examples:
+  * ''chmod 777 file'': user has ''rwx'' access to the file, group has ''rwx'' access and also others (''-rwxrwxrwx'')
+  * ''chmod 664 file'': user has ''rw-'' access to the file, group has ''rw-'' access and others has ''r--'' access (''-rw-rw-r--'')
+  * ''chmod 754 file'': user has ''rwx'' access to the file, group has ''r-x'' access and others has ''r--'' access (''-rwxr-xr--'')
+Options:
+  * ''-R'': recursive, the command works recursively for each subdirectory
+===== The ls Command =====
+<code bash>
+ls [-options] [files]
+</code>
+  * list directory contents
+  * Options are:
+    * ''-a'': all, list also hidden files
+    * ''-l'': long, long format output
+    * ''-r'': reverse, reverse order of output
+    * ''-t'': time, files are sorted by modification time
+    * ''-R'': recursive, the command works recursively for each subdirectory
+===== ls command example =====
+<code bash>
+user@pc:~$ ls -al ~/tmp
+drwxr-xr-x  3 user user  4096 2009-10-13 15:10 ./
+drwxr-xr-x 64 user user 12288 2009-10-13 15:10 ../
+drwxr-xr-x  2 user user  4096 2009-10-13 15:10 adir/
+-rw-r--r--  1 user user    29 2009-10-13 15:10 afile
+lrwxrwxrwx  1 user user     5 2009-10-13 15:24 alink -> afile
+</code>
+This command lists the ''tmp'' directory which is in the home directory showing also the hidden files with a long output format:
+  * the first character is the file type (''-'' standard file, ''d'' directory, ''l'' link)
+  * ''rwxr-xr-x'' are the protections
+  * the number of the second column is 1 for files and the number of file contained in the directory for directories
+  * the first ''user'' is the owner, the second is the group
+  * the second number is the dimension in byte
+  * date and hour are the creation time
+  * the last is the //filename//, in the case of soft link the linked file is reported
+===== Viewing a textual file =====
+Using visual editor like ''gedit'', [[http://www.vim.org/|vim]] (or its oldest version ''vi'') or [[http://www.gnu.org/software/emacs/|emacs]]
+With ''cat'' command
+<code bash>
+cat file1 file2 ...
+</code>
+  * print in order file1 and file2 content
+With ''tail'' command
+<code bash>
+tail [-n number] file1 file2 ...
+</code>
+  * print the last number rows of ''file1'' and ''file2'' (default is 10 rows)
+  * with ''+number'' prints the rows from number to the end of ''file1'' and ''file2'' (ex. ''tail -n +15)''
+With ''head'' command
+<code bash>
+head [-n number] file1 file2 ...}
+</code>
+  * like ''tail'', but it prints the first number rows of ''file1'' and ''file2''
+With ''more'' command
+<code bash>
+more file
+</code>
+  * prints a file page by page. Use space to go to the next page
+With ''diff'' command
+<code bash>
+diff file1 file2
+</code>
+  * displays differences between ''file1'' and ''file2''
+===== The find command =====
+''find <dir> [-opt]'': it finds files present in the ''dir'' directory and in each subdirectory
+Options are:
+  * ''-name pattern'': search ''pattern''. Pattern can be a filename or a part of filename using the symbol ''\*'' to denote the rest of the file
+  * examples of ''pattern'':
+    * ''abc'': the file ''abc'' is searched
+    * ''\*ab'': files that end with ''ab'' are searched
+    * ''bc\*'': files that begin with ''bc'' are searched
+    * ''\*dd\*'': files that contain ''dd'' are searched
+  * ''-type [b c d l]'': ''b''=block file, ''c''=character file, ''d''=directory, ''l''=link
+  * ''-exec command \;'': for each file found executes ''command''
+  * ''-exec command \{} \;'': for each file found executes ''command file_found''. ''\{}'' is the file found.
+===== The grep command =====
+<code bash>
+grep [-opt] pattern file1 file2 ...
+</code>
+The ''grep'' command prints lines matching a pattern found in ''file1'', ''file2''...
+Its options are:
+  * ''-c'': prints the number of matching lines
+  * ''-i'': case insensitive
+  * ''-l'': prints only names of files containing the pattern
+  * ''-n'': prints line number of matched lines
+  * ''-v'': prints only lines that non match pattern
+''pattern'' can be a regular expression:
+  * ''.'': represents any character
+  * ''^'': the begin of the row
+  * ''$'': the end of the row
+  * ''*'': zero or more repetitions
+  * ''+'': one or more repetitions
+===== The tar command =====
+<code bash>
+tar [options] tarfile [file1 file2 ...] [dir1 dir2 ...]
+</code>
+The ''tar'' command is used for the compression or decompression of files and directories
+**Compression**:
+  * ''c'': //creation// of a new tar file
+  * ''f'': //file//, must be present both in the compression phase, both in the decompression phase
+  * ''v'': //verbose// mode
+  * ''z'': //zipped//, the file is compressed
+Example:
+<code bash>
+tar cvzf /tmp/file.tgz /home/user/Documents /home/user/a.txt
+</code>
+it //creates// a //verbose//, //zipped// //file//. The Documents directory and the file ''a.txt'' are compressed in the archive ''/tmp/file.tgz''.
