Less (Unix)


less is a terminal pager program on Unix, Windows, and Unix-like systems used to view the contents of a text file one screen at a time. It is similar to more, but has the extended capability of allowing both forward and backward navigation through the file. Unlike most Unix text editors/viewers, less does not need to read the entire file before starting, resulting in faster load times with large files.

History

initially wrote less during 1983–85, in the need of a version of more able to do backward scrolling of the displayed text. The name came from the joke of doing "backwards more." Originally, less was developed for Unix, but it has been ported to a number of other operating systems, including MS-DOS, Microsoft Windows, OS/2, and OS-9, as well as Unix-like systems such as Linux.
To help remember the difference between less and more, a common joke is to say, "less > more," implying that less has greater functionality than more. A similar saying is that "less is more, more or less".

Usage

less can be invoked with options to change its behaviour, for example, the number of lines to display on the screen. A few options vary depending on the operating system. While less is displaying the file, various commands can be used to navigate through the file. These commands are based on those used by both more and vi. It is also possible to search for character patterns in the file.
By default, less displays the contents of the file to the standard output. If the file name argument is omitted, it displays the contents from standard input. If the output is redirected to anything other than a terminal, for example a pipe to another command, less behaves like cat.
The command-syntax is:
less

Frequently used options

KeyCommand
Next Page
Next half Page
Previous Page
Previous half Page
Edit Content
or Next Line
Previous Line
Top of file
End of file
Follow Mode. Interrupt to abort.
or First Line
or Last Line
Line
Forward Search for . Text is interpreted as a regex.
Backward Search like
Next Search Match
Previous Search Match
Turn off Match Highlighting
Toggle option , e.g., toggles option to match case in searches
Set Mark
Go to Mark
or File information
Next file
Previous file
Help. This is presented with less, to quit.
Quit

Examples


less -M readme.txt # Read "readme.txt"
less +F /var/log/mail.log # Follow mode for log
file * | less # Easier file analysis
less -I -g void *.c # Case insensitive search for "void" in all.c files

Memory considerations

The --buffers=n and --auto-buffers options control how much memory less may use to buffer inputs. This is most relevant when less is directly accessing a named file that is modified or deleted while less is still running, and when less is receiving data from a pipe and the data can not be randomly accessed or regenerated. On the other hand, unlimited buffering means that less will request as much memory as it is fed data, which could drive the system into using virtual memory and swapping a lot of data between RAM and disks, or even further into memory exhaustion where any application on the host requesting memory may have that request denied, or may crash when attempting to access memory that the OS promised but can't find when the application actually attempts to use it and a page-fault occurs. For this reason, some companies/organisations insist that less be used only with fixed buffering - or not at all - on production machines.