Red Hat System Administration I

The date command displays the current date and time. The superuser or a privileged user can also use the date command to set the system clock. Use the plus sign (+) as an argument to specify a format string for the date command.

[user@host ~]$ date
Sun Feb 27 08:32:42 PM EST 2022
[user@host ~]$ date +%R
20:33
[user@host ~]$ date +%x
02/27/2022

The passwd command with no options changes the current user's password. To change the password, first specify the original password for the account. By default, the passwd command is configured to require a strong password, to consist of lowercase letters, uppercase letters, numbers, and symbols, and not to be based on a dictionary word. A superuser or privileged user can use the passwd command to change another user's password.

[user@host ~]$ passwd
Changing password for user user.
Current password: old_password
New password: new_password
Retype new password: new_password
passwd: all authentication tokens updated successfully.

Linux does not require file name extensions to classify files by type. The file command scans the compiled header of a file for a 2-digit magic number and displays its type. Text files are recognized because they are not compiled.

[user@host ~]$ file /etc/passwd
/etc/passwd: ASCII text
[user@host ~]$ file /bin/passwd
/bin/passwd: setuid ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=a467cb9c8fa7306d41b96a820b0178f3a9c66055, for GNU/Linux 3.2.0, stripped
[user@host ~]$ file /home
/home: directory

The cat command is often used in Linux. Use this command to create single or multiple files, view the contents of files, concatenate the contents from various files, and redirect contents of the file to a terminal or to files.

The following example shows how to view the contents of the /etc/passwd file:

[user@host ~]$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
...output omitted...

To display the contents of multiple files, add the file names to the cat command as arguments:

[user@host ~]$ cat file1 file2
Hello World!!
Introduction to Linux commands.

Some files are long and might need more space to be displayed than the terminal provides. The cat command does not display the contents of a file as pages. The less command displays one page of a file at a time and you can scroll at your leisure.

Use the less command to page forward and backward through longer files than can fit on one terminal window. Use the UpArrow key and the DownArrow key to scroll up and down. Press q to exit the command.

The head and tail commands display the beginning and the end of a file, respectively. By default, these commands display 10 lines of the file, but they both have a -n option to specify a different number of lines.

[user@host ~]$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
[user@host ~]$ tail -n 3 /etc/passwd
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
gnome-initial-setup:x:980:978::/run/gnome-initial-setup/:/sbin/nologin
dnsmasq:x:979:977:Dnsmasq DHCP and DNS server:/var/lib/dnsmasq:/sbin/nologin

The wc command counts lines, words, and characters in a file. Use the -l, -w, or -c options to display only the given number of lines, words, or characters, respectively.

[user@host ~]$ wc /etc/passwd
41   98 2338 /etc/passwd
[user@host ~]$ wc -l /etc/passwd ; wc -l /etc/group
41 /etc/passwd
63 /etc/group
[user@host ~]$ wc -c /etc/group /etc/hosts
883 /etc/group
114 /etc/hosts
997 total

With tab completion, users can quickly complete commands or file names after typing enough at the prompt to make it unique. If the typed characters are not unique, then pressing the Tab key twice displays all commands that begin with the typed characters.

[user@host ~]$ pasTab+Tab
passwd       paste        pasuspender
[user@host ~]$ passTab 
[user@host ~]$ passwd
Changing password for user user.
Current password:

Tab completion helps to complete file names when typing them as arguments to commands. Press Tab to complete as much of the file name as possible. Pressing Tab a second time causes the shell to list all files that the current pattern matches. Type additional characters until the name is unique, and then use tab completion to complete the command.

[user@host ~]$ ls /etc/pasTab 
[user@host ~]$ ls /etc/passwdTab 
passwd   passwd-

Use the useradd command to create users on the system. The useradd command has many options that might be hard to remember. By using tab completion, you can complete the option name with minimal typing.

[root@host ~]# useradd --Tab+Tab 
--badnames              --gid                   --no-log-init           --shell
--base-dir              --groups                --non-unique            --skel
--btrfs-subvolume-home  --help                  --no-user-group         --system
--comment               --home-dir              --password              --uid
--create-home           --inactive              --prefix                --user-group
--defaults              --key                   --root
--expiredate            --no-create-home        --selinux-user

Commands with many options and arguments can quickly become long and are automatically wrapped by the command window when the cursor reaches the right margin. Instead, type a long command by using more than one line for easier reading.

To write one command in more than one line, use a backslash character (\), which is referred to as the escape character. The backslash character ignores the meaning of the following character.

Previously, you learned that to complete a command entry, you press the Enter key, the newline character. By escaping the newline character, the shell moves to a new command line without executing the command. This way, the shell acknowledges the request by displaying a continuation prompt on an empty new line, which is known as the secondary prompt, and uses the greater-than character (>) by default. Commands can continue over many lines.

One issue with the secondary prompt's use of the greater-than character (>) is that new learners might mistakenly insert it as part of the typed command. Then, the shell interprets a typed greater-than character as output redirection, which the user did not intend. Output redirection is discussed in an upcoming chapter. This course book does not show secondary prompts in screen outputs, to avoid confusion. A user still sees the secondary prompt in their shell window, but the course material intentionally displays only the characters to be typed, as demonstrated in the following example.

[user@host ~]$ head -n 3 \
/usr/share/dict/words \
/usr/share/dict/linux.words
==> /usr/share/dict/words <==
1080
10-point
10th

==> /usr/share/dict/linux.words <==
1080
10-point
10th

The history command displays a list of previously executed commands that are prefixed with a command number.

The exclamation point character (!) is a metacharacter to expand previous commands without retyping them. The !number command expands to the command that matches the specified number. The !string command expands to the most recent command that begins with the specified string.

[user@host ~]$ history
...output omitted...
   23  clear
   24  who
   25  pwd
   26  ls /etc
   27  uptime
   28  ls -l
   29  date
   30  history
[user@host ~]$ !ls
ls -l
total 0
drwxr-xr-x. 2 student student 6 Feb 27 19:24 Desktop
...output omitted...
[user@host ~]$ !26
ls /etc
abrt                     hosts                     pulse
adjtime                  hosts.allow               purple
aliases                  hosts.deny                qemu-ga
...output omitted...

The arrow keys help to navigate through previous commands in the shell's history. The UpArrow edits the previous command in the history list. The DownArrow edits the next command in the history list. The LeftArrow and RightArrow move the cursor left and right in the current command from the history list so that you can edit the command before running it.

Use either the Esc+. or Alt+. key combination simultaneously to insert the last word of the previous command at the cursor's current location. The repeated use of the key combination replaces that text with the last word of earlier commands in history. The Alt+. key combination is particularly convenient, because you can hold down Alt and press . repeatedly to quickly cycle earlier commands.

When used interactively, bash has a command-line editing feature. Use the text editor commands to move around and modify the currently typed command. Using the arrow keys to move within the current command and to step through the command history was introduced earlier in this section. The following table shows further powerful editing commands.

These command-line editing commands are the most helpful for new users. For other commands, refer to the bash(1) man page.