Linux Bible. Christopher Negus
. w q resendit
With these lines added to a script run by the root user, the ed text editor adds the IP address of a DNS server to the /etc/resolv.conf file.
Using brace expansion characters
By using curly braces ({}), you can expand out a set of characters across filenames, directory names, or other arguments to which you give commands. For example, if you want to create a set of files such as memo1 through memo5, you can do that as follows:
$ touch memo{1,2,3,4,5} $ ls memo1 memo2 memo3 memo4 memo5
The items that are expanded don't have to be numbers or even single digits. For example, you could use ranges of numbers or digits. You could also use any string of characters, as long as you separate them with commas. Here are some examples:
$ touch {John,Bill,Sally}-{Breakfast,Lunch,Dinner} $ ls Bill-Breakfast Bill-Lunch John-Dinner Sally-Breakfast Sally-Lunch Bill-Dinner John-Breakfast John-Lunch Sally-Dinner $ rm -f {John,Bill,Sally}-{Breakfast,Lunch,Dinner} $ touch {a..f}{1..5} $ ls a1 a3 a5 b2 b4 c1 c3 c5 d2 d4 e1 e3 e5 f2 f4 a2 a4 b1 b3 b5 c2 c4 d1 d3 d5 e2 e4 f1 f3 f5
In the first example, the use of two sets of braces means John, Bill, and Sally each have filenames associated with Breakfast, Lunch, and Dinner. If I had made a mistake, I could easily recall the command and change touch to rm -f to delete all of the files. In the next example, the use of two dots between letters a and f and numbers 1 and 5 specifies the ranges to be used. Note the files that were created from those few characters.
Listing Files and Directories
The ls command is the most common command used to list information about files and directories. Many options available with the ls command allow you to gather different sets of files and directories as well as to view different kinds of information about them.
By default, when you type the ls command, the output shows you all non-hidden files and directories contained in the current directory. When you type ls, however, many Linux systems (including Fedora and RHEL) assign an alias ls to add options. To see if ls is aliased, enter the following:
$ alias ls alias ls='ls --color=auto'
The --color=auto option causes different types of files and directories to be displayed in different colors. So, return to the $HOME/test directory created earlier in the chapter, add a couple of different types of files, and then see what they look like with the ls command.
$ cd $HOME/test $ touch scriptx.sh apple $ chmod 755 scriptx.sh $ mkdir Stuff $ ln -s apple pointer_to_apple $ ls apple pointer_to_apple scriptx.sh Stuff
Although you can't see it in the preceding code example, the directory Stuff shows up in blue, pointer_to_apple (a symbolic link) appears as aqua, and scriptx.sh (which is an executable file) appears in green. All other regular files show up in black. Typing ls -l to see a long listing of those files can make these different types of files clearer still:
$ ls -l total 4 -rw-rw-r--. 1 joe joe 0 Dec 18 13:38 apple lrwxrwxrwx. 1 joe joe 5 Dec 18 13:46 pointer_to_apple -> apple -rwxr-xr-x. 1 joe joe 0 Dec 18 13:37 scriptx.sh drwxrwxr-x. 2 joe joe 4096 Dec 18 13:38 Stuff
As you look at the long listing, notice that the first character of each line shows the type of file. A hyphen (-) indicates a regular file, d indicates a directory, and l (lowercase L) indicates a symbolic link. An executable file (a script or binary file that runs as a command) has execute bits turned on (x). See more on execute bits in the upcoming section “Understanding File Permissions and Ownership.”
You should become familiar with the contents of your home directory next. Use the -l and -a options to ls.
$ ls -la /home/joe total 158 drwxrwxrwx 2 joe sales 4096 May 12 13:55 . drwxr-xr-x 3 root root 4096 May 10 01:49 .. -rw------- 1 joe sales 2204 May 18 21:30 .bash_history -rw-r--r-- 1 joe sales 24 May 10 01:50 .bash_logout -rw-r--r-- 1 joe sales 230 May 10 01:50 .bash_profile -rw-r--r-- 1 joe sales 124 May 10 01:50 .bashrc drw-r--r-- 1 joe sales 4096 May 10 01:50 .kde -rw-rw-r-- 1 joe sales 149872 May 11 22:49 letter ^ ^ ^ ^ ^ ^ ^ col 1 col 2 col 3 col 4 col 5 col 6 col 7
Displaying a long list (-l option) of the contents of your home directory shows you more about file sizes and directories. The total line shows the total amount of disk space used by the files in the list (158 kilobytes in this example). Adding the all files option (-a) displays files that begin with a dot (.). Directories such as the current directory (.) and the parent directory (..)—the directory above the current directory—are noted as directories by the letter d at the beginning of each entry. Each directory begins with a d and each file begins with a dash (-).
The file and directory names are shown in column 7. In this example, a dot (.) represents /home/joe and two dots (..) represent /home—the parent directory of /joe. Most of the files in this example are dot (.) files that are used to store GUI properties (.kde directory) or shell properties (.bash files). The only non-dot file in this list is the one named letter. Column 3 shows the directory or file owner. The /home directory is owned by root, and everything else is owned by the user joe, who belongs to the sales group (groups are listed in column 4).
In addition to the d or -, column 1 on each line contains the permissions set for that file or directory. Other information in the listing includes the number of hard links to the item (column 2), the size of each file in bytes (column 5), and the date and time each file was most recently modified (column 6).
Here are a few other facts about file and directory listings:
The number of characters shown for a directory (4096 bytes in these examples) reflects the size of the file containing information about the directory. Although this number can grow above 4096 bytes for a directory that contains lots of files, this number doesn't reflect the size of files contained in that directory.
The format of the time and date column can vary. Instead of displaying “May 12,” the date might be displayed as “2019-05-12,” depending upon the distribution and the language setting (LANG variable).
On occasion, instead of seeing the execute bit (x) set on an executable file, you may see an s in that spot instead. With an s appearing within either the owner (-rwsr-xr-x) or group (-rwxr-sr-x) permissions, or both (-rwsr-sr-x), the application can be run by any user, but ownership of the running process is assigned to the application's user/group instead of that of the user launching the command. This is referred to as a set UID or set GID program, respectively. For example, the mount command has permissions set as -rwsr-xr-x. This allows any user to run mount to list mounted filesystems (although you still have to be root to use mount to actually mount filesystems from the command line, in most cases).
If a t appears at the end of a directory, it indicates that the sticky bit is set for that directory (for example, drwxrwxr-t). By setting the sticky bit on a directory, the directory's owner can allow other users and groups to add files to the directory but prevent users from deleting each other's files in that directory. With a set GID assigned to a directory, any files created in that directory are assigned the same group as the directory's group. (If you see a capital S or T instead of the execute bits on a directory, it means that the set GID or sticky bit permission, respectively, was set, but for some reason the execute bit was not also turned on.)