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The Shell

• The shell is a program that reads commands and interprets them.
• Shell commands interact with the file system.

Shell Commands

cat: Prints out the content of a file or combine multiple files to standard output

• cat file1.txt prints out file1.txt
• cat file1.txt file2.txt combines both files
• cat/cat - takes in keyboard input and outputs it (ctrl-D ends)
• cat file1.txt - file2.txt takes in file1.txt, then keyboard input (end with ctrl-D), then file2.txt
• man cat provides info about cat command

head: Displays first few (10) lines of a file
tail: Displays last few (10) lines of a file
-n[# of lines]/-[# of lines]: specify number of lines

ls: lists the contents of the current directory
ls -a: lists all contents including hidden files (files that start with '.')
ls -l: lists the contents of a directory in a long listing format (ls -l [filename] displays info for a file in long listing format)

pwd: shows the current working directory

cd: change directory

• cd/cd ~ home directory
• cd [directory] specified directory
• cd .. parent directory
• cd / root directory
• cd - previous directory
• Absolute directories begin with /
• Relative directories begin with
  • ./ (current)
  • ../ (parent)
  • ~/ (home)

exit: terminates the shell session
echo: write arguments, separated by spaces and terminated with a newline
rm: remove
rm -f: force remove
less: loads a text file one page at a time rather than all at once
wc: displays the number of words, bytes, and lines in a file
wc -w: word count
wc -l: line count
sort: sort lines of text file (alphabetical)

• Lines starting with a number will appear before lines starting with a letter.
• Sorted in increasing alphabetical order based on first character
• Lines starting with a lowercase letter will appear before lines starting with the same letter in uppercase.
• sort -r: sorts in reverse order

Standard File Streams

There are 3 standard file streams:

1. stdin with descriptor value of 0 (e.g. keyboard)
2. stdout with descriptor value of 1 (e.g. terminal)
3. stderr with descriptor value of 2 (e.g. log file)

Input Redirection: stdin is often supplied by directing an input to come from a file or previous output command using Pipe.

• doCommand < input_file
• e.g. cat < file1.txt redirects keyboard input from file1.txt. Note that this is different from cat file1.txt which opens the file and prints out content

Output Redirection: stdout is often redirected to a file, stderr is usually printed to terminal but sometimes redirected to a log file

• doCommand > output_file for stdout
• doCommand 2> error_file for stderr
• doCommand > output_file 2>&1 redirects the error messages to the same location where output is (or in bash: doCommand >% output_file)
• doCommand >> output_file appends to start of file
• e.g. cat > output_file.txt saves content of keyboard input to file
• e.g. cat file1.txt file2.txt combines file1 and file2 to new file

Wildcards

• used for pattern matching and searching for files names with specific characters
• ? replaces a single character or digit
• * replaces a string of characters of digits
• e.g. cat file?.txt shows the content of all files that start with the word 'file' then a single character or digit of type .txt
• e.g. ls *file*.txt list all files that has the word 'file' anywhere in its name

Pipes

• can pipe the output of certain commands into the input of another command (i.e. chains commands)
• commandA | commandB | commandC
• head -n11 file3.txt | tail -n2 displays lines 10 and 11 of file3.txt

Users and Groups

Each user is assigned a userid

who: lists the currently logged-in users to the system
whoami: lists current user

File Permissions

Files have three types of permissions:

1. read (r)
2. write (w)
3. execute (x)

There are 4 main types of users:

1. user (u)
2. group (g)
3. others (o)
4. all (a)

File permissions are ordered by user, group, others. For example,
drwxr-xr-x means:

