Linux Substitution Characters

• Coursework covers Unix and Linux operating systems, Linux basics, file and disk management, text editors, shell scripting, system administration, process and system monitoring, networking/ security, and Linux environment configuration.

Substitution Characters

• Substitution characters, including wildcards and special characters, streamline command execution, expand filenames, and enable pattern matching in Linux.
• The * character matches zero or more characters.
  • ls *.txt lists all .txt files.
  • ls *.* lists all files with extensions.
  • ls * lists directory content and is equivalent to ls.
• The ? character matches exactly one character.
  • ls file?.txt matches file1.txt or fileA.txt.
• The [] matches one character from a set.
  • ls file[123].txt matches file1.txt, file2.txt, and file3.txt.
• The {} matches a list of values.
  • cp {file1,file2}.txt backup/ copies file1.txt and file2.txt to the backup directory.
• The {start..end} generates a numeric or alphabetic sequence.
  • echo {1..5} outputs 1 2 3 4 5.
• The {start..end..step} generates a sequence with a step value.
  • echo {1..10..2} outputs 1 3 5 7 9.

Creating and Modifying Timestamps

• The touch command creates files and updates timestamps in Linux.
• If a file doesn't exist, touch creates an empty file. If it exists, it updates the access time (atime) and modification time (mtime).
• touch newfile.txt creates an empty file.
• touch existingfile.txt updates timestamps without modifying content.
• The -c option with touch prevents file creation if it doesn't exist.
  • touch -c file.txt
• The -a option with touch updates only the access time.
• The -m option with touch updates only the modification time.
• The -t option with touch sets a custom timestamp.
  • touch -t 202401011200 file.txt sets the timestamp to January 1, 2024, 12:00 PM.
• Combine -a or -m with -t to change access or modification times, respectively.
• The -r option with touch copies timestamps from another file.
  • touch -r file1.txt file2.txt makes file2.txt have the same timestamps as file1.txt.
• Substitution characters generate multiple files with touch.
  • touch file{1..3}.txt creates file1.txt, file2.txt, and file3.txt.
  • touch file{1,2}.{txt,csv} creates file1.txt, file1.csv, file2.txt, and file2.csv.
  • touch file{A..C}{1..3}.txt creates fileA1.txt, fileA2.txt, fileA3.txt, fileB1.txt, fileB2.txt, fileB3.txt, fileC1.txt, fileC2.txt, and fileC3.txt.

Searching for Files

• The which command locates executables by searching directories listed in the PATH environment variable.
• The whereis command finds all files related to an executable, including source code and documentation, by searching in standard paths.
• which finds executables, searches the $PATH, and shows only the first match
• whereis finds source code and man pages, searches standard paths, and shows all matches.

Searching for Files with find

• The find command searches files and directories recursively based on specified criteria.
• By default, find searches the current directory and its subdirectories.
• Searches in another directory are achieved by following find with the directory path.
• Searches by name: find /path/to/search -name "filename".
  • find ~/ -name "myfile.txt" searches for myfile.txt in the home directory and its subdirectories.
• Searches by name ignoring case: find /path/to/search -iname "filename".
  • find ~/ -iname "myfile.txt" searches for myfile.txt (case-insensitive) in the home directory and its subdirectories.
• Searches by extension: find /path/to/search -name "*.txt".
  • sudo find /var/log -name "*.log" finds all .log files in /var/log and its subdirectories.
• Searches by file size: find /path/to/search -size +100M.
  • sudo find / -size +100M finds files larger than 100 MB in the current filesystem.
• Size units include c for bytes, k for kilobytes, M for megabytes, and G for gigabytes.
• Omitting the + searches for the exact size specified.
• Searches by modification time: find /path/to/search -mtime -n.
  • find ~/ -mtime -7 finds files modified in the last 7 days.
  • find ~/ -mtime 7 finds files modified exactly 7 days ago.
  • find ~/ -mtime +7 finds files modified over 7 days ago.
• Searches by access time: find /path/to/search -atime -n.
  • find ~/ -atime -7 finds files accessed in the last 7 days.
  • find ~/ -atime 7 finds files accessed exactly 7 days ago.
  • find ~/ -atime +7 finds files accessed over 7 days ago.
• Combining access and modification time: find /var -mtime -7 -atime +3 finds files in /var modified in the last week but not accessed in the last 3 days.
• -ctime searches for files whose metadata has been changed.
• -mmin and -amin search for files by modification and access times in minutes instead of days.
• -mtime checks when the file content was last modified.
  • find /home -mtime -7 lists files last modified in the last 7 days in the /home directory.
• -atime checks when the file was last accessed (read or opened).
  • find /home -atime -3 lists files last accessed in the last 3 days in the /home directory.
• -mmin checks if the file was modified in the last X minutes.
  • find /var -mmin -60 will find files that were modified in the lest hour
• -amin checks if the file was accessed in the last X minutes.
  • find /tmp -amin -30 will find files accesses in the ladt 30 minutes
• find -path pattern searches for files whose full name matches a specified pattern.
• find -type t finds files of a specific type, such as d for directory, f for regular file, and l for symbolic link.
• find -user user_name searches for files owned by a specified user.
• find -group grp searches for files belonging to a specified group.
• Finding all directories contained in /etc and sorting them alphabetically.
  • find /etc -type d | sort
• Finding all files in /home belonging to the users group whose metadata was modified in the last 5 days.
  • find /home -type f -group users -ctime -5
• Finding all files in / of a size greater than 10M.
  • sudo find / -type f -size +10M
• The -exec option runs a command on each file found (e.g., find /path -name "filename" -exec command {} \;).
  • The {} acts as a placeholder for the found file
• The -ok action is similar to -exec, but asks for confirmation before execution.
• The command, find ~/Documents -name "*.txt" -exec rm {} \;, removes all .txt files from the Documents directory.
• The find ~/Documents -name "*.sh" -exec chmod +x {} \;, will make all shell scripts executable.
• xargs takes input from find and runs commands while reducing process creation.
• Finding the file name using xargs find /path -name "filename" | xargs command
• Removing the file name using xargs find ~/Documents -name "*.txt" | xargs rm
• Display all files contained in / of a size greater than 10 Mo on screen at a time
  • sudo find / -type f -size +10M | xargs ls -l | less

