Erle Robotics Unix Introduction Gitbook Free

Other useful Unix commands

df

The df command reports on the space left on the file system. For example, to find out how much space is left on the fileserver, type

df .

du

The du command outputs the number of kilobyes used by each subdirectory. If you want to find out which directory has the most files. In your home-directory, type

du -s *

The -s flag will display only a summary (total size) and the * means all files and directories.

working with erlerobot:

gzip

This reduces the size of a file, thus freeing valuable disk space. For example, type

ls -l science.txt

and note the size of the file using ls -l. Then to compress science.txt, type

gzip science.txt

This will compress the file and place it in a file called science.txt.gz

To see the change in size, typels -l again.

To expand the file, use the gunzip command.

gunzip science.txt.gz

zcat

zcat will read gzipped files without needing to uncompress them first.

zcat science.txt.gz

If the text scrolls too fast for you, pipe the output though less .

zcat science.txt.gz | less

file

file classifies the named files according to the type of data they contain, for example ascii (text), pictures, compressed data, etc.. To report on all files in your home directory, type

file *

diff

This command compares the contents of two files and displays the differences. Suppose you have a file called file1 and you edit some part of it and save it as file2. To see the differences type

diff file1 file2

Lines beginning with a < denotes file1, while lines beginning with a > denotes file2.

find

This searches through the directories for files and directories with a given name, date, size, or any other attribute you care to specify. It is a simple command but with many options - you can read the manual by typing man find.

To search for all files with the extention .txt, starting at the current directory (.) and working through all sub-directories, then printing the name of the file to the screen, type

find . -name "*.txt" -print

To find files over 1Mb in size, and display the result as a long listing, type

find . -size +1M -ls

working with erlerobot:

history

The C shell keeps an ordered list of all the commands that you have entered. Each command is given a number according to the order it was entered.

history (show command history list)

If you are using the C shell, you can use the exclamation character (!) to recall commands easily.

!! (recall last command)

!-3 (recall third most recent command)

!5 (recall 5th command in list)

!grep (recall last command starting with grep)

You can increase the size of the history buffer by typing

set history=100

By using the navigation keys on your keyboard (up, down) you can recall the previous commands.

You can also use the recursive search. If you type ^R ([control +R]) and afterward the letter c, the shell shows you the previus commands beginning with c.

Shutdown

If you want to turn off a Linux machine, type:

shutdown -h now

bc

The command bc open an arbitrary precision calculator language. To use it type:

 bc
 (a text should appear on yout screen)
 7*2
 (the result should be 14)
 quit (you get out of the calculator)

Another possibility of evaluating this is typing:

 echo '7*2' | bc -l

Also, it can evaluate expressions like exp1 < exp2 returning a 1 if it is true and a 0 if it is false. For furtherinformation type:

man bc