• d: directory
• rwx: user has rwx access
• r-x: group has r and x access
• r-x: others have r and x access

chmod: change permissions of a file

• + adds permission
• - removes permission
• = sets permission
• e.g. chmod u-r file removes read permission from user for file
• e.g. chmod o+x file adds execute permission to others
• e.g. chmod a=rwx file gives everybody full control

which: search where an application resides (e.g. which ls searches where the ls command resides)
whereis: search broader range of directories and locate the source and man packages of the application
tree: displays tree view of file system (e.g. tree cs246 displays tree view of cs246 folder)
tac: start from last line of a file and displays its contents backwards (e.g. tac file1.txt file2.txt file3.txt displays the contents of each file backwards, in order)
touch: update the file timestamp to match the current time

• can be used to update the access, change and modify time
• if file doesnt exist, it creates an empty file
• touch -t sets the time
• Create a file called tomato and set the time stamp of tomato to June 6, 2006, 6:07

  touch tomato
  touch -t 0606060606 tomato
  touch tomato

stat: displays the detailed status of a particular file or file system
mkdir: creates new directory
rmdir: removes empty directory
mv: move or/and rename a file (e.g. mv alice.txt cs246/alicewonderland.txt moves alice.txt into cs246 and rename to alicewonderland.txt)
sudo: runs a program by the use of security privilege of another user often the root or super user
locate: search through a previously created database of files and directory to find a match of your entry

Move alice.txt into cs246 and rename to alicewonderland.txt. Update the directory to reflect changes and locate the new location of alicewonderland

mv alice.txt cs246/alicewonderland.txt
updatedb
locate alicewonderland

awk: extract and then print specific content of the file to generate reports

e.g. output the whole line

awk '{ print $0 }' fileName.txt

cal: displays the calendar
find: searches for file

• find [dir] -name file_name
• e.g. find / -name ubunto > ubuntofiles.txt 2> logfile.txt less ubuntofiles.txt less logfile.txt finds file named ubunto in the root dir and save results into a file, save errors into log file and display the first page of the two new files
• -delete removes all files named file_name that are in the specified directory
• -exec [command] executes command on specified files. All following arguments are args to command until a ;. {} is the variable for each file
• e.g. find / -name "*.txt" -exec rm {} \; searches all txt files and deletes them.
• -size [size] searches for all files of a specific size.
• b: 512-byte blocks (default if no suffix is used)
• c: bytes
• w: two-byte words
• k: Kilobytes (units of 1024 bytes)
• M: Megabytes (units of 1048576 bytes)
• G: Gigabytes (units of 1073741824 bytes)
• +: bigger than specified size
• -: smaller than specified size
• e.g. find / -size +100M: searches for all files bigger than 100MB

Difference between find and locate:

1. locate is faster than find
2. find has more functionality (can select files more than just name)
3. find doesn't necessarily search the entire filesystem, points to a subdirectory
4. find can perform operations to files it finds (exec, delete, size)
5. locate's output can be outdated (needs to be updated by updatedb)

uniq: removes repeated, adjacent lines in a fil
e.g.

• sort file3.txt file4.txt | uniq sorts file3 and file4 and removes duplicates
• sort file3.txt file4.txt | uniq | wc -l counts number of lines in unique combination of file3 and file4

grep: searches for a pattern in a file name and prints all matching lines

• grep [pattern] directory
• -v print line NOT matched by the pattern
• -i case-insensitive
• -egrep/grep -E uses extended regular expressions, such as +, ?, |, (, )
• e.g. egrep "(T|t)he" 1001nights.txt | wc -l counts the number of lines where the word 'the' or 'The' appeared in 1001night.txt

shutdown: halts the machine

• -h schedules shutdown at current time
• e.g. shutdown -h 10:00 schedules shut down at 10am
• -r restarts machine

Regular Expressions (RegEx)

• . matches any single character
• | matches either
• $ matches end of string
• ^ matches beginning of string
• * matches preceding item 0 or more times
• + matches preceding item 1 or more times
• ? matches preceding item 0 or 1 times
• {n} matches preceding item exactly n occurences
• [...] matches any character in set
• e.g. (W|w)onderland matches Wonderland or wonderland
• [C|c][S|s][0-9]{3} matches any CS course