Searching with Locate

• locate pattern searches for files by matching the full filename against the specified pattern.
  • The command locate *.py will search for all files with the .py extension.
• The locate command accesses a database containing all filenames on the system, updated regularly, instead of browsing the filesystem directly.
• A manual database update can be launched with sudo updatedb.

Searching for Patterns with grep

• grep scans a text file for a pattern and returns lines containing that pattern.
• grep pattern myfile returns all lines from myfile that contain the searched pattern.
• -v option in grep excludes lines that contain the specified pattern.
• -c option returns the number of lines that contain the search pattern.
• -l option returns the names of files containing the pattern.
• -i option ignores case distinctions.
• -w option matches only whole words.
• -n option shows line numbers containing the pattern.
• -r option searches files in all subdirectories.
• The command grep 'Hello' *.txt searches for "Hello" in all .txt files in the current directory, displaying matching lines with filenames.
  • To search in all subdirectories: grep -r 'Hello' *.txt
• The command grep -v student /etc/passwd > npasswd returns all lines from /etc/passwd that do not contain "student" and saves the result to npasswd.
• Combining find and grep enables effective searches:
  • -q option in grep suppresses output.
  • -print action in find shows the full name of matching files, followed by a line feed.
  • The command find -name '*.txt' -size -10k -exec grep -q password {} \; -print displays all files smaller than 10KB in the current directory containing the pattern "password".

