Using the Shell in Red Hat Linux

Command-line editing

If you type something wrong on a command line, the bash shell ensures that you don't have to delete the entire line and start over. Likewise, you can recall a previous command line and change the elements to make a new command.

By default, the bash shell uses command-line editing that is based on the emacs text editor. So, if you are familiar with emacs, you probably already know most of the keystrokes described here.

set -o vi

To do the editing, you can use a combination of control keys, meta keys, and arrow keys. For example, Ctrl+f means to hold the control key and type f. Alt+f means to hold the Alt key and type f. (Instead of the Alt key, your keyboard may use a Meta key or the Esc key instead. On a Windows keyboard, use the Windows key.)

To try out a bit of command-line editing, type the following command:

$ ls /usr/bin | sort -f | more

This command lists the contents of the /usr/bin directory, sorts the contents in alphabetical order (regardless of upper- and lowercase), and pipes the output to more (so you can page through the results). Now, suppose you want to change /usr/bin to /bin. You can use the following steps to change the command:

1. Press Ctrl+a. This moves the cursor to the beginning of the command line.

2. Press Ctrl+f or the right arrow (?) key. Repeat this command a few times to position the cursor under the first slash (/).

3. Press Ctrl+d. Type this command four times to delete /usr.

4. Press Enter. This executes the command line.

As you edit a command line, at any point you can type regular characters to add those characters to the command line. The characters appear at the location of your cursor. You can use right (?) and left (() arrows to move the cursor from one end to the other on the command line. You can also press the up (() and down (() arrow keys to step through previous commands in the history list to select a command line for editing. (See the discussion on command recall for details on how to recall commands from the history list.)

There are many keystrokes you can use to edit your command lines. Table 4-1 lists the keystrokes that you can use to move around the command line.

Table 4-2 lists the keystrokes for editing command lines.

Table 4-3 lists the keystrokes for cutting and pasting text on a command line.

Command line completion

To save you a few keystrokes, the bash shell offers several different ways of completing partially typed values. To attempt to complete a value, type the first few characters, and then press Tab. Here are some of the values you can type partially:

• Environment variable — If the text begins with a dollar sign ($), the shell completes the text with an environment variable from the current shell.

• User name — If the text begins with a tilde (~), the shell completes the text with a user name.

• Command, alias, or function — If the text begins with regular characters, the shell tries to complete the text with a command, alias, or function name.

• Host name — If the text begins with an at (@) sign, the shell completes the text with a host name taken from the /etc/hosts file.

  Tip?

  To add host names from an additional file, you can set the HOSTFILE variable to the name of that file. The file must be in the same format as /etc/hosts.

Here are a few examples of command completion. (When you see <Tab>, it means to press the Tab key on your keyboard.) Type the following:

$ echo $OS<Tab>
$ cd ~ro<Tab>
$ fing<Tab>
$ mailx root@loc<Tab>

The first example causes $OS to expand to the $OSTYPE variable. In the next example, ~ro expands to the root user's home directory (~root/). Next, fing expands to the finger command. Finally, the address of root@loc expands to computer name localhost.

Of course, there will be times when there are several possible completions for the string of characters you have entered. In that case, you can check the possible ways text can be expanded by pressing Esc+? (or by pressing Tab twice) at the point where you want to do completion. This shows the result you would get if you checked for possible completions on $P.

$ echo $P<Esc+?>
$PATH $PPID $PS1 $PS2 $PS4 $PWD
$ echo $P

In this case, there are six possible variables that begin with $P. After possibilities are displayed, the original command line returns, ready for you to complete it as you choose.

If text you are trying to complete is not preceded by a $, ~, or @, you can still try to complete the text with a variable, user name, or host name. Press the following to complete your text:

• Alt+~ — Complete the text before this point as a user name.

• Alt+$ — Complete the text before this point as a variable.

• Alt+@ — Complete the text before this point as a host name.

• Alt+! — Complete the text before this point as a command name (alias, reserved word, shell function, built-in, and filenames are checked in that order). In other words, complete this key sequence with a command that you previously ran.

• Ctrl+x+/ — List possible user name text completions.

• Ctrl+x+$ — List possible environment variable completions.

• Ctrl+x+@ — List possible host name completions.

• Ctrl+x+! — List possible command name completions.

Command line recall

After you type a command line, that entire command line is saved in your shell's history list. The list is stored in a history file, from which any command can be recalled to run again. After it is recalled, you can modify the command line, as described earlier.

