The UNIX philosophy is based on the DRY principle which declares: “Don't repeat yourself”. Therefore, a program just does what it's made for and uses libraries or even other programs on your system to do a more advanced job. Some of these auxiliary programs are user definable and probably you already know the
EDITOR environment variable, which specifies your default editor for the console. This is used, e.g., for the command
visudo, which opens your
/etc/sudoers file safely, or by
svn to input a commit comment. But another important component is your console pager, which is used to display textual content on the console.
As a system administrator or a more advanced Linux user you surely know
man and how to use it.
man shows you the (probably localized) contents of your system's manual pages for a specific command or file, at least you might think so, but actually that's wrong or let's say: not completely the truth.
man selects which manpage to show, but the display itself is managed by your pager. Of course, not only
man makes use of the pager, also commands like
I assume, you have already used your pager very often, which is nothing else than
lv (which is also a good pager but very seldom installed by default and therefore rarely known). Most of the time, you use them to display the contents of log files or configurations, which you don't want to alter, but that's not all. As mentioned before, other programs use your pager as well and there is a way to configure which one to take and of course also how to use it.
Setting a pager
For this blog entry let's stay with
man, which is probably the most important program or at least the most often used program that uses a pager. By default,
/usr/bin/less -is as display engine, which starts
less in in-case-sensitive mode and collapses sequent empty lines to one single line. On most distributions, the configuration for your manpage viewer is found at
/etc/man.conf but we will leave this file untouched as it's only specific for the
man program. There is another more common way to set your pager all over your system and that's the
PAGER environment variable. In this variable you set which command to use as your pager. To use
more, you would fill this variable as follows:
If you now start
man, you should see
more as your manpage viewer. You could also use
cat as your pager, that's completely up to you. But how to pass parameters to this instance of
more? I mentioned earlier in this text that
PAGER contains a command, so you could do something like this:
export PAGER="/usr/bin/less -isM
but there is actually a better way. All three pager programs,
lv, support passing parameters via separate environment variables. These variables are named
LV, respectively. These variables can contain all the parameters passed to the specific program by default, not affecting your setting for
PAGER. These variables will also be evaluated, when you execute your pager program independently (stand-alone). So to assign the parameters
less, type in
Then start less (either stand-alone or with
Note: specifically for
man, you can also define the variable
MANPAGER, which overrides
Making the settings persistent
As yet you'll loose your settings as soon as you close your current terminal. Thus, you should save your settings. You can do this by writing your variable export commands into your own
.profile file for your native tty login shell or into
.bashrc for non-login shells (like your Xterm). To make your settings available system wide, create a proper file in
/etc/profile.d/ or directly edit
/etc/profile, depending on your distribution. To see an effect for your current X session, you have indeed to re-login.
There might be some issues when using a custom manpager. One big problem that could occur when using
lv as a pager, is that your manual pages are displayed as garbage. That could look something like this:
MOUNT(8) Linux Programmer's Manual MOUNT(8) ESC[1mNAMEESC[0m mount - mount a filesystem ESC[1mSYNOPSISESC[0m ESC[1mmount ESC[22m[ESC[1m-lhVESC[22m] ESC[1mmount -a ESC[22m[ESC[1m-fFnrsvwESC[22m] [ESC[1m-t ESC[4mESC[22mvfstypeESC[24m] [ESC[1m-O ESC[4mESC[22moptlistESC[24m] ESC[1mmount ESC[22m[ESC[1m-fnrsvwESC[22m] [ESC[1m-o ESC[4mESC[22moptionESC[24m[ESC[1m,ESC[4mESC[22moptionESC[24m]...] ESC[4mdeviceESC[24m|ESC[4mdirESC[0m ESC[1mmount ESC[22m[ESC[1m-fnrsvwESC[22m] [ESC[1m-t ESC[4mESC[22mvfstypeESC[24mESC[1m] ESC[22m[ESC[1m-o ESC[4mESC[22moptionsESC[24m] ESC[4mdeviceESC[24m ESC[4mdirESC[0m ESC[1mDESCRIPTIONESC[0m All files accessible in a Unix system are arranged in one big tree, the file hierarchy, rooted at ESC[1m/ESC[22m. These files can be spread out over several devices. The ESC[1mmount ESC[22mcommand serves to attach the filesystem found on some device to the big file tree. Conversely, the ESC[1mumountESC[22m(8) command will detach it again. The standard form of the ESC[1mmount ESC[22mcommand, is ESC[1mmount -t ESC[4mESC[22mtypeESC[24m ESC[4mdeviceESC[24m ESC[4mdirESC[0m This tells the kernel to attach the filesystem found on ESC[4mdeviceESC[24m (which is of type ESC[4mtypeESC[24m) at the directory ESC[4mdirESC[24m. The pre‐ vious contents (if any) and owner and mode of ESC[4mdirESC[24m become invisible, and as long as this filesystem remains mounted, the pathname ESC[4mdirESC[24m refers to the root of the filesystem on ESC[4mdeviceESC[24m. ESC[1mThe listing and help.ESC[0m Three forms of invocation do not actually mount anything: ESC[1mmount -hESC[0m prints a help message ESC[1mmount -VESC[0m prints a version string ESC[1mmount ESC[22m[ESC[1m-lESC[22m] [ESC[1m-t ESC[4mESC[22mtypeESC[24m] lists all mounted filesystems (of type ESC[4mtypeESC[24m). The option -l adds the labels in this listing. See below.
