Learning Linux

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1. Basic commands

Navigate the filesystem & manipulate directories and files.

View the contents of files.

Monitor system status.

Use commands.

2. Run commands in shell

2.1 IO redirection

Redirect output.

# overwriting the file
command > file
# appending output to the file
command >> file
# suppressing output.
command > /dev/null

Redirect input.

command < inputfile
# inline input redirection
command << marker
data
marker

There is no dedicated redirection operator for redirecting errors. We have to refer to its file descriptor. The bash shell reserves the first three file descriptors as STDIN (0), STDOUT (1) and STDERR (2).

command 2> file

Redirect both output and errors.

command 1> file1 2> file2
command &> file

2.2 Pipes

command1 | command2

Use tee to construct a T pipe.

# the output of command1 will be saved to file and passed to command2
command1 | tee file | command2

Some commands can be a filter in pipes.

2.3 Expansion

Wildcards.

Class include [[:alnum:]], [[:alpha:]], [[:digit:]], [[:lower:]] and [[:upper:]].

Pathname expansion.

# list all txt files
ls *.txt
# tilde for home directory
cd ~

Arithmetic expansion. It only supports integer arithmetic.

$((expression))
# the result is 2
echo $((5/2))

Brace expansion.

# the result is "file1 file2 file3"
echo file{1,2,3}
# use range
echo file{1..3}
# nested brace expansion
# the result is "file13 file14 file23 file 24"
echo file{1{3,4},2{3,4}}

Variable expansion.

$var

Command substitution.

$(command)
`command`

Double quoting.

# all characters are treated as ordinary characters
# except dollar sign, backslash and backquote
echo "~ will not be expanded as home directory"
echo "$var still show the value of var"
echo "backslash is used to escape special characters such as \$"

Single quoting.

# suppress all expansion
echo '$(ls)'

2.4 Jobs

Run commands one after one.

command1; command2

Commands are run in foreground mode by default. Run commands in background mode.

# run in background
command &
# show jobs launched from current terminal
jobs
# return the job to foreground
fg %n

If you exit the terminal session, all jobs, even in background mode, will be interrupted. To avoid this, run scripts without a hang-up.

nohup command &

The nohup command disassociates the process from the terminal, thus it redirects STDOUT and STDERR to nohup.out file. Be careful if you run multiple commands using nohup. All output is appended to nohup.out.

Use Ctrl+C to interrupt the current-running job. Use Ctrl+Z to stop the current running job. Usually we use Ctrl+Z to move the current-running job to background.

command
# stop by Ctrl+Z
# show the job number
jobs
# move to background
bg %n
# or return to foreground
fg %n

Use kill and killall to send other signals to processes.

3. Environment variables

Linux environment variables help to store system information, temporary data and configuration information.

Define a environment variable.

variable_name=variable_value
echo $variable_name

Define a global environment variable.

variable_name=variable_value
export variable_name
echo $variable_name

Add new path.

PATH=$PATH:/your/new/path

4. File permissions

The permission of each file is in form of -rwxrwxrwx. The first character defines the type of the file.

• - for files
• d for directories
• l for links
• c for character devices
• b for block devices
• n for network devices

After that define the permissions of the owner, the group owning the file, and everyone else.

5. Filesystem

The default filesystem of linux is ext filesystem. The ext filesystem uses an inode system to create a inode table, to store file information, including filename, file size, the owner & group, file permission and pointers to disk block.

The ext4 filesystem, the newest ext filesystem, is a journaling filesystem. It write file changes into a temporary file first. After data is successfully written to the storage device and the inode table is updated, the journal file is deleted. Other journaling filesystems include XFS and ReiserFS.

An alternative to journaling is copy-on-write. For modifying data, a snapshot is used. Even when data modification is completed, the old data is never overwritten. COW filesystems include ZFS and Btrfs.

Linux Logical Volume Manager (LVM) is used for managing logical volumes.

6. Utilities in Linux world

6.1 Package management system

Use packege management system to install software.

6.2 Network

6.3 Archive

6.4 Find files

APPENDIX

Common Linux Directory Names

Linux Process Signals