Introduction to Bash | HP-UX 11i Systems Administration Handbook and Toolkit (2nd Edition)

With many developers using the Open Source development tools in HP-UX 11i, there is a lot of interest in Bash. Bash is widely used, although not required, when using GNU tools. In addition, the Linux-based UNIX operating systems I have used configure Bash as the default shell. Bash is covered in this section on a Linux system, however, the features of Bash covered exist on HP-UX as well. Bash possesses many of the fine features of other shells , and in fact derives its name from B ourne A gain SH ell, which is a dead giveaway that it possesses at least some of the features of the Bourne shell. Bash is similar to other shells in that it provides a user interface to UNIX. You can use the Bash shell in the following three ways:

Interactively type commands on the command line.
Group commonly executed sets of commands into command files that you can execute by typing the name of the file.
Create Bash shell programs using the structured programming techniques of the shell.

These three techniques are listed in the order in which you'll probably use them. First, you log in and use interactive commands. Then you group together commonly used commands and execute them with a single command. Finally, you may want to create sophisticated shell scripts.

For this reason, I'll describe these aspects of the Bash shell in the order in which they are listed. The command file and programming aspects of the Bash shell are covered in Chapter 28. Bash is very similar to the KornShell, which is the shell used in the shell programming chapter. You can, therefore, use the shell programming chapter as an introduction to programming with Bash as well. Keep in mind, however, that differences always occur when programming with one shell rather than another.

Issuing Commands

The first activity you perform after you log into the system is to issue commands at the prompt. A command you may want to issue immediately is ls -al . Here is what I see on my system after executing this command to check my present working directory and producing a long listing of all files when logged in as root:

 #  pwd  #  ls -al  total 46 drwxr-xr-x   5 root     root         1024 Nov 26 19:40 . drwxr-xr-x  20 root     root         1024 Nov  8 20:10 .. -rw-r--r--   1 root     root          964 Nov 26 19:40 .bash_history -rw-r--r--   1 root     root          674 Feb  5  1997 .bashrc -rw-r--r--   1 root     root          602 Feb  5  1997 .cshrc -rw-r--r--   1 root     root        14815 Nov  8 20:09 .fvwmrc.menus.prep -rw-r--r--   1 root     root          116 Feb  5  1997 .login -rw-r--r--   1 root     root          234 Feb  5  1997 .profile drwxr-xr-x   2 root     root         1024 Nov  8 14:10 .seyon -rw-r--r--   1 root     root         4276 Nov  8 20:09 XF86Config -r--r--r--   1 root     root        13875 Nov  8 20:05 XF86Config.bak drwxrwxrwx   2 root     root         1024 Nov 26 19:40 book drwxr-xr-x   5 root     root         1024 Nov 14 18:12 lg -rw-r--r--   1 root     root            0 Nov 26 19:40 typescript #

Among the files produced in the long listing of all files is a Bash startup file called .bashrc . The following shows the contents of the .bashrc file:

 #  cat .bashrc  # ~/.bashrc -- #   The individual per-interactive-shell startup file for bash . /etc/profile # try solve this tedious 'Backspace vs. Delete' problem... if [ -z "$TERM" ]; then   echo ".bashrc: TERM empty: this shouldn't happen!" 1>&2   echo "   Please contact 'support@lst.de'" 1>&2 else   case $TERM in   linux*)     stty erase '^?'     ;;   *)     stty erase '^H'     ;;   esac fi # general environment settings #export GROFF_TYPESETTER=latin1 #export LC_CTYPE=iso-8859-1 export LESSCHARSET=latin1 #export METAMAIL_PAGER=less HISTSIZE=100 alias which='type -path' alias h=history alias j="jobs -l" alias l="ls -Fax" alias ll="ls -Alg" alias pd=pushd alias z=suspend #

The . bashrc file has some interesting contents. Among them is a value for HISTSIZE, which we'll get into shortly, and a set of aliases. These aliases are "shortcuts" for long commands. When I issue the ll command, for instance, I am really issuing the ls -Alg command.

