Files and directories

Streams

STDIN, STDOUT, STDERR

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

Stream	File descriptor	Description
`stdin`	0	The default source of input to a program
`stdout`	1	The default destination for program output
`stderr`	2	The default destination for error messages

Redirection and piping

Redirection lets you change where a program reads input from and sends output to, without modifying the program itself:

Operator	Description	Example
`>`	Redirects STDOUT to a file	`ls -la > listing.out`
`<`	Sends a file to a program as STDIN	`program < input.txt`
`2>`	Redirects STDERR to a file	`cp -r /etc/a /etc/b 2> err.msg`
`2>&1`	Redirects STDERR to the same location as STDOUT	`program > output.out 2>&1`
`&>`	Shorthand to redirect both STDOUT and STDERR	`program &> output.out`
`>` and `2>`	Redirect STDOUT and STDERR to separate files	`find / -name 'syslog' > stdout.txt 2> stderr.txt`

Redirect STDIN and STDOUT

The following command sends input.txt to program as STDIN and redirects STDOUT to output.out:

program < input.txt > output.out

Redirect to separate files

To separate STDOUT and STDERR into different files, combine both operators:

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

Combine STDOUT and STDERR

To send STDERR to the same destination as STDOUT, append 2>&1. The &> shorthand is equivalent:

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

Discard STDOUT

To discard STDOUT entirely, redirect it to /dev/null:

program < input.txt > /dev/null

Split output with tee

The tee command sends output to both STDOUT and a file. Use the -a flag to append instead of overwrite:

program < input.txt | tee results.out
program < input.txt | tee -a results.out

Append output

To append STDOUT to a file, use >>. To append both STDOUT and STDERR, use &>>:

program < input.txt >> appended.file
program < input.txt &>> appended.file

Running commands in the background

Run a command in the background

Append & to any command to run it in the background. Redirect both STDOUT and STDERR to a file so output is captured and not lost:

ping 10.20.30.40 &> ping.log &

Bring a background job to the foreground

Run jobs to list background tasks, then run fg with the job number to bring it forward:

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

Suspend and resume a job

Stop a running foreground job with Ctrl+z. Send it to the background with bg, then bring it forward again with fg:

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

Linux supports two types of links: hard links and symbolic links. Both let you access a file from more than one location, but they work differently at the filesystem level.

An inode is a filesystem object that stores metadata about a file or directory, including the owner, permissions, last modified date, and file type. Understanding inodes is key to understanding how both link types work.

Hard link

A hard link is a filename that points to the same inode as another file. Both filenames reference the same data on disk. Deleting one does not remove the data as long as the other filename still exists.

Hard links have the following constraints:

You can only create hard links to files, not directories
Both filenames must exist on the same filesystem
Moving either filename anywhere on the filesystem does not break the link

File backup

Hard links work well as file backups when disk space is limited. If one filename is deleted, the data remains accessible through the other.

Syntax

ln ORIGINAL LINKED-FILE
unlink LINKED-FILE

Example

Create the original file and a separate file for comparison:
```
touch original-file.txt
touch single-file.txt
```

Create a hard link:

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

View the inodes. Notice that hard-link-file.txt and original-file.txt share inode 4868127:

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

Remove the hard link:
```
unlink hard-link-file.txt
```

The original file remains and the link count drops to 1:

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 another file’s path, not a copy of its data. Because it references a path rather than an inode, it has a unique inode and can point to files on other filesystems.

Symbolic links have the following characteristics:

Linked files have different inodes
You can link to files or directories
You can link across filesystems and disk volumes
Moving or deleting the original file breaks the link, but does not delete the symbolic link itself

Dangling symlinks

If the original file is deleted and a malicious file is placed at the same path, the symbolic link will point to it. Audit dangling symlinks regularly.

Apache virtual hosts

For Apache, you can disable a site by removing the symlink in /etc/apache2/sites-enabled/ without deleting the config in /etc/apache2/sites-available/. Re-add the link to reactivate the site.

Syntax

ln -s ORIGINAL LINKED-FILE
unlink LINKED-FILE

Example

View the current directory contents:

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

Create a symbolic link:

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

View the inodes. Notice that soft-link-file.txt has a different inode from original-file.txt:

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

Remove the symbolic link:
```
unlink soft-link-file.txt
```

The original file remains:

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