Regular Expressions

• A regular expression (Regex) is a set of characters that define a search pattern.
• They are used to find specific lines of text containing a particular pattern in ASCII files, one line at a time and cannot start or end on one line.
• Regular Expressions consist of Anchors, Character Sets and Modifiers.
• Anchors specify the position of the pattern in the line.
• Character Sets match one or more characters in a single position.
• Modifiers indicate how many times the previous character set is repeated.
• The character^ is the starting anchor, and the character$ is the end anchor.
• ^A matches all lines that start with a capital A.
• A$ matches all lines that end with a capital A.
• A^ matches lines that contain 'A^' anywhere in a string
• $A matches lines that contain '$A' anywhere in a string
• ^^ matches lines that start with '^'
• $$ matches lines that end with '$'
• The simplest character set is a character, like in the expression "the" comprised of "t","h", and "e".
• The metacharacter . matches any character except the new line character.
  • ^.$
• Specify the exact characters to search for using Ranges ([...]),
  • ^[0123456789]$
  • ^[0-9]$ matches all lines containing exactly one number
  • [A-Za-z0-9_]matches a single character that is a letter, number, or underscore.
  • ^T[a-z][aeiou] matches any 3 letters strings: a lowercase letter followed by a vowel
• Character sets such as alpha, [[:alpha:]] matches any letter.
• Character sets such as digits, [[:digit:]] matches a digit.
• Character sets such as alphanum, [[:alnum:]] matches a letter or number.
• Character sets such as punctuation, [[:punct:]] matches a punctuation character.
• Character sets such as a blank spaces, [[:blank:]] matches a horizontal space or tab.
• Character sets such as any spaces, [[:space:]] matches all forms of spaces characters.
• Exceptions in a character set, allows you to search for characters except those in square brackets [^aeiou].
• If "]" and "-" immediately follow "[", they don't have special meanings:
  • [] matches the characters "[]".
  • [0] matchs the character "0"
  • [0-9] matches any number
  • [^0-9] matches any character that isn't a number
  • [-0-9] matches a number or "-"
  • [0-9\-] matches a number or "-"
  • [^-0-9] matches a character that isn't a "-" or a number
  • []0-9] matches a number or "]"
  • [0-9]] any number followed by "]"
  • [0-9\\-a\\]] Any number, or a "-", a "a", or a "]"
• ** is used to specify how may times you expect to see the previous character set.
• Repeating characters sets matches with *.
  • '0*' matches none or sets of zero
  • '[0-9]*' means none or a set of numbers.
  • '[0-9][0-9]*' means one or several numbers.
• To matching a specific number of sets:
  • \{n\} matches n repetitions of the previous expression.
  • \{n,\} matches at least n occurrences of the previous expression.
  • \{n,m\} matches between n and m occurrences of the previous expression.
  • \(pattern\)\n matches (n+1) occurrences of the element captured by pattern.
• Examples of modifiers in regular expressions
  • '[a-z]\{4,8\}'matches four to eight lowercase letter.
  • ’\([a-z]\)\1’, matches 2 occurrences of the same lower-case character.
• Modifiers (*, \{, \}) only act as modifiers if they follow a character set; at the beginning of a pattern, they are not modifiers.
• Regular Expression Examples
  • * matches a line with an asterisk
  • \* matches a line with an asterisk
  • \\ matches a line with a backslash.
  • ^* matches a line that starts with an asterisk.
  • ^A* matches any A line
  • ^A\* matches any line stating with "A*"
  • ^AA*\ matches any line if if starts with one "A"
  • ^AA*B matches any line with one or more A's followed by B.
  • ^A\{4,8\}Bmatches any line starting with four to eight A's followed by a B
  • ^A\{4,\}B matches any line starting with 4 four or more A's followed by B -^A\{4\}B matches any line starting with "AAAAB"
  • \{4,8\} matches any line with {4,8}
  • A{4,8} matches any line with A{4,8}

Extended Regular Expressions

• ? matches the preceding expression zero or one time.
• + matches the preceding expression one or more times.
• {n} matches n occurrences of the previous expression.
• {n,} matches at least n occurrences of the previous expression.
• {n,m} matches between n and m occurrences of the previous expression.
• | corresponds to the choice between several expressions.
• (pattern) group to manage operator precedence
• grep uses basic regular and extended expressions.
• grep '..te' myfile returns all lines from myfile that contain two arbitrary characters followed by te.
• grep '[[:alpha:]]te’ myfile returns all lines of myfile that contain letters followed by te.
• grep '^[[:digit:]]' myfile returns all lines of myfile that start with a number.
• grep -v '[[:punct:]]$' myfile returns all lines that do not end with a punctuation mark.
• grep 's[ea]s' myfile returns all lines from myfile which contain the strings sees or seas
• grep '[^t]ion' myfile returns all lines in myfile that containion preceded by anything other than `t.
• grep '([A-Za-z]*)' myfile will return all the lines that come enclosed in parentheses in side the file.
• All lines of myfile that end with 4 digits grep '[0-9]\{4\}$' myfile
• All lines of myfile containing the character string 00,11,22 ect grep '([0-9])\1' myfile
• A Unix Administration command that displays if there are one occurrences of four numbers that end with 4 digits
  • grep -E '[0-9]{4}$' myfile
• If one starts with a number , follows to the lines it uses the following code grep -E '^([[:digit:]]\.)' myfile
• Lines that contains any of the strings with"CCE | GIC" has to use the following function grep -E 'CCE | GIC' myfile
• If it has the strings possible or impossible it uses, grep -E '(im)? possible' myfile