I may execute both the local .profile shown in the earlier long listing as well as /etc/profile . /etc/profile usually performs setup for all users who log into the system. The following is a listing of /etc/profile :

 #  cat /etc/profile  # /etc/profile # System wide environment and startup programs # Functions and aliases go in $HOME/.bashrc PATH="/bin:/usr/bin:/opt/bin:/usr/X11R6/bin:/usr/openwin/bin:/usr/TeX/bin:/usr/local/bin" umask 022 if [ 'id -gn' = 'id -un' ] && [ 'id -u' != 0 ]; then   umask 002 fi if [ -z "$UID" ]; then   UID='id -u' fi if [ "$UID" = 0 ]; then   PATH=/sbin:/usr/sbin:$PATH else   PATH=$PATH: fi USER='id -un' LOGNAME=$USER export PATH USER LOGNAME HOSTNAME='/bin/hostname' MAIL="/var/spool/mail/$USER" export HOSTNAME MAIL if [ -n "$BASH_VERSION" ]; then   # (aliases now in $HOME/.bashrc, resp. /etc/skel/.bashrc)   export PS1="[\u@\h \W]\$ "   export HISTSIZE=100 fi #

We'll also cover some of the contents of /etc/profile .

Initializing the History List in .bashrc

The Bash shell can keep a history list of the commands you have issued. If you wish to reissue a command or view a command you earlier issued, you can use the history list.

You can specify any number of commands to be included in the history list. The following line in .bashrc sets the history list to 100:

set history = 100

One hundred commands will be saved in the history list. When you log, out the last 100 commands you have issued are stored in the history list. The next time you log in you can view these 100 commands; however, as you issue commands, the oldest commands fall off the history list. This fact is shown in the following example:

 #  history  2  more history     3   ll     4  cd ..     5  pwd     6  cd ..     7  ll     8  cd ..     9  ll    10  ll log    11  cd log    12  more *    13  l    14  ll    15  cd /    16  ll    17  cd    18  XF86Setup    19  XF86Setup    20  startx    21  ll    22  pwd    23  ll    24  ll /    25  XF86Setup    26  ll    27  startx    28  find / -name XF86Config*    29  cp /usr/X11R6/lib/X11/XF86Config.eg .    30  ll    31  XF86Setup    32  XF86Setup    33  startx    34  ll    35  mv XF86Config.eg XF86Config    36  XF86Setup    37  startx    38  shutdown -h now    39  man ls    40  man ll    41  man ls    42  man file    43  lsr    44  man chmod    45  man chmod    46  shutdown -h now    47  pwd    48  ls -l    49  pwd    50  ls -a    51  ls -al    52  pwd    53  ls -al    54  more .profile    55  more .bashrc    56    57  alias    58  ll    59  pwd    60  script    61  script    62  scrit    63  script    64  more .bashrc    65  more .bashrc    66   ll    67  more .profile    68   ll    69  more .bashrc  grep P    70  more .profile  grep P    71  env    72  more /.profile    73  more /etc/profile    74  more /etc/profile  grep PS    75  find / -name *profile* -print    76  more .bashrc    77   more .bashrc    78  ll /etc/profi*    79  cp /etc/profile /etc/profile.orig    80  vi /etc/profile    81  exit    82  cp /etc/profile.orig /etc/profile    83  history    84    85  exit    86  history    87  history    88    ll    89    ll    90  history    91  ll /etc/profi*    92  ll /etc/profi*    93   more .bashrc    94    history    95  ll    96  cd /root    97  ll    98  history  more    99   history  more   100   exit   101  history

Notice in this example that command number 100 is the exit , or command to log out, from the last session. Command number 101 is the history command I issued immediately upon establishing the next session.

Recalling from the History List

All these commands ( cp, more, find, ll ) are in the history list with their corresponding numbers . You can repeat the last command with !! , the 89th command with !89 , and the last command that started with "m" with !m , all of which are shown in the following example:

