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[[Category:OPS102]]
== Standard File Descriptors ==
The same example on Windows:
> '''date /t >now.txt ''' > '''type now.txt'''
2028-09-26
> '''date /t >>now.txt ''' > '''type now.txt'''
2028-09-26
2028-09-26
> '''date /t >>now.txt''' > '''type now.txt'''
2028-09-26
2028-09-26
2028-09-26
> '''date /t >now.txt ''' > '''type now.txt'''
2023-09-26
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 one
-rw-r--r--. 1 chris chris 0 Sep 26 13:17 two
== Piping ==
Piping is a special case of redirection, where the output of one command is connected to the input of another command. This is set up using the vertical-bar (pipe) symbol: <code>|</code> (this may look like a solid or a dashed vertical line, depending on the terminal font in use).
For example, on Windows, the output of the <code>help</code> command is more than one screen long. You could pipe the output of the <code>help</code> command into the input of the <code>more</code> command to view one screen of text at a time:
> '''help|more'''
For more information on a specific command, type HELP command-name
ASSOC Displays or modifies file extension associations.
ATTRIB Displays or changes file attributes.
BREAK Sets or clears extended CTRL+C checking.
BCDEDIT Sets properties in boot database to control boot loading.
CACLS Displays or modifies access control lists (ACLs) of files.
CALL Calls one batch program from another.
CD Displays the name of or changes the current directory.
CHCP Displays or sets the active code page number.
CHDIR Displays the name of or changes the current directory.
CHKDSK Checks a disk and displays a status report.
CHKNTFS Displays or modifies the checking of disk at boot time.
CLS Clears the screen.
CMD Starts a new instance of the Windows command interpreter.
COLOR Sets the default console foreground and background colors.
COMP Compares the contents of two files or sets of files.
COMPACT Displays or alters the compression of files on NTFS partitions.
CONVERT Converts FAT volumes to NTFS. You cannot convert the
current drive.
-- More --
Press the ENTER key to scroll by one line, or the spacebar to scroll by one screen.
Similarly, on Linux, a long directory listing could be redirected to <code>more</code>, or better yet, the more advanced <code>less</code> command:
$ '''ls -l /etc | less'''
total 2160
drwxr-xr-x. 3 root root 4096 Jun 29 20:00 abrt
-rw-r--r--. 1 root root 18 May 30 03:03 adjtime
-rw-r--r--. 1 root root 1529 Nov 27 2022 aliases
drwxr-xr-x. 2 root root 4096 May 30 09:32 alliance
drwxr-xr-x. 3 root root 4096 Sep 3 20:00 alsa
drwxr-xr-x. 2 root root 4096 Aug 9 09:01 alternatives
drwxr-xr-x. 4 root root 4096 Apr 13 17:47 anaconda
-rw-r--r--. 1 root root 541 Jan 18 2023 anacrontab
-rw-r--r--. 1 root root 269 Jan 17 2023 anthy-unicode.conf
-rw-r--r--. 1 root root 833 Feb 10 2023 appstream.conf
-rw-r--r--. 1 root root 55 Sep 3 20:00 asound.conf
-rw-r--r--. 1 root root 1 Jan 17 2023 at.deny
drwxr-x---. 4 root root 4096 Aug 5 20:00 audit
drwxr-xr-x. 3 root root 4096 May 30 03:03 authselect
drwxr-xr-x. 4 root root 4096 Mar 18 2023 avahi
drwxr-xr-x. 2 root root 4096 May 30 09:33 avrdude
drwxr-xr-x. 2 root root 4096 Aug 8 10:57 bash_completion.d
-rw-r--r--. 1 root root 2638 Nov 27 2022 bashrc
-rw-r--r--. 1 root root 535 Aug 6 20:00 bindresvport.blacklist
drwxr-xr-x. 2 root root 4096 Sep 17 20:00 binfmt.d
drwxr-xr-x. 2 root root 4096 Aug 24 20:00 bluetooth
drwxr-xr-x. 2 root root 4096 May 30 09:32 bonobo-activation
:
Like <code>more</code>, you can press ENTER to scroll by one line or SPACE to scroll by one screen; but you can also use the up/down arrow keys, or the PgUp/PgDn keys, to scroll in either direction by one line or one screen.
On Windows, to see the <code>help</code> lines that mention color, you could feed the output of the <code>help</code> command into the <code>find</code> command:
> '''help | find /i "color" '''
COLOR Sets the default console foreground and background colors.
You can accomplish complex tasks by connecting a series of simple commands together using pipes. For example, the Linux <code>ls -l</code> command displays permissions in the second through tenth columns of output. You could pipe the output of this command through <code>cut</code> and cut out just those columns:
$ '''ls -l'''
total 0
dr-xr-xr-x. 2 chris chris 100 Sep 26 14:22 apple
-rw-r--r--. 1 chris chris 0 Sep 26 14:22 one
-rw-r--r--. 1 chris chris 0 Sep 26 14:22 orange
-rw-r--r--. 1 chris chris 0 Sep 26 14:22 plum
-rw-r--r--. 1 chris chris 0 Sep 26 14:22 three
-rw-r--r--. 1 chris chris 0 Sep 26 14:22 two
$ '''ls -l | cut -c 2-10'''
otal 0
r-xr-xr-x
rw-r--r--
rw-r--r--
rw-r--r--
rw-r--r--
rw-r--r--
However, this displays the "total ..." line at the top of the output, missing the "t" letter in the first column. We can eliminate this with the command <code>tail -n +2</code> which will give us the last part of the output starting at line 2 (therefore skipping the first line):
$ '''ls -l | cut -c 2-10 | tail -n +2'''
r-xr-xr-x
rw-r--r--
rw-r--r--
rw-r--r--
rw-r--r--
rw-r--r--
If we wanted to see the filenames displayed alongside the permissions, we could take the output of <code>ls -l</code>, sliminate the first line (with <code>tail</code> as above), squeeze out multiple consecutive spaces so that they become a single space using <code>tr -s " "</code>, then use <code>cut -d " " -f 1,9</code> to separate each line into fields delimited by the space (" ") character, selecting just fields 1 and 9:
$ '''ls -l | tail -n +2 | tr -s " " | cut -d " " -f 1,9'''
dr-xr-xr-x. apple
-rw-r--r--. one
-rw-r--r--. orange
-rw-r--r--. plum
-rw-r--r--. three
-rw-r--r--. two
However, this displays an extra character in front of the file permission mode, and another extra character after the file permission mode (.), which can be eliminated by selecting the columns with another <code>cut</code> command:
$ '''ls -l | tail -n +2 | tr -s " " | cut -d " " -f 1,9 | cut -c2-10,12-'''
r-xr-xr-x apple
rw-r--r-- one
rw-r--r-- orange
rw-r--r-- plum
rw-r--r-- three
rw-r--r-- two