To view your history list, use the history command. Type the command without options or followed by a number to list that many of the most recent commands. For example:

$ history 8
 382 date
 383 ls /usr/bin | sort -a | more
 384 man sort
 385 cd /usr/local/bin
 386 man more
 387 useradd -m /home/chris -u 101 chris
 388 passwd chris
 389 history 8

A number precedes each command line in the list. There are several ways to run a command immediately from this list, including:

• Run Command Number (!n) — Replace the n with the number of the command line, and the command line indicated is run.

• Run Previous Command (!!) — Runs the previous command line.

• Run Command Containing String (!?string?) — Runs the most recent command that contains a particular string of characters.

Instead of just running a history command line immediately, you can recall a particular line and edit it. You can use these keys to do that:

• Step (Arrow Keys) — Press the up (() and down (() arrow keys to step through each command line in your history list to arrive at the one you want. (Ctrl+p and Ctrl+n do the same functions, respectively.)

• Reverse Incremental Search (Ctrl+r) — After you press these keys, you are asked to enter a search string to do a reverse search. As you type the string, a matching command line appears that you can run or edit.

• Forward Incremental Search (Ctrl+s) — After you press these keys, you are asked to enter a search string to do a forward search. As you type the string, a matching command line appears that you can run or edit.

• Reverse Search (Alt+p) — After you press these keys, you are asked to enter a string to do a reverse search. Type a string and press Enter to see the most recent command line that includes that string.

• Forward Search (Alt+n) — After you press these keys, you are asked to enter a string to do a forward search. Type a string, and press Enter to see the most recent command line that includes that string.

• Beginning of History List (Alt+<) — Brings you to the first entry of the history list.

• Beginning of History List (Alt+>) — Brings you to the last entry of the history list.

Another way to work with your history list is to use the fc command. Type fc followed by a history line number, and that command line is opened in a text editor. Make the changes that you want. When you exit the editor, the command runs. You could also give a range of line numbers (for example, fc 100 105). All the commands open in your text editor, and then run one after the other when you exit the editor.

The history list is stored in the .bash_history file in your home directory. Up to 1000 history commands are stored for you by default.

Piping commands

The pipe (|) metacharacter connects the output from one command to the input of another command. This lets you have one command work on some data, then have the next command deal with the results. Here is an example of a command line that includes pipes:

$ cat /etc/password | sort | more

This command lists the contents of the /etc/password file and pipes the output to the sort command. The sort command takes the user names that begin each line of the /etc/password file, sorts them alphabetically, and pipes the output to the more command. The more command displays the output one page at a time, so that you can go through the output a line or a page at a time.

Pipes are an excellent illustration of how UNIX, the predecessor of Linux, was created as an operating system made up of building blocks. A standard practice in UNIX was to connect utilities in different ways to get different jobs done. For example, before the days of graphical word processors, users created plain-text files that included macros to indicate formatting. To see how the document really appeared, they would use a command such as the following:

$ gunzip < /usr/share/man/man1/grep.1.gz | nroff -c -man | less

In this example, the contents of the grep man page (grep.1.gz) are directed to the gunzip command to be unzipped. The output from gunzip is piped to the nroff command to format the man page using the manual macro (-man). The output is piped to the less command to display the output. Because the file being displayed is in plain text, you could have substituted any number of options to work with the text before displaying it. You could sort the contents, change or delete some of the content, or bring in text from other documents. The key is that, instead of all those features being in one program, you get results from piping and redirecting input and output between multiple commands.

Sequential commands

Sometimes you may want a sequence of commands to run, with one command completing before the next command begins. You can do this by typing several commands on the same command line and separating them with semicolons (;):

$ date ; troff -me verylargedocument | lpr ; date

In this example, I was formatting a huge document and wanted to know how long it would take. The first command (date) showed the date and time before the formatting started. The troff command formatted the document and then piped the output to the printer. When the formatting was done, the date and time was printed again (so I knew how long the troff command took to complete).

Background commands

Some commands can take a while to complete. Sometimes you may not want to tie up your shell waiting for a command to finish. In those cases, you can have the commands run in the background by using the ampersand (&).

Text formatting commands (such as nroff and troff, described earlier) are examples of commands that are often run in the background to format a large document. You also may want to create your own shell scripts that run in the background to check continuously for certain events to occur, such as the hard disk filling up or particular users logging in.

Here is an example of a command being run in the background:

$ troff -me verylargedocument | lpr &

There are other ways of managing background and foreground processes (described in the "Managing background and foreground processes" section).