 #  !!  history     3   ll     4  cd ..     5  pwd     6  cd ..     7  ll     8  cd ..     9  ll    10  ll log    11  cd log    12  more *    13  l    14  ll    15  cd /    16  ll    17  cd    18  XF86Setup    19  XF86Setup    20  startx    21  ll    22  pwd    23  ll    24  ll /    25  XF86Setup    26  ll    27  startx    28  find / -name XF86Config*    29  cp /usr/X11R6/lib/X11/XF86Config.eg .    30  ll    31  XF86Setup    32  XF86Setup    33  startx    34  ll    35  mv XF86Config.eg XF86Config    36  XF86Setup    37  startx    38  shutdown -h now    39  man ls    40  man ll    41  man ls    42  man file    43  lsr    44  man chmod    45  man chmod    46  shutdown -h now    47  pwd    48  ls -l    49  pwd    50  ls -a    51  ls -al    52  pwd    53  ls -al    54  more .profile    55  more .bashrc    56    57  alias    58  ll    59  pwd    60  script    61  script    62  scrit    63  script    64  more .bashrc    65  more .bashrc    66   ll    67  more .profile    68   ll    69  more .bashrc  grep P    70  more .profile  grep P    71  env    72  more /.profile    73  more /etc/profile    74  more /etc/profile  grep PS    75  find / -name *profile* -print    76  more .bashrc    77   more .bashrc    78  ll /etc/profi*    79  cp /etc/profile /etc/profile.orig    80  vi /etc/profile    81  exit    82  cp /etc/profile.orig /etc/profile    83  history    84    85  exit    86  history    87  history    88    ll    89    ll    90  history    91  ll /etc/profi*    92  ll /etc/profi*    93   more .bashrc    94    history    95  ll    96  cd /root    97  ll    98  history  more    99   history  more   100   exit   101  history   102  history #  !89  ll total 44 -rw-r--r--   1 root     root          956 Nov 26 19:33 .bash_history -rw-r--r--   1 root     root          674 Feb  5  1997 .bashrc -rw-r--r--   1 root     root          602 Feb  5  1997 .cshrc -rw-r--r--   1 root     root        14815 Nov  8 20:09 .fvwmrc.menus.prep -rw-r--r--   1 root     root          116 Feb  5  1997 .login -rw-r--r--   1 root     root          234 Feb  5  1997 .profile drwxr-xr-x   2 root     root         1024 Nov  8 14:10 .seyon -rw-r--r--   1 root     root         4276 Nov  8 20:09 XF86Config -r--r--r--   1 root     root        13875 Nov  8 20:05 XF86Config.bak drwxrwxrwx   2 root     root         1024 Nov 13 21:25 book drwxr-xr-x   5 root     root         1024 Nov 14 18:12 lg -rw-r--r--   1 root     root            0 Nov 26 19:36 typescript #  !m  more .bashrc # ~/.bashrc -- #   The individual per-interactive-shell startup file for bash . /etc/profile # try solve this tedious 'Backspace vs. Delete' problem... if [ -z "$TERM" ]; then   echo ".bashrc: TERM empty: this shouldn't happen!" 1>&2   echo "   Please contact 'support@lst.de'" 1>&2 else   case $TERM in   linux*)     stty erase '^?'     ;;   *)     stty erase '^H'     ;;   esac fi # general environment settings #export GROFF_TYPESETTER=latin1 #export LC_CTYPE=iso-8859-1 [7m--More--(70%)[m export LESSCHARSET=latin1 #export METAMAIL_PAGER=less HISTSIZE=100 alias which='type -path' alias h=history alias j="jobs -l" alias l="ls -Fax" alias ll="ls -Alg" alias pd=pushd alias z=suspend [root@nycald1 /root]# Script done on Thu Nov 26 19:39:51 1998

Table 27-8 includes some of the more commonly used history list recall commands:

Table 27-8. Recalling from the History List

Command	Description	Example
! N	Issue command N	!2
!!	Issue last command	!!
!- N	Issue Nth command from last command issued	!-N
! str	Issue last command starting with str	!c
!? str ?	Issue last command that had str anyplace in command line	!?cat?
!{ str1 } str2	Append str2 to last command with str1	!{cd} /tmp
^ str1 ^ str2 ^	Substitute str2 for str1 in last command	^cat^more^

Editing on the Command Line

Using the history list is a great way of viewing and reissuing commands. Bash also supports command-line editing. You can use the up arrow key to move back one command in the history list. When you press the up arrow key, the last command from the history list appears on the command line. Every time you press the up arrow key, you move back one more command in the history list. When a command appears on the command line, you can press the "Enter" key to issue the command. You can modify the command by using the left and right arrow keys to move to a point in the command line and type additional information, or use the "backspace" and "delete" keys to remove information from the command line.

Aliases in .bashrc

An alias is a name that you select for a frequently used command or series of commands. You can use the .bashrc file as a place where your aliases are stored and read every time you log in. In the earlier .bashrc file, seven aliases were already set up. You can add additional aliases in the .bashrc file or define aliases at the command-line prompt, but these will be cleared when you log out.