Access Rights

• Files/directories have: owner, group (users with common interests), and access bits (r, w, x).
• Access is separated into three categories: owner, group members, and other system users.
• Read (r), denoted by an 'r', the file/directory can be read
• Write (w), denoted by a 'w', the file/directory can be written to
• Execute (x), denoted by an 'x', the file/directory can be executed
• For files with rw-r--r-- the owner can read and modify while group and other users can only read
• For directories with access bits rwxr-xr-x, all users can list and navigate, but only the owner can modify.
• ls -l shows access bits, owner, and group, the file type indicated in the first column (- for file, d for directory, l for link),
• Next nine characters indicate the access bits to the file or directory
• The second column indicates the number of hard links
• The third column indicates the owner of the file
• The fourth column indicates the group associated with the file

Change Access Rights

• The chmod command alters file and directory access rights:
  • chmod users+rights file adds rights to users on the file.
  • chmod users-rights file revokes rights from users on the file.
  • chmod users=rights file assigns rights to users on the file.
  • users can be u (owner),'g' (group), o (others), or a (all).
  • rights: r for reading, w for writing and x for executing.
  • -R recursively changes permissions in directories and their contents.To allow all users to read 'myfile':
• chmod a+r myfile.
• To restrict the modification of 'myfile' to its owner, chmod og -w myfile.
• The representation of permissions can be represented numerically using octal notation, where each permission has a value
  • r (read) = 4
  • 'w' (write) = 2
  • 'x' (execute) = 1
• If one to describe the directory by numbers in access rights you need to know the list the binary values to represent those, examples:
  • Permissions of rwx you need to specify 111 therefore the octal is 7
  • Permissions of rw- you need to specify 110 therefore the octal is 6
  • Permissions of r-x you need to specify 101 therefore the octal is 5
  • Permissions of r-- you need to specify 100 therefore the octal is 4
  • Permissions of -wx you need to specify 011 therefore the octal is 3
  • Permissions of -w- you need to specify 010 therefore the octal is 2
  • Permissions of --x you need to specify 001 therefore the octal is 1
  • Permissions of --- you need to specify 000 therefore the octal is 0
• Change rights through numbers instead of letters:
  • A number is corresponding to the access rights of the owner.
  • A number that corresponds to the access rights of the groups and a number for the other users.
  • Any numbers that is an addition of the value as the following, 4,2 and 1 for rwx
  • symbolic numeric
  • rwxrwxrwx 777 Full permissions for all
  • rwxr-xr-x 755 Owner full, group/others read & execute
  • rw-r--r-- 644 Owner read/write, group/others read-only
  • rw------- 600 Owner read/write, no access for others
  • rwx------ 700 Owner full, no access for others
• Examples:
  • To add file access for read access using the code is chmod u+r file.txt
    • Therefore the owner can read the file
  • To add write access using the code is chmod u+w file.txt
    • Therefore the owner can write to the file
  • To add execute access using the code is chmod u+x script.sh
    • Therefore the owner can execute the file
  • To remove write access to prevent modification chmod u-w file.txt
    • Therefore the owner can no longer modify the file
  • To determine owner full permissions using the code chmod u=rwx file.txt
    • Therefore the owner gets full accesses.
  • This command restricts all accesses chmod u= file.txt
    • Therefore the owner gets no more accesses
  • u is replaced by g for group, o for others, and a for all users.

Advanced Permission

• To give an owner rwx permissions, a group rx group and other group a r type access: chmod u=rwx,g=rx,o=r file.txt
• It's a Granular permission setting in the script
• To give a permission for owner and the group to execute an instruction chmod ug+x script.sh
  • Owner and group can access the files
• If one needs to remove the write from a group type this code chmod go-w file.txt
  • Group looses any type of access.
• To grant and write an owner or a group a script chmod ug+rw file.txt
  • Owner and group can read and write.
• The chown command changes the owner and group of files and directories:
• chown user1 myfile sets user1 as the owner of myfile.
• chown -R user1 dir1 sets user1 as the owner of directory dir1 and its content.
• chown :grp1 myfile sets group grp1 as the group associated with myfile.
• chown user1:grp1 myfile sets user1 as the owner of myfile and grp1 as the associated group.
• chown -R user1:grp1 dir1 sets user1 as the owner of dir1 and its content, with grp1 as the associated group.
• The chgrp command changes the group association:
• chgrp grp1 myfile associates group grp1 with myfile.
• chgrp -R grp1 dir1 associates group grp1 with dir1 and its content.