MOUNT(8) Linux Programmer's Manual MOUNT(8) ^[[1mNAME^[[0m mount - mount a filesystem ^[[1mSYNOPSIS^[[0m ^[[1mmount ^[[22m[^[[1m-lhV^[[22m] ^[[1mmount -a ^[[22m[^[[1m-fFnrsvw^[[22m] [^[[1m-t ^[[4m^[[22mvfstype^[[24m] [^[[1m-O ^[[4m^[[22moptlist^[[24m] ^[[1mmount ^[[22m[^[[1m-fnrsvw^[[22m] [^[[1m-o ^[[4m^[[22moption^[[24m[^[[1m,^[[4m^[[22moption^[[24m]...] ^[[4mdevice^[[24m|^[[4mdir^[[0m ^[[1mmount ^[[22m[^[[1m-fnrsvw^[[22m] [^[[1m-t ^[[4m^[[22mvfstype^[[24m^[[1m] ^[[22m[^[[1m-o ^[[4m^[[22moptions^[[24m] ^[[4mdevice^[[24m ^[[4mdir^[[0m ^[[1mDESCRIPTION^[[0m All files accessible in a Unix system are arranged in one big tree, the file hierarchy, rooted at ^[[1m/^[[22m. These files can be spread out over several devices. The ^[[1mmount ^[[22mcommand serves t o attach the filesystem found on some device to the big file tree. Conversely, the ^[[1mumount^[[22m(8) command will detach it again. The standard form of the ^[[1mmount ^[[22mcommand, is ^[[1mmount -t ^[[4m^[[22mtype^[[24m ^[[4mdevice^[[24m ^[[4mdir^[[0m This tells the kernel to attach the filesystem found on ^[[4mdevice^[[24m (which is of type ^[[4mtype^[[24m) at the directory ^[[4mdir^[[24m. The previous contents (if any) and owner and mode of ^[[4mdi r^[[24m become invisi‐ ble, and as long as this filesystem remains mounted, the pathname ^[[4mdir^[[24m refers to the root of the filesystem on ^[[4mdevice^[[24m. ^[[1mThe listing and help.^[[0m Three forms of invocation do not actually mount anything: ^[[1mmount -h^[[0m prints a help message ^[[1mmount -V^[[0m prints a version string ^[[1mmount ^[[22m[^[[1m-l^[[22m] [^[[1m-t ^[[4m^[[22mtype^[[24m] lists all mounted filesystems (of type ^[[4mtype^[[24m). The option -l adds the labels in this listing. See below.
lv. Why does this happen? Actually, that's no great mystery, it's just that your pager does not display the escape sequences used for formatting properly. To get rid of this odd looking character soup, you have to pass the parameter
# For less (additionally passed parameters -i, -s, -M and --shift 5) export LESS="-isMR --shift 5" # For lv export LV="-c"
You'll notice that you won't have this problem with
cat because these two programs don't buffer your text for the sake of scrolling up and down, so the contents are directly passed to the console. The price you have to pay for this advantage is the ability of scrolling bidirectional.
Special case: sudo
You might also encounter that your man pages work pretty well but display the garbage above when switching to a
sudo environment – even if you've explicitly set proper parameters. That's because
sudo does not inherit all your environment variables. To make your pager settings also available in
sudo environment, edit your
/etc/sudoers file. Type in
LV to the
env_keep directive, so this line looks like this:
Defaults env_keep="DISPLAY XAUTHORITY EDITOR LESS LV PAGER"
This will preserve all the specified variables.