Here is a list of the aliases that are already set up for us in the .bashrc file and an example of running the aliases l and ll :

 #  alias  alias h='history' alias j='jobs -l' alias l='ls -Fax' alias ll='ls -Alg' alias pd='pushd' alias which='type -path' alias z='suspend' # #  l  ./                ../                 .bash_history        .bashrc .cshrc            .fvwmrc.menus.prep  .login               .profile .seyon/           XF86Config          XF86Config.bak       book/ lg/               typescript # #  ll  total 44 -rw-r--r--   1 root     root          970 Nov 26 21:35 .bash_history -rw-r--r--   1 root     root          674 Feb  5  1997 .bashrc -rw-r--r--   1 root     root          602 Feb  5  1997 .cshrc -rw-r--r--   1 root     root        14815 Nov  8 20:09 .fvwmrc.menus.prep -rw-r--r--   1 root     root          116 Feb  5  1997 .login -rw-r--r--   1 root     root          234 Feb  5  1997 .profile drwxr-xr-x   2 root     root         1024 Nov  8 14:10 .seyon -rw-r--r--   1 root     root         4276 Nov  8 20:09 XF86Config -r--r--r--   1 root     root        13875 Nov  8 20:05 XF86Config.bak drwxrwxrwx   2 root     root         1024 Nov 26 19:41 book drwxr-xr-x   5 root     root         1024 Nov 14 18:12 lg -rw-r--r--   1 root     root            0 Nov 26 21:35 typescript #

These are all very useful indeed, but let's now set up our own alias. Suppose that we want to know how many processes are running on the system. We'll create an alias called "procs" that does this for us. The ps command produces a list of processes. We'll issue ps and pipe () this output to wc with the "l" option to tell us how many lines are present. The pipe () directs the output of ps to be used as the input to wc . The ps command produces a list of processes, and wc -l gives us a count of the number of lines. Therefore, we'll know the total number of processes running. The following example first shows the output of ps , then our alias command, and finally the output produced by the alias command:

 #  ps  PID TTY STAT  TIME COMMAND   188   2 S    0:00 /sbin/getty tty2 VC linux   189   3 S    0:00 /sbin/getty tty3 VC linux   190   4 S    0:00 /sbin/getty tty4 VC linux   191   5 S    0:00 /sbin/getty tty5 VC linux   192   6 S    0:00 /sbin/getty tty6 VC linux   619   1 S    0:00 login root   620   1 S    0:00 -bash   642   1 S    0:00 script   643   1 S    0:00 script   644  p0 S    0:00 bash -i   656  p0 R    0:00 ps # #  alias procs='echo "Number of processes are: ";ps  wc -l'  # #  procs  Number of processes are: 11 #

This alias works wonderfully. All we have to type is "procs" to see the number of processes running on our system.

A lot of quoting takes place in this command line. To understand what is taking place on this line, consult Table 27-9:

Table 27-9. Shell Quoting

Character(s)	Description
'cmd'	Single quote means to take the string character literally
"str"	Double quote means allow command and variable substitution
\c	Escape character prevents everything following it from printing, including new line
'str'	Grave means to execute command and substitute output

Applying Table 27-9 to the earlier procs alias, we can see what comprises this alias. The alias begins with a single quote, which means to execute the command(s) within the single quotes. The first command is echo , which uses double quotes to specify the characters to echo . We could have added the escape character \c , which would have prevented a new line from being printed. The semicolons separate commands. ps is then run to produce a list of processes, and the output is piped ( ) to word count ( wc ), which produces a count of the number of lines, as shown in Figure 27-9:

Figure 27-9. Quoting Example

graphics/27fig09.gif

As you can see in Figure 27-9, some of the quoting becomes tricky. An understanding of quoting is important if you wish to modify and reuse existing shell scripts or craft your own.

Command and Path Completion

Bash sometimes knows what you're thinking. You can type part of a command or pathname, and Bash can complete the remainder for you. You can type part of a command or pathname and use the "tab" key to complete the command. If, for instance, you wish to issue the runlevel command to view the current system run level, but can't remember the full command, you can type "run" and press the tab key and the command is completed for you, as shown in the following example:

 #  run  <tab key>  level  N 3

Bash determined that the only command that starts with "run" is runlevel and completed the command.

As long as you issue the command or pathname to the extent that it is unique, then Bash completes it for you. If the command or pathname is not unique, then Bash shows you the options for completing the command. The following example shows typing "ru" and two tabs to get a list of commands that start with "ru":

 #  ru  <tab key><tab key> runlevel  rusers

You can see from this example that typing "ru" produced two possible commands - runlevel and rusers .

This great completion also works for path names . If you change directory to "/b," you get the following result:

 #  cd /b  <tab key><tab key> bin  boot

Because two directories at the root level begin with "b," Bash could not determine which of the two you wanted and listed both.