Unix systems provide a number of commands to manage files and directories. Their strong point is the ability to use them in a rather simple manner against a group of files/directories meeting certain conditions. For example all the files satisfying specific criteria can be deleted or have their names changed en masse.

1. ls

It is very frequently used command. It redirects directory contents to standard output. If issued without any path it will list current directory (where we are at present).

adam@laptop:~/Documents/polishlinux.org$ pwd
/home/adam/Documents/polishlinux.org/

adam@laptop:~/Documents/polishlinux.org$ls
example.txt  all_about_console.txt

adam@laptop:~/Documents/polishlinux.org$ls /var/
backups  cache  crash  games  lib  local  lock  log  mail  opt  run  spool
tmp

Second example in the listing above shows ls invoked with „/var/” directory path. The ls command can reveal more detailed data if run with the „-l” parameter.

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
total 0
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 example.txt
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 all_about_console.txt

The first row of ls output shows total memory blocks taken by files from the directory. The next rows are ordered as follows:

• -rw-r--r-- file and directory permissions (more about permissions in the next part of this guide),
• number of hard links to the file,
• file owner then a group the owner belongs to,
• file length,
• time of latest modification,
• file/directory name.

ls command can display hidden files (so called „dot files”) as well. Names of the hidden files begin with a dot .. To show the files we need a -a parameter.

adam@laptop:~/Documents/polishlinux.org/examples$ ls -a
.  ..  .hidden_file  example.txt  all_about_console.txt

We are able to sort files using the ls command. The following parameters are available:

• -t – displays contents sorted according to modification time (from latest to oldest),
• -S – displays contents according to size (from biggest to smallest),
• -r – reverses sort order.

There is also a possibility to list directories recursively, that is, to display contents of all subdirectories found in a given directory – it is served by -R parameter.

adam@laptop:~/Documents/polishlinux.org/examples$ ls -R
.:
directory1  directory2  example.txt  all_about_console.txt

./directory1:
file1  file2

./directory2:
file3  file4

2. cd

Command change directory (cd) allows us to move through directory trees. It has one parameter – target directory path.

adam@laptop:~$cd /usr/bin
adam@laptop:/usr/bin$pwd
/usr/bin

It is worth noting the tilde character ~ here. It is one of the special characters recognized by all system shells. It denotes a path to the user’s home directory.

adam@laptop:/usr/bin$cd ~
adam@laptop:~$pwd
/home/adam

When we enter cd ~user's_name the system will move us to that user’s home directory.

adam@laptop:~$cd ~zoidberg
adam@laptop:/home/zoidberg$pwd
/home/zoidberg

Another special character, namely -, denotes former directory. To jump to parent directory one needs to issue cd .. at the prompt. Every directory in Unix systems, even empty one, comprises of at least two parts . (dot) .. (double dot). One dot . means the same directory, double dots .. is a link to the parent directory in the directory tree.

adam@laptop:~$cd ..
adam@laptop:/home$pwd
/home

Together with cd command it is worthwhile to introduce two new expressions – the relative path and the absolute path.

• Absolute path is a path started from root of a directory tree „/”, e.g. /home/adam.
• Relative path is a path whose name begin with current directory, e.g. adam/ from /home/.

3. mkdir, rmdir

Both command deal with directories – the first one creates them, the second one deletes them. Both make use of one parameter – a directory path. rmdir can only be performed on empty directories.

adam@laptop:~/Documents/polishlinux.org/examples$ mkdir directory
adam@laptop:~/Documents/polishlinux.org/examples$ ls
directory  example.txt  all_about_console.txt

adam@laptop:~/Documents/polishlinux.org/examples$ rmdir directory/
adam@laptop:~/Documents/polishlinux.org/examples$ ls
example.txt  all_about_console.txt

4. cp, mv, rm

• cp – (copy) is self explanatory, it uses at least two parameters: a source file and a target location to which the file will be copied,
• mv – (move) is used to change the location of directories and files or renaming them. It works in similar way as the cp command,
• rm – (remove) removes files/directories.