Setuid/Setgid Bits

• The setuid and setgid bits grant rights to users without needing privileges.
• The setuid (or suid) bit allows unprivileged users to execute a file with the file owner's privileges:
  • ls -l shows s instead of x in the access bits for the owner.
• This bit is ineffective on directories.
• The setgid bit lets unprivileged users execute a file as members of the associated group.
  • The commandls -l shows an s instead of x in the access bits for the group.
• When applied this can be used to create shareable directories.
• The sticky bit renders modifications for only the owner in the directory , only if this is apply to the directory.
  • User has the right w and x.
  • ls -l displays the letter t has to be set in order of the systems.
  • Prevents users from accidentally or maliciously deleting or modifying each other's files in shared directories.
• This can be commonly seen in the command /tmp can be deleted for the owner but all the users has the permission to write access files
• It does not have any effect on the files
• chmod also enables setuid (u+s), setgid (g+s) and sticky bits (+t).
• setuid, setgid and sticky bits have values of"4000, 2000 and 1000" respectively.
• The command chmod -R 1777 dir1assigns all accesses to dir1 and its contents, but only with "the sticky bit"
• /bin/passwd

Setuid/Setgid Useful Example

• A common example is the passwd command, which enables users to modify their passwords.
  • The command for it is -rwsr-xr-x 1 root root 54256 Jan 10 12:00 /bin/passwd
• backup.sh it belongs to
• user the user is "root".
• The group belongs to backup.
• Just with g+x the command means the members with back up user can exectute it.
• In order to perform operations only the the back the permissions for the the backup can you use you the g+s.

###File Compression

• The gzip command compresses files:
• gzip myfile compresses myfile and generates myfile.gz.
• gunzip myfile.gz or gzip -d myfile.gz decompresses myfile.gz.
• Primarily effective for text files, less so for audio or video.
• The bzip2 command, is another way to compress a file:
• bzip2 myfile compresses myfile into myfile.bz2.
• bunzip2 myfile.bz2 decompresses myfile.bz2.
• The advantage of bzip2 vs gzip is that has a better compressing files but is slower
• The xz commandis another way to compress files.
• xz myfile compresses myfile into myfile.xz.
• unxz myfile.xz decompresses myfile.xz.
• Out of all all these the highest percentage compression ration has xz.
• The tar command archives multiple files, then compresses them using gzip, bzip2, or zx.
  • tar -czf dir.tgz dir groups files from the dir directory into a compressed archive named dir.tgz.
• Then -c option for the tar has been created to archive files.
• The -z option can be used to compress files with gzip.
• When using the command you also have the option with bzip2 to use the command -j.
• The command you also have the option with xz to use the command-J.
• In order to specify, is to the make name with a the same extentions with .tar.gz. or tgz has to be used.
• Show the contents to show less data in order:
  • tar -tzf.
  • tar -xzf.
• The -xzf command is used to unpack the command that has to the the archived format, the command is : tar -xzf dir.tgz`

Windows Compatible File Compression

• Preferred Linux archive format: Tar.
• The zip command to has a compressed archive that is compatible.
  • `zip dir.zip *.html, the code in that html directory can be transfer to the a zip file.
• In order to transfer all data into the zip file to zip -r dir zup dir.

###Zip files

• To archive them in order the zip format needs to be archived.

• The information of the archived file can be displayed and.

• In order to decompressed the archive a code has to be implement that

  • unzip dir is the unzipped version ###Comparing Directories

• Comparing code and to check all directories you need use code

• the command need to be diff.

• It creates the content in order and and there has be need to context after the file different.

  • There are other options to indicate the code, the option has to be.

• An important point the to not to have access point using an code

• -in order to ignore code and all other has white code

  • If they the there is a command to see if they are there or not.

• There are commands:

• The -i option may be used to ignore the differences between upper and lower case

• The -w option ignores white spaces when comparing lines.

• The -q option only indicates if the files differ, not the differences themselves.

• The -u option uses the unified output format.

• The -r option allows, when directories are compared, to compare also all subdirectories recursively.

• The -N option allows, when directories are compared, to consider absent files as present and empty.

• One to reference the commands with the code

• This command diff3 file1 file ref file2

• If those all commands are not implemented implement to the patch codes, the following code in order to

  • run diff Old and new diff old-file new-file > patch-file
  • run the pacht code: patch old-file patch-file

File synchronization

• The Rsync command. Rsync file directory For example
• rsync dir1/*py copy rsync dir1/*py dir
• Then dir2 created
• Not present copies files present
• Not presented directories
• Command to run rsync: rsync -a dir1 dir2 copy the directory and its subdirectories.
• It has all the files that are been altered. Command to give fee:
• -v give code and speed up delivery.
• To prevent the directory for to remote files:
• delete, to copy new files, and code and information
• rsync code and rsync copy code.