Files and directories

vim

vim.nox is a minimal flavor of vim for server environments that has scripting language support.

command mode lets you move around the file
insert mode lets you edit text
visual mode lets you select text for copy/paste
Add default settings to .vimrc in /home dir. Ex: set number

CTRL + z        # put vim session in background
fg              # bring vim session to foreground
A               # insert mode at the end of the current line
gg              # go to beginning of first line of file
G               # go to beginning of last line of file
:%s/original string/new string/g    # global replace
:! <shell command>                  # run <shell command> while vim is open
:set number     # add line numbers
:set nonumber   # remove line numbers

# --- Split files --- #
:sp /path/to/file                   # split current window with file window
:vs /path/to/file                   # vertical split window and file
CTRL + w, CTRL + w                  # switch between open files

Streams

STDIN, STDOUT, STDERR

Every program is a process, and every process has 3 distinct file descriptors:

stdin: File descriptor 0. Default source for input to a program
stdout: File descriptor 1. Default place for sending output (the console)
stderr: File descriptor 2. Where error messages are written

Redirection and piping

Redirection is when you change the input and outputs of a program without modifying the program.

Operator	Description	Example
`>`	Sends the output of the value on the left to the value on the right.	`ls -la > listing.out`
`<`	Sends the value on the right to the STDIN of the value on the left.	`program < input.txt`
`2>`	Redirect STDERR messages to the value on the right.	`cp -r /etc/a /etc/b 2> err.msg`
`2>&1`, `&>`	Send output to STDOUT and STDERR. Place this	`XXXXXXXXXXX`
`1> file 2>`	Redirect more than one stream	`find / -name 'syslog' > stdout.txt 2> stderr.txt`

The following command sends input.txt to the STDIN of program, and sends the output to output.out:

program < input.txt > output.out

To distinguish between STDOUT and STDERR, use the file descriptor 2>. The following command redirects error messages to err.msgs:

program 2> err.msgs

Or, combine all redirection methods:

program < input.txt > output.out 2> err.msgs

Combined redirect: To send STDERR to the same location as STDOUT, combine the error messages with the standard output:

program < input.txt > output.out 2>&1
# shorthand
program < input.txt &> output.out

The previous command sends input.txt to program, then sends the STDOUT and STDERR to output.out. The shorthand is more clear.

Commonly, you can discard STDOUT by sending it to /dev/null:

program < input.txt > /dev/null

The tee command sends output to STDOUT and the file that follows the command:

program < input.txt | tee results.out # the -a option allows tee to append to the file

To append to a file, use the >> operator:

program < input.txt >> appended.file

To append STDOUT and STDERR, use &>>:

program < input.txt &>> appended.file

Running commands in the background

Use the & operator at the end of the command to run it in the background:

ping 10.20.30.40 > ping.log &

When you run a task in the background, send both STDOUT and STDERR so the task doesn’t log everything to the console:

ping 10.20.30.40 &> ping.log &

To bring a job back to the foreground, use jobs to list running tasks, then use fg with the corresponding task number:

ping 192.168.10.56 &> ping.log &
[1] 7452
jobs
[1]+  Running                 ping 192.168.10.56 &> ping.log &
fg 1
ping 192.168.10.56 &> ping.log
^C

Stop a running job with Ctrl+z, and send it to the background with bg:

ping 192.168.10.56 &> ping.log &
[1] 7572
fg 1
ping 192.168.10.56 &> ping.log
^Z
[1]+  Stopped                 ping 192.168.10.56 &> ping.log
bg 1
[1]+ ping 192.168.10.56 &> ping.log &
fg 1
ping 192.168.10.56 &> ping.log
^C

Links

Links let you reference a file somewhere else on the system to create shortcuts:

inode is a database object that contains metadata about a file or directory, such as the woner, permissions, last modified date, type,…

Hard link

Hard links are duplicates of the original file:

Both files point to the same inode
Because they share an inode, you cannot reference files on another filesystem
Can only create hard links to file
You can move either file anywhere in the fs and it will not break the link bc hard links use inodes

An object that points to another file on the same disk or volume

A hard link is a file that has one inode number but at least two different filenames.
- One inode means its a single data file on the filesystem (single filesystem location on a disk partition)
- Two filenames means that it can be accessed in multiple ways
References physical location on disk
Shares inode with original file
Is not deleted if you delete original file and references the same data
Original file must exist, linked file cannot exist
Both files share the same data, exist on same filesystem in any directory
Unlink the linked file with unlink LINKED-FILE

Use case: file backup when there is not enough space to backup the file. If someone deletes one of the files, its not permanently deleted.

ln ORIGINAL LINKED-FILE
unlink LINKED-FILE

# create original
touch original-file.txt
original-file.txt

# link files
ln original-file.txt hard-link-file.txt 

# different file for comparison
touch single-file.txt

# view inode and links
ls -iog
total 0           (*)
4868127 -rw-rw-r-- 2 0 Mar 17 09:32 hard-link-file.txt
4868127 -rw-rw-r-- 2 0 Mar 17 09:32 original-file.txt
4868128 -rw-rw-r-- 1 0 Mar 17 09:33 single-file.txt

# unlink
unlink hard-link-file.txt 

# linked file is gone
ls -iog
total 0           (*)
4868127 -rw-rw-r-- 1 0 Mar 17 09:32 original-file.txt
4868128 -rw-rw-r-- 1 0 Mar 17 09:33 single-file.txt

Symbolic (soft) link

A symbolic link is a pointer to the original file’s path, not a clone. It’s an object that points to an object somewhere else on the system:

Linked files have different inodes
Cannot move original file, because symlinks just point to a path location
Can link across filesystems
Has unique inode
Reference abstractions, not physical place on disk
Can point to file or dir
Can reference an object on another disk or volume
Is not deleted if you delete the original file, but won’t reference anything
Typically a soft link is a pointer to a file or directory that might be on another filesystem
Different inodes bc they point to different data
If a soft link points to a file that was deleted or removed, that is a security risk in the event that a malicious file is put in the original file’s place.
View or execute a resource from the local dir

Use case

For apache, you can delete the symlink in /sites-enabled/ instead of the config in /sites-avaiable/, then readd the link when you want to reactivate the site.

ln -s ORIGINAL LINKED-FILE

# current contents
ls -iog
total 0
4868127 -rw-rw-r-- 1 0 Mar 17 09:32 original-file.txt

# create soft link
ln -s original-file.txt soft-link-file.txt

# linked, but different inode and link numbers
ls -iog
total 0           (*)
4868127 -rw-rw-r-- 1  0 Mar 17 09:32 original-file.txt
4868128 lrwxrwxrwx 1 17 Mar 17 09:45 soft-link-file.txt -> original-file.txt

# rm link
unlink soft-link-file.txt

# linked file is deleted
ls -iog
total 0
4868127 -rw-rw-r-- 1 0 Mar 17 09:32 original-file.txt