Basic toolkit

Objectives

• Let's dig into the most important BASH commands
• We'll do a type-along session

Like videos?

Below usage of the command line is discussed in text. If you prefer video, here is how to use the command-line on the UPPMAX Bianca cluster.

We will cover these commands

Navigation and file management

1. pwd   present directory
2. ls  list content
3. cd  change directory
4. mkdir  make directory
5. cp  copy
6. scp  securely remotely copy
7. mv  move
8. rm  remove
9. rmdir  remove empty directory

Read files and change file properties

1. cat  print content on screen
2. head  print first part
3. tail  print last part
4. less  browse content
5. tar  compress or extract file
6. chmod  change file permissions
7. man  info about a command

File system Navigation

pwd — where are you now? “Print name of current/Working Directory`

$ pwd

$ pwd -P

• -P gives you the physical path,
  • ignores how you got there

ls — list directory contents

Type ls to display the contents of the current directory.

$ ls -a

-a also shows hidden files and directories.

$ ls -l

-l gives you listed and detailed information.

$ ls -lt

-lt sorts things by time modified.

$ ls -lrt

-r gives reversed order, so in this case newest in last line.

$ man ls

• for complete information about a command.
• TIP: -$ man <command> works for almost any command!
  • scroll with arrows and quit with q.

cd — Change the shell working Directory

• To change directory, use cd <target>

Warning

• Some of following steps will only be available for the Introduction course members.
• These involve the `/proj/introtouppmax`` directory

$ cd /proj/introtouppmax

$ pwd

$ ls

$ cd labs

$ pwd

Challenge

• Experiment with cd.
• Try adding <spaces> or extra / in various places.

• Use tab completion to avoid typos and typing ls a lot.

• Figure out the use of the following:

$ cd -

$ cd ..

$ cd

$ cd ~

Solution

• cd - : goes back to your last directory

• cd .. : goes a level up in the hierarchy

• cd : goes to home directory

• cd ~ : also goes to home directory

Copy, Create, Move

mkdir — make directories

Warning

• Make sure you’re in your home directory by cd ~

• Create a new directory uppmax-intro

$ cd ~
$ mkdir uppmax-intro

• Go in there:

$ cd uppmax-intro/

cp — copy files and directories

• Copy files with: cp <source> <target>
• Set target to . to keep name and to point at present directory.

$ cp /proj/introtouppmax/labs/linux_tutorial/ .

• Well, that didn’t work. What does the error say?
• So... try

$ cp -r /proj/introtouppmax/labs/linux_tutorial/ .

-r is for recursive, meaning including files and subdirectories!

• Move to your just created linux_tutorial/

$ cd linux_tutorial

• Make a copy of the file newfile in the same directory:

$ cp newfile copyfile

scp — secure copy (remote file copy program)

• Linux/MacOS: To copy data to/from Rackham, you can use scp from the terminal on your local machine:

Download from Rackham

• Download

[bob@macbook]$ scp bob@rackham.uppmax.uu.se:~/mydata copyofmydata

[bob@macbook]$ scp bob@rackham.uppmax.uu.se:~/mydata .                      # (keeping file name)

Example

Download the file first.txt

• In your local terminal:

[bob@macbook]$ scp <username>@rackham.uppmax.uu.se:~/first.txt .                      # (keeping file name)

Upload to Rackham

• Upload from present directory on local machine to your home directory on cluster.
  • Example:

[bob@macbook]$ scp myinput bob@rackham.uppmax.uu.se:~/copyofmyinput

[bob@macbook]$ scp myinput bob@rackham.uppmax.uu.se:~/                      # (keeping filename)

Example

upload the file first.txt after some modification

1. Open the file you just downloaded in any editor.
2. Add a row, like: A new row
3. Save and quit.
4. Upload your file but save it as second.txt on Rackham. In your local terminal:

[bob@macbook]$ scp first.txt <username>@rackham.uppmax.uu.se:~/second.txt                     # (new filename)

Seealso

• Rackham file transfer using scp

mv — move/rename file

• Moving files works just like copying files:
• mv <source> <target>
• Move the copy you just made to another place:

$ mv copyfile ../

• Rename it.

$ mv ../copyfile ../renamedfile

Archiving

tar — archiving and compression

• We’re going to need more files. Let's extract the tar.gz file (tared and gzipped file)

$ tar -vxzf files.tar.gz

• The flags mean: - v*erbosely - extract - gzipped - f**ilename
• Order of flags may matter!
  • f should be in the start or in the end!
• You should see a list of files being extracted

Tip

• To compress use the flag -cinstead of -x

$ tar -czfv <tar file> <path/to/directory/file(s)-or-directory>

Deleting

rm — delete files or directories

Note

• Tip: make "rm" ask if you really want to erase:
• Within a session: Type in the command prompt

alias rm='rm -i'

- Override asking with

rm -f <>

- Do you want this to be the case everytime you start a new session? - Edit file .bashrc in /home directory by adding the above alias line on any but the first line. - These steps will also work for mv and cp.

• Deleting files works just like copying or moving them: rm <target>

• Try it out:

$ rm ../renamedfile

$ rm this_is_empty

• hmmmm...

rmdir — delete an empty directory

• We need another command to delete directories

$ rmdir this_is_empty

$ rmdir this_has_a_file

• Problem again??

• Is there a way to use rm to delete directories?