Expanding commands

With command substitution, you can have the output of a command interpreted by the shell instead of by the command itself. In this way, you can have the standard output of a command become an argument for another command. The two forms of command substitution are $(command) or 'command'.

The command in this case can include options, metacharacters, and arguments. Here is an example of using command substitution:

$ vi $(find /home | grep xyzzy)

In this command line, the command substitution is done before the vi command is run. First, the find command starts at the /home directory and prints out all files and directories below that point in the file system. This output is piped to the grep command, which filters out all files except for those that include the string xyzzy. Finally, the vi command opens all filenames for editing (one at a time) that include xyzzy.

This particular example may be useful if you knew that you wanted to edit a file for which you knew the name but not the location. As long as the string was uncommon, you could find and open every instance of a filename existing beneath a point you choose in the file system.

Expanding arithmetic expressions

There may be times when you want to pass arithmetic results to a command. There are two forms you can use to expand an arithmetic expression and pass it to the shell: $[expression] or $((expression)). Here is an example:

$ echo "I am $[2003 - 1957] years old."
I am 46 years old.

In this example, the shell interprets the arithmetic expression first (2003 – 1957), and then passes that information to the echo command. The echo command displays the text, with the results of the arithmetic (46) inserted.

Expanding variables

Environment variables that store information within the shell can be expanded using the dollar sign ($) metacharacter. When you expand an environment variable on a command line, the value of the variable is printed instead of the variable name itself, as follows:

$ ls -l $BASH
-rwxr-xr-x 1 root  root  625516 Dec 5 11:13 /bin/bash

Using $BASH as an argument to ls -l causes a long listing of the bash command to be printed. For more information on shell environment variables, see the following section.

Common shell environment variables

When you start a shell (by logging in or opening a Terminal window), a lot of environment variables are already set. Here are some variables that are either set when you use a bash shell or that can be set by you to use with different features.

• BASH — Contains the full path name of the bash command. This is usually /bin/bash.

• BASH_VERSION — A number of the current version of the bash command.

• EUID — This is the effective user ID number of the current user. It is assigned when the shell starts, based on the user's entry in the /etc/passwd file.

• FCEDIT — If set, this indicates the text editor used by the fc command to edit history commands. If this variable isn't set, the vi command is used.

• HISTFILE — The location of your history file. It is typically located at $HOME/.bash_history.

• HISTFILESIZE — The number of history entries that can be stored. After this number is reached, the oldest commands are discarded. The default value is 1000.

• HISTCMD — This returns the number of the current command in the history list.

• HOME — This is your home directory. It is your current working directory each time you log in or type the cd command with any options.

• HOSTTYPE — A value that describes the computer architecture on which the Linux system is running. For Intel-compatible PCs, the value is i386, i486, i586, or i686.

• MAIL — This is the location of your mailbox file. The file is typically your user name in the /var/spool/mail directory.

• OLDPWD — The directory that was the working directory before you changed to the current working directory.

• OSTYPE — A name identifying the current operating system. In our case, this will say linux-gnu. (Bash can run on other operating systems as well.)

• PATH — The colon-separated list of directories used to find commands that you type. The default value for regular users is:

  /bin:/usr/bin:/usr/local/bin:/usr/bin/X11:/usr/X11R6/bin:~/bin

  For the root user, the value also includes /sbin, /usr/sbin, and /usr/local/sbin.

• PPID — The process ID of the command that started the current shell (for example, its parent process).

• PROMPT_COMMAND — Can be set to a command name that is run each time before your shell prompt is displayed. Setting PROMPT_COMMAND=date lists the current date/time before the prompt appears.

• PS1 — Sets the value of your shell prompt. There are many items that you can read into your prompt (date, time, user name, host name, and so on). Sometimes a command requires additional prompts, which you can set with the variables PS2, PS3, and so on. (Setting your prompt is described later in this chapter.)

• PWD — This is the directory that is assigned as your current directory. This value changes each time you change directories using the cd command.

• RANDOM — Accessing this variable causes a random number to be generated. The number is between 0 and 99999.

• SECONDS — The number of seconds since the time the shell was started.

• SHLVL — The number of shell levels associated with the current shell session. When you log in to the shell, the SHLVL is 1. Each time you start a new bash command (by, for example, using su to become a new user, or by simply typing bash), this number is incremented.

• TMOUT — Can be set to a number representing the number of seconds the shell can be idle without receiving input. After the number of seconds is reached, the shell exits. This is a security feature that makes it less likely for unattended shells to be accessed by unauthorized people. (This must be set in the login shell for it to actually cause the shell to log out the user.)

