Red Hat System Administration I

Objectives

After completing this section, students should be able to save time running commands from a shell prompt using Bash shortcuts.

Basic command syntax

The GNU Bourne-Again Shell (bash) is a program that interprets commands typed in by the user. Each string typed into the shell can have up to three parts: the command, options (that begin with a - or --), and arguments. Each word typed into the shell is separated from each other with spaces. Commands are the names of programs that are installed on the system. Each command has its own options and arguments.

The Enter key is pressed when a user is ready to execute a command. Each command is typed on a separate line and the output from each command displays before the shell displays a prompt. If a user wants to type more than one command on a single line, a semicolon, ;, can be used as a command separator. A semicolon is a member of a class of characters called metacharacters that has special meanings for bash.

Examples of simple commands

The date command is used to display the current date and time. It can also be used by the superuser to set the system clock. An argument that begins with a plus sign (+) specifies a format string for the date command.

[student@desktopX ~]$ date
Sat Apr  5 08:13:50 PDT 2014
[student@desktopX ~]$ date +%R
08:13
[student@desktopX ~]$ date +%x
04/05/2014

The passwd command changes a user's own password. The original password for the account must be specified before a change will be allowed. By default, passwd is configured to require a strong password, consisting of lowercase letters, uppercase letters, numbers, and symbols, and is not based on a dictionary word. The superuser can use the passwd command to change other users' passwords.

[student@desktopX ~]$ passwd
Changing password for user student.
Changing password for student.
(current) UNIX 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 beginning of a file's contents and displays what type it is. The files to be classified are passed as arguments to the command.

[student@desktopX ~]$ file /etc/passwd
/etc/passwd: ASCII text
[student@desktopX ~]$ file /bin/passwd
/bin/passwd: setuid ELF 64-bit LSB shared object, x86-64, version 1
(SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32,
BuildID[sha1]=0x91a7160a019b7f5f754264d920e257522c5bce67, stripped
[student@desktopX ~]$ file /home
/home: directory

The head and tail commands display the beginning and end of a file respectively. By default, these commands display 10 lines, but they both have a -n option that allows a different number of lines to be specified. The file to display is passed as an argument to these commands.

[student@desktopX ~]$ 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
[student@desktopX ~]$ tail -n 3 /etc/passwd
gdm:x:42:42::/var/lib/gdm:/sbin/nologin
gnome-initial-setup:x:993:991::/run/gnome-initial-setup/:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin

The wc command counts lines, words, and characters in a file. It can take a -l, -w, or -c option to display only the lines, words, or characters, respectively.

[student@desktopX ~]$ wc /etc/passwd
  39   70 2005 /etc/passwd
  [student@desktopX ~]$ wc -l /etc/passwd ; wc -l /etc/group
  39 /etc/passwd
  63 /etc/group
[student@desktopX ~]$ wc -c /etc/group /etc/hosts
 843 /etc/group
 227 /etc/hosts
1070 total

Tab completion

Tab completion allows a user to quickly complete commands or file names once they have typed enough at the prompt to make it unique. If the characters typed are not unique, pressing the Tab key twice displays all commands that begin with the characters already typed.

[student@desktopX ~]$ pas<Tab><Tab>
passwd       paste        pasuspender  
[student@desktopX ~]$ pass<Tab>
[student@desktopX ~]$ passwd 
Changing password for user student.
Changing password for student.
(current) UNIX password: 

Tab completion can be used to complete file names when typing them as arguments to commands. When Tab is pressed, it will complete the file name as much as it can. Pressing Tab a second time causes the shell to list all of the files that are matched by the current pattern. Type additional characters until the name is unique, then use tab completion to finish off the command line.

[student@desktopX ~]$ ls /etc/pas<Tab>
[student@desktopX ~]$ ls /etc/passwd<Tab>
passwd   passwd-

Arguments and options can be matched with tab completion for many commands. The useradd command is used by the superuser, root, to create additional users on the system. It has many options that can be used to control how that command behaves. Tab completion following a partial option can be used to complete the option without a lot of typing.

[root@desktopX ~]# useradd --<Tab><Tab>
--base-dir        --groups          --no-log-init     --shell
--comment         --help            --non-unique      --skel
--create-home     --home-dir        --no-user-group   --system
--defaults        --inactive        --password        --uid
--expiredate      --key             --root            --user-group
--gid             --no-create-home  --selinux-user    
[root@desktopX ~]# useradd --

Command history

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

The exclamation point character, !, is a metacharacter that is used to expand previous commands without having to retype them. !number expands to the command matching the number specified. !string expands to the most recent command that begins with the string specified.

[student@desktopX ~]$ history
...Output omitted...
   23  clear
   24  who
   25  pwd
   26  ls /etc
   27  uptime
   28  ls -l
   29  date
   30  history
[student@desktopX ~]$ !ls
ls -l
total 0
drwxr-xr-x. 2 student student 6 Mar 29 21:16 Desktop
...Output omitted...
[student@desktopX ~]$ !26
ls /etc
abrt                     hosts                     pulse
adjtime                  hosts.allow               purple
aliases                  hosts.deny                qemu-ga
...Output omitted...

The arrow keys can be used to navigate through previous command lines in the shell's history. Up Arrow edits the previous command in the history list. Down Arrow edits the next command in the history list. Use this key when the Up Arrow has been pressed too many times. Left Arrow and Right Arrow move the cursor left and right in the current command line being edited.

The Esc+. key combination causes the shell to copy the last word of the previous command on the current command line where the cursor is. If used repeatedly, it will continue to go through earlier commands.

Editing the command line

When used interactively, bash has a command line-editing feature. This allows the user to use text editor commands to move around within and modify the current command being typed. Using the arrow keys to move within the current command and to step through the command history was introduced earlier in this session. More powerful editing commands are introduced in the following table.

There are several other command line-editing commands available, but these are the most useful commands for beginning users. The other commands can be found in the bash(1) man page.