Solution

• Recursive commands -r are applied to directories and their contents

  $ rm -r this_has_a_file
  

Help

man — manual, look up the right flags

• Nobody can remember whether it’s -R or -r for recursive, or if -f lets you choose a file or forces an action.

$ man ls

• shows you how to use ls and all its options
• Type /<keyword> to search for a keyword, use n (forward) and ´N` (backward) to scan through hits.
• Scroll with arrows.
• Type q to quit.

Challenge

• Spend some time now to browse the man pages for the commands you’ve just learned!

Let’s get wild with Wildcards

$ ls many_files

$ ls many_files/*.txt

$ ls many_files/file_1*1.docx

• Want to clean out temporary files ending in .tmp in all the subdirectories?

Warning

• It could be wise to do ls -a */*.tmp first to see what will be deleted...

$ rm */*.tmp

Challenge

• Exercise: Create a new directory and move all .txt files in many_files to it.

Reading files

• In Linux, you can (if you wish) also display files without being able to change them

$ cd old_project

$ ls

• Hmm, which of these files are useful?

cat - concatenate files and print on the standard output

• cat dumps the contents of files to the terminal as text

$ cat the_best

• Yummy!

$ cat a

• What's this????

• Concatenate files with this wizardry:

$ cat a the_best > combinedfiles.txt

• File a is written first and the_best is appended

head — display the top (heading) of a file

$ head a

• You can choose how many lines to display (default 10)

$ head -n 4 a

tail — display the end of a file

• Tail is the same as head, but for the other end.

$ tail -n 5 a

• Handy to look at log files or to figure out the structure of a text file.

less — read a whole file

• cat doesn’t really work for long files

 $ less a

• Search with /<keyword> and n/N
• Hit q to quit.
• scroll with arrows.

• man uses `less!

  “less is more`

History

• history shows previous commands
• You can rerun earlier commands by:
  • copy-pasting and pressing <enter>
  • !990 will run the command of line 990 of last history output.
• Search for earlier commands you just remember parts of:
  • history | grep 'jobstats'
• More info

File permissions

Example

$ ls -l

drwxrwxr-x 2 marcusl marcusl 4096 Sep 19 2012 external_hdd
-rwxr-xr-x 1 marcusl marcusl 17198 Jul 16 14:12 files.tar.gz

• Leading symbol:
  • d directory
  • - regular file
  • l symbolic link (more on this tomorrow)
  • Others exist, but you can ignore them for now

$ ls -l

  drwxrwxr-x 2 marcusl marcusl 4096 Sep 19 2012 external_hdd

  -rwxr-xr-x 1 marcusl marcusl 17198 Jul 16 14:12 files.tar.gz

• Three sets of “rwx` permissions

  • rwx: r ead, w rite, ex ecute
  • User: the user account that owns the file (usually the one that created it)
  • Group: the group that owns the file (usually the project group in /proj/xyz or the user’s group elsewhere)
  • Others: everyone else on the system (literally a thousand strangers)

• r - read

  • Files: Read the contents of the file
  • Directories: List the files in the directory

• w - write

  • Files: Modify the file
  • Directories: Add, rename, or delete files in the directory

• x - execute

  • Files: Run the file as a program
  • Directories: Traverse the directory (e.g. with “cd`)

Changing permissions

chmod — change file mode bits

If you own, i.e. created, the file or directory, you can modify the content.

Common issues

• Files with w can be modified and destroyed by accident. Protect your data!
• If you want to share data or scripts with a person not in your project (e.g. support staff like me), you can!
• If you want to keep non-members from even seeing which files you have, you can!

Syntax

chmod <mode> <files>

• <mode> is of the form: For whom, Modify, What permission(s)
• For whom?
  • u: user/owner
  • g: group, often the members to a certain project
  • o: others
  • a: all
  • if not set changes are applied for user AND group
• Modify?
  • +: add permissions,
  • -: remove
  • =: set equal to
    • = usually causes unmentioned bits to be removed except that a directory's unmentioned set user and group ID bits are not affected.
• What permissions?
  • r, w, x, i.e. the actual permission

Examples

• <mode> can be e.g.:

  • u+x : lets You (owner) run a script you just wrote
  • -w : no write permissions for owner+group
    • warning: if w was already set for others it will be kept!!
  • +rw : let user and group members read and edit this file, not others if not already set
  • =xw : let group members go into your directory and put files there, but not see which files are there, others are not affected
  • a=xw : set xw for everyone

• chmod takes flags as usual, e.g.

  • -R for recursive (i.e. all files and sub-directories therein)

chmod 755 style — binary sum — octal bit mask

• Online, you will come across e.g. chmod 755 <file/dir>. What does this mean? It’s an "octal bit mask`:

• Each digit corresponds to the binary sum for the owner, group and others, respectively.

  • 7 = 4 + 2 + 1 = r + w + x All permissions
  • 5 = 4 + 0 + 1 = r + + x Read and execute permission

• 755 then means all permissions for owner, but limiting write permissions for the group and all others

• What number would rw be?

Solution

6

chmod — Hands-on

• In your locally created linux_tutorial directory, find important files and old saved data that you wouldn’t want to lose (imagine).
• Directories: important_results/, old_project/
• File: last_years_data
• Use chmod to remove write permission from those files and directories (use the -R flag (not -r) to also do the files in the directories).
• Take a moment to play around with chmod and explore the effects of permissions on files and directories.

Solution

$ chmod -wR <target>

Links

• A free online book about Linux: 'The Linux Command Line'.