• UID — The user ID number assigned to your user name. The user ID number is stored in the /etc/password file.

Set your own environment variables

Environment variables can provide a handy way of storing bits of information that you use often from the shell. You can create any variables that you want (avoiding those that are already in use) so that you can read in the values of those variables as you use the shell. (The bash man page lists variables already in use.)

To set an environment variable temporarily, you can simply type a variable name and assign it to a value. Here is an example:

$ AB=/usr/dog/contagious/ringbearer/grind ; export AB

This example causes a long directory path to be assigned to the AB variable. The export AB command says to export the value to the shell so that it can be propagated to other shells you may open. With AB set, you can go to the directory by typing the following:

$ cd $AB

The problem with setting environment variables in this way is that as soon as you exit the shell in which you set the variable, the setting is lost. To set variables more permanently, you should add variable settings to a bash configuration file, as described later in this section.

If you want to have other text right up against the output from an environment variable, you can surround the variable in braces. This protects the variable name from being misunderstood. For example, if you wanted to add a command name to the AB variable shown earlier, you could type the following:

$ echo ${AB}/adventure
/usr/dog/contagious/ringbearer/grind/adventure

Remember that you must export the variable so that it can be picked up by other shell commands. You must add the export line to a shell configuration file for it to take effect the next time you log in. The export command is fairly flexible. Instead of running the export command after you set the variable, you could do it all in one step, as follows:

$ export XYZ=/home/xyz/bin

You can override the value of any environment variable. This can be temporary by simply typing the new value. Or you can add the new export line to your $HOME/.bashrc file. One useful variable to update is PATH. Here is an example:

$ export PATH=$PATH:/home/xyz/bin

In this example, I added the /home/xyz/bin directory to the PATH, a useful technique if you want to run a bunch of commands from a directory that is not normally in your PATH, without typing the full or relative path each time.

If you decide that you no longer want a variable to be set, you can use the unset command to erase its value. For example, you could type unset XYZ, which would cause XYZ to have no value set. (Remember to remove the export from the $HOME/.bashrc file — if you added it there — or it will return the next time you open a shell.)

Starting background processes

If you have programs that you want to run while you continue to work in the shell, you can place the programs in the background. To place a program in the background at the time you run the program, type an ampersand (&) at the end of the command line. For example:

$ find /usr > /tmp/allusrfiles &

This command finds all files on your Red Hat Linux system (starting from /usr), prints those filenames, and puts those names in the file /tmp/allusrfiles. The ampersand (&) runs that command line in the background. To check which commands you have running in the background, use the jobs command, as follows:

$ jobs
[1]  Stopped (tty output)  vi /tmp/myfile
[2]  Running        find /usr -print > /tmp/allusrfiles &
[3]  Running        nroff -man /usr/man2/* >/tmp/man2 &
[4]- Running        nroff -man /usr/man3/* >/tmp/man3 &
[5]+ Stopped        nroff -man /usr/man4/* >/tmp/man4

The first job shows a text-editing command (vi) that I placed in the background and stopped by pressing Ctrl+z while I was editing. Job two shows the find command I just ran. Jobs three and four show nroff commands currently running in the background. Job five had been running in the shell (foreground) until I decided too many processes were running and pressed Ctrl+z to stop job five until a few processes had completed.

The plus sign (+) next to number 5 shows that it was most recently placed in the background. The minus sign (-) next to number 4 shows that it was placed in the background just before the most recent background job. Because job 1 requires terminal input, it cannot run in the background. As a result, it is Stopped until it is brought to the foreground again.

Using foreground and background commands

Continuing with the example, you can bring any of the commands on the jobs list to the foreground. For example, to edit myfile again, type:

$ fg %1

As a result, the vi command opens again, with all text as it was when you stopped the vi job.

To refer to a background job (to cancel or bring it to the foreground), use a percent sign (%) followed by the job number. You can also use the following to refer to a background job:

• % — A percent sign alone refers to the most recent command put into the background (indicated by the plus sign). This action brings the command to the foreground.

• %string — Refers to a job where the command begins with a particular string of characters. The string must be unambiguous. (In other words, typing %vi when there are two vi commands in the background results in an error message.)

• %?string — Refers to a job where the command line contains a string at any point. The string must be unambiguous or the match will fail.

• %-- — Refers to the previous job stopped before the one most recently stopped.

If a command is stopped, you can start it running again in the background using the bg command. For example, take job number 5 from the jobs list in the previous example:

[5]+ Stopped         nroff -man man4/* >/tmp/man4

Type the following:

$ bg %5

After that, the job runs in th