The commands: cp, mv, and rm have the following common parameters:

• -f force – forces removing a file, even if user has no rights to write the file,
• -i interactive – user will be asked to conform the operation,
• -b -backup – creates backup copy of a file to be overwritten (for cp and mv),

cp and rm commands can work in recursive mode – thanks to -r (-R, --recursive parameters.

adam@laptop:~/Documents/polishlinux.org/examples$ ls
directory1  directory2  directory3  example.txt  all_about_console.txt

adam@laptop:~/Documents/polishlinux.org/examples$ cp -R directory2/ directory3/
adam@laptop:~/Documents/polishlinux.org/examples$ ls directory3/
directory2

adam@laptop:~/Documents/polishlinux.org/examples$ rm -r directory3/
adam@laptop:~/Documents/polishlinux.org/examples$ ls
directory1  directory2  example.txt  all_about_console.txt

Beware! Command rm -r will remove ALL nested directories (not only empty ones), as in the above example.

5. ln

The „ln” utility creates a new directory entry (linked file) which has the same properties as the original file. Symlink (for short) can be thought as a sort of a short-cut known from Windows systems.

There are two kind of such links in Unix/Linux systems:

• symbolic links – referring to a file – they can be considered as Windows short-cuts,
• hard links – referring to a disk area, in other words to a physical disk area where a file is located.

If a file to which a symbolic link was created is removed (deleted), the symlink will be listed in red (provided the shell supports colors). If we delete a file with a hard link nothing will happen. The file won’t be erased until the number of hard links equals zero.

All symlinks are created by ln command issued with -s parameter.

adam@laptop:~/Documents/polishlinux.org/examples$ cat file
Symlink example.

adam@laptop:~/Documents/polishlinux.org/examples$ ln -s file
symlink_file

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
lrwxrwxrwx 1 adam adam    4 2007-06-01 19:11 symlink_file -> file
-rw-r--r-- 1 adam adam   22 2007-06-01 19:10 file

adam@laptop:~/Documents/polishlinux.org/examples$ rm file 

adam@laptop:~/Documents/polishlinux.org/examples$ ls
symlink_file

The above listing shows symbolic link in action. ls (with -l) tells us the number of hard links for a „file” file remained the same. Contrary to the below example where number of two hard links had diminished to one after a file deletion and the „file” file was still present.

adam@laptop:~/Documents/polishlinux.org/examples$ ln file hardlink_file

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
-rw-r--r-- 2 adam adam   22 2007-06-01 19:11 hardlink_file
-rw-r--r-- 2 adam adam   22 2007-06-01 19:11 file

adam@laptop:~/Documents/polishlinux.org/examples$ rm file

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
-rw-r--r-- 1 adam adam   22 2007-06-01 19:11 hardlink_file

6. touch

This simple command has two applications. If we add as a parameter an existing file, the command will change the file’s modification time. If the file does not exist it will be created.

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l example.txt
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 example.txt

adam@laptop:~/Documents/polishlinux.org/examples$ touch example.txt
adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
total 0
-rw-r--r-- 1 adam adam 0 2007-06-07 13:27 example.txt
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 all_about_console.txt

adam@laptop:~/Documents/polishlinux.org/examples$ touch new.txt
adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
total 0
-rw-r--r-- 1 adam adam 0 2007-06-07 13:28 new.txt
-rw-r--r-- 1 adam adam 0 2007-06-07 13:27 example.txt
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 all_about_console.txt

When touch is invoked with -c or --no-create parameters we will prevent the file creation. Other parameters -d and -t change the access and modification times to the specified time. The „-t” needs to have a date specified in the form of [[CC]YY]MMDDhhmm[.ss].