+**Decompression**:
+  * ''x'': //extraction// of files from an archive
+  * ''t'': //test// the content of an archive
+Example 1:
+<code bash>
+tar tvzf /tmp/file.tgz
+</code>
+it //tests// a //verbose// //zipped// file, i.e., it tests the archive previously compressed
+Example 2:
+<code bash>
+tar xvzf /tmp/file.tgz
+</code>
+it //extracts// a //verbose//, //zipped// //file//, i.e., it extracts in the current directory the archive previously compressed
+===== The sort command =====
+<code bash>
+sort [-opt] file1 file2 ...
+</code>
+it sorts ''file1'', ''file2'', ...
+Options are:
+  * ''-b'': ignores blanks spaces at the beginning of a line
+  * ''-f'': case insensitive
+  * ''-r'': reverses the result
+  * ''-n'': numeric sort (for numbers)
+===== The bash shell =====
+  * A //shell// is a textual interface between the user and the operating system
+  * The //shell// is not part of the Linux kernel, it is a normal userspace program
+  * Since the //shell// is a normal process, different shells has been developed. The most commons are Bourne shell (''sh''}), C-shell (''csh''), Tahoe C-shell (''tcsh'') and Bourne again shell (''bash'')
+The focus of this document is on ''bash''
+Each shell, when opened, search a configuration script in the home directory and run it
+  * it can be used to customize the shell behavior or to initialize environment variables
+  * in ''bash'' this file is ''.bashrc'' (it is located in the home directory and can be edited)
+===== Completion and History =====
+  * Using TAB the shell automatically complete file names (using the file present in the actual directory or in the directories listed in the environment variable ''$PATH'').
+  * ↓ or ↑: use arrows to scroll commands previously performed
+  * use the ''history'' command to show the history buffer that lists the last executed commands associating them with a number
+  * ''!n'' executes a command present in the buffer identified with the number ''n''
+  * ''!$'' is the last parameter of the command previously performed
+  * ''!*'' are all the parameter of the command previously performed
+  * ''!string'' perform the last command that begin with ''string''
+===== Pipe =====
+<code bash>
+command1 | command2
+</code>
+A ''pipe'' is a connection between the first command //stdout// and the second command //stdin//
+This linking is performed by the operator ''|''
+Examples:
+  * ''ls | head -n 5''
+  * ''cat file | tail -n 25'' that does the same thing of ''tail -n 25 file''
+  * ''cat file | sort -n -u | head -n 5''
+===== Process Management =====
+  * **Batch execution**: ''comand1; command2'', ''command1'' is executed. When ''command1'' is finished ''command2'' is executed.
+  * **Concurrent execution**: ''command1 \&; command2'', ''command1'' and ''command2'' are executed concurrently
+    * ''command1'' is executed in background (symbol ''&'').
+    * it returns immediately the control to the shell that executes ''command2''.
+Summarizing:
+  * normally a command is run foreground (//fg//)
+  * with the symbol ''&'' after the command it can be run background (//bg//)
+  * A program that run //fg// can be suspended pressing //CTRL-Z//
+  * A suspended program can be restarted foreground using the command ''fg'' or background using the command ''bg''
+In the following figure the process states and the transitions between them are illustrated:
+{{:cs:process_graph.png?direct&350}}
+===== Scripts - Shell commands in a File =====
+A sequence of commands can be written in a file and they can be executed directly calling it
+It can be executed indirectly:
+<code bash>
+source <sciptfile> <args>
+</code>
+It can be executed directly:
+  * the script file must have the //execution// permissions
+  * the first row must begin with ''#!'' and with the absolute path of the shell used (example: ''\#!/bin/tcsh'')
+  * to run the script type ''./scriptfile'' in the shell prompt
+===== Shell Variables =====
+<code bash>
+variableName=value
+</code>
+  * instantiates a variable with the name ''variableName'', setting it to ''value''
+  * e.g., ''a=10'', ''b="pippo"''
+<code bash>
+$variableName
+</code>
+  * used to access the variable
+  * to print a variable the command ''echo $variableName'' can be used
+  * to print the value of variable ''b'' for example:
+<code bash>
+user@pc:~$ echo $b
+pippo
+</code>
+===== Shell Environment Variables =====
+  * Environment variables are written in upper-case and are used for the shell configuration
+  * ''env'': displays shell environment variables
+  * ''printenv var'': prints the value of ''var'' shell environment variables
+  * ''export var'': makes the variable ''var'' available to all the processes launched with the shell
+===== Useful Environment Variables =====
+  * ''HOME'': the home directory
+  * ''PATH'': directory where the shell finds shell commands
+  * ''SHELL'': the used shell
+  * ''LD_LIBRARY_PATH'': it shows the directories where shared libraries are located
+Example: add to the environment variable ''PATH'' the directory ''/home/user/bin'':
+<code bash>
+export PATH=\$PATH:/home/user/bin
+</code>
+====== References ======
+  - Slides form which this page is derived: {{:cs:bash_shell_intro.pdf|}} (Screen version), {{:cs:bash_shell_intro_printable.pdf|}} (Printable version)
+  - Some examples regarding the //bash// shell scripting language: [[cs:Bash language]]
+  - List of resources regarding the Linux operating system: [[cs:linux operating system]]

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