Unix has several commands for various purposes. They can be slightly modified using “options”.
An example of a command is ls which lists all the files in a particular directory (not mentioning any directory lists all the files in the current directory). This can be modified as
ls -ltr Documents
which lists all the contents of the Documents directory in the long format (l), sorted by time (t), and in reverse order (r).
Each of l, t, and r is an option. We can also write these separately or in a verbose function (like ls -l -t --reverse Documents).
If we want to find information about a particular command, we use the man command (Example: man ls).
You can interact by two means:
In a directory, we use . to represent the directory itself and .. to represent the parent directory.
We can set up shortcuts as alias <shortcut>='<desired command>'.
mkdir: Makes a directory with the given name.
Usage: mkdir dirname creates a directory with name dirname.
cat: Concatenate files and display result.
Usage: cat file1 file2 appends file2 to file1. If we only use one file, it will just display the file. If we want to write the concatenation into a new file, we can use cat file1 file2 > file to “redirect” the output (that would go to the console) to file.
cp: Copies from source file/folder(s) to destination file/folder(s).
Usage: cp src file copies the content of src to file. If we use something like cp src1 src2 folder, src1 and src2 are copied into folder. To copy a directory (and recursively all the directories inside it), we need to use something like cp -rR srcdir folder.
mv: Similar to cp except that the original file/folder is removed.(mv means “move”)
Usage: mv src file.
tree: Prints the folder structure in a very pretty organized tree-like structure.
Usage: tree dirname.
ls: Discussed earlier. See the permissions section.
chmod: If as an owner, I want to change the permissions, I can use this command.
Usage: Say I want to set the permissions of a directory dirname to rwxrw-r-x. The method to do this is to split this into three triads (rwx, rw-, and r-x), convert each one to a binary representation depending on whether or not it is set (111, 110, and 101), and convert each number to decimal form (7, 6, and 5). Then the resulting command I must use is chmod 765 dirname. (Experiment with this!)
Let us consider the long format of ls in particular.
Consider the long format displayed as
drwxr-xrw- 4 amit amit 4096 Aug 16 10:27 'CS251'
The data displayed is the permissions, number of links, owner, group, size, date, and name in that order.
Permission: The first triad is what the owner can do. The second triad is what the group members can do. The third triad is what the third triad can do.
If we are considering a file, then r means readable, w means writable, and x means executable. The first character represents what the thing under consideration is (f for file, d for directory, l for link, etc.)
On the other hand, if we are considering a directory, r means that you can list the files in the directory, w means that you can add/delete the files in the directory, and Execution of a directory just means that you can step into a directory. Note that the w bit is meaningless if the x bit is not set.
So in the example given, CS251 is a directory. The owner, amit can list and add/delete the files in it. Members of the group amit can list the files in the directory and step into it, but cannot add/delete files. Note that they may be able to edit individual files in the directory depending on the individual permissions, but they cannot add/delete them. Other users can only list the files in the directory (Despite the w bit being set, they cannot add/delete files since the x bit isn’t set).
team, I can change the group from amit to team and give other people the permission to do whatever.sA directory is just a table of pairs: the name of the file/directory and a pointer. The pointer points to something called an inode, which just has all the information displayed when we use ls -l: permissions, owner/group, link, timestamp, size, and finally a pointer to where the file content is actually stored.
The size of a directory is the size of the table, as a result of which it is either 4096 or a multiple of it. When allocating memory for the table, it is done so in chunks of 4096.
ln: Sometimes, we create shortcuts to a file (on our Desktop for example). To do this, we use the ln command. This can be implemented in two ways:
inode. A symbolic link to a.txt can be created using ln -s slink a.txt.inode of the original file. A hard link to a.txt can be created using ln hlink a.txt.An important distinction arises when we delete the original file (which removes its name and pointer from the table) but not the shortcut. The soft link will break, since the entry will have a dangling pointer. However, the hard link will continue to work. This is because the inode and the file content is still there and undeleted.
Look at links in particular after doing ls -l.
cmp: This is used to compare two files byte by byte.
Usage: cmp file1 file2
diff: This is used to compare two files line by line.
Usage: diff file1.txt file2.txt. Compares the two files and indicates changes to be made to file1.txt to make it identical to file2.txt. In the output, a is “add”, c is “change”, and d is “delete”. A < indicates a line in the first file and a > indicates a line in the second file.
find: Search the system for file(s) with particular properties and take possible actions.find <starting point> <expression>. For example, if we want to search for a file named notes1.md within the directory /Documents, we write find /Documents -name notes1.md. Some possible things to include in the expression are
-name: Name of the file. If we want a particular type of file, say .txt , we can search for *.txt.-type: Type of the file (d for directory, f for file).-perm: Permissions of the file (in a format similar to that used in chmod such as 765)-size: Find files of a particular size. We use c to mean bytes. So if we want a file with size greater than 100 bytes, we write -size +100c. If we want say between 50 and 100 bytes, we can write -size +50c -size -100c.-exec: Action to be taken on all the file(s) returned.So if we want to list in long format all .txt files in the current directory that are at least 100 bytes, we write
find . -name '*.txt' -size +100c -exec ls -al {} \;
egrep: This is equivalent to grep -E. egrep stands for “Extended Global Regular Expression Print”.egrep <regex> <filename>. Prints every line in filename containing the regular expression regex.
egrep wurd grep.txt prints all lines in grep.txt that contains the string wurd.egrep 'a b' grep.txt prints all lines that contains the string a b. We use the quotations because there is a space in the string.egrep 'hi|hey' grep.txt prints all lines that contain either the string hi or hey.egrep '(Th)?is' grep.txt prints all lines that contain is or This.egrep '[a-z]{2,5}' grep.txt prints all lines that contain strings made of characters from a to z of length between 2 and 5 (inclusive). If we want only length 5, we can use {5} instead of {2,5}.egrep -v <expression> grep.txt prints all lines that don’t contain strings satisfying <expression>.ls -al | egrep '^d.*' finds directories. ^d means that it begins with d (We can similarly use $ to mean that it ends there). The output of ls -al is “piped” to the egrep by the |. Piping is often used to give the output of one command as input to another.cut: Cuts each line into several “fields” using a given delimiter. For example, if we want to cut up each line of a .csv file into several fields (separated by ,) and output the third field in each line, we can use
cut -d "," -f 3 csvfile.csv
If we want everything but the third field, we can add the --complement option.