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
total 0
-rw-r--r-- 1 adam adam 0 2007-06-07 13:28 new.txt
-rw-r--r-- 1 adam adam 0 2007-06-07 13:27 example.txt
-rw-r--r-- 1 adam adam 0 2007-05-30 11:31 all_about_console.txt

adam@laptop:~/Documents/polishlinux.org/examples$ touch -t
200706101200 new.txt

adam@laptop:~/Documents/polishlinux.org/examples$ touch -d
"last monday" example.txt

adam@laptop:~/Documents/polishlinux.org/examples$ touch -d
"2 days ago 12:00" all_about_console.txt

adam@laptop:~/Documents/polishlinux.org/examples$ ls -l
total 0
-rw-r--r-- 1 adam adam 0 2007-06-10 12:00 new.txt
-rw-r--r-- 1 adam adam 0 2007-06-04 00:00 example.txt
-rw-r--r-- 1 adam adam 0 2007-06-05 12:00 all_about_console.txt

7. df, du

Next shell commands – df and du – display free disk space – df for the whole filesystem and du for a given file. If run with -h attribute they will show the size in human readable format rather than blocks. For example:

adam@laptop:~/Documents/polishlinux.org/examples$ df -h
Filesystem    Size     Used  Avail Capacity  Mounted on
/dev/sda5      40G     34G   4,0G     90%     /
varrun        502M    136K   502M      1%     /var/run
varlock       502M       0   502M      0%     /var/lock
procbususb    502M    148K   502M      1%     /proc/bus/usb
udev          502M    148K   502M      1%     /dev
devshm        502M       0   502M      0%     /dev/shm

And example for the du command:

adam@laptop:~/Documents/polishlinux.org/examples$ du -h error.txt
4,0K    error.txt

8. echo

This simple command returns a text or a message attached to it as a parameter.

adam@laptop:~$echo Hello World!
Hello World!
adam@laptop:~$

„Echo” adds at the end of a line a newline character. To get rid of it one should run the command with -n. For example:

adam@laptop:~$echo -n Hello World!
Hello World!adam@laptop:~$

When the text is enclosed in quotation marks it will be interpreted directly (but we will tell about it later).

9. pwd

pwd (print working directory) shows full path of the current directory.

adam@laptop:~$pwd
/home/adam
adam@laptop:~$

10. cat

cat command can be used for a file creation. To put it more correctly it can be used to redirect standard input to a file and to display it on standard output. Files are created in the following way:

adam@laptop:~$cat > file.txt
Very interesting text.
<Ctrl+D>

We will expand on the redirection operator > later in this guide. To display file contents it suffices to type in cat file_name.

adam@laptop:~$cat file.txt
Very interesting text.

One of its parameters prints line numbers when „cat” is used to shows a file contents.

adam@laptop:~$cat -n file.txt
     1  Very interesting text.

cat enables us to concatenate several files together, indeed cat is shorthand for concatenate. In the following example five files are to be merged in one „file.iso” file.

cat file1 file2 file3 file4 file5 > file.iso

11. wc, head, tail

These three commands are used to process text strings. wc command displays a number of lines, a number of words, and a number of bytes for a given file.

adam@laptop:~/Documents/polishlinux.org$ wc all_about_console.txt
94  908 6828 all_about_console.txt

The numbers in the above example refer to (from left to right) – lines, words, and bits – found in all_about_console.txt file.

Two successive commands display accordingly: the head – a file beginning, and the tail – a file tail (ending). Both commands show 10 rows of a file by default. The number can be changed with the -n number parameter.

adam@laptop:~/Documents/polishlinux.org$ head -n 1 example.txt
The first row of the text.

adam@laptop:~/Documents/polishlinux.org$ tail -n 2 example.txt
The last but one row of the text.
The last row of the text.

12. less

less enables us to scroll a text up and down on the screen.

adam@laptop:~$less file.txt

Running the command as shown above will display file.txt contents on a screen. Entering :f during browsing the file will show interesting details, for example the line number of the line displayed at the upper edge of monitor’s screen or the file size. Joining the command with the „cat -n” command will display the contents supplemented by line numbers placed at the left side of a screen.

adam@laptop:~$cat -n file.txt | less

Entering :q quits the program (command).

This article is part of the Command line tricks series.
Go back to: Shell, terminal, console — the basics »
Go further to: UNIX Pipes, Streams and Redirections Explained »

Translated by P2O2, Proof-read by trashcat