Output

Standard File Descriptors

Standard File Descriptors.

Name	Number	Description
STDIN	0	Standard Input
STDOUT	1	Standard Output
STDERR	2	Standard Error

Standard Input

The cat command reads content from the standard input by default. If you run cat without specifying a filename, it will wait for user input.

cat

Redirection is done using <.

cat < foo

wc < foo

Standard Output

The default is the terminal or screen.

echo "hello world"

Redirection is done using > or 1>.

echo "hello world" > foo

Standard Error

The default is the terminal or screen.

ls 404

Redirection is done using 2>.

ls 404 2> foo

Using &> can redirect both standard output and standard error output simultaneously.

ls 404 &> foo

EXEC Command

Using in Command Line

exec ls

The Shell process will be replaced by the new command's process, and after execution, it will not return to the original Shell.

Using in Script

#!/usr/bin/env bash

exec /bin/date
echo "This will never be executed"

The script process will be replaced by the date command, and echo will not be executed.

File Descriptor Redirection

#!/usr/bin/env bash

echo "This will output to the terminal"
exec 1> output.txt
echo "This will be written to output.txt"

When modifying file descriptors, it will not replace the current process, but will affect the input/output of subsequent commands.

Closing File Descriptors

Redirecting to &- will close it. After closing, no more data can be written.

#!/usr/bin/env bash

exec 3> testfile
echo "apple" >&3
exec 3>&-

/dev/null

The null device, a bit bucket.

crw-rw-rw- 1 root root 1, 3 Aug 14 23:16 /dev/null

Redirecting both standard output and standard error to the null device.

command > /dev/null 2>&1

MKTEMP Command

Creates a temporary file based on a filename template.

filename.XXXXXX

The command will replace the X with random characters, and the template must have at least 6 X's.

Creating Files

Files created by this command only have permissions for the Owner.

mktemp foo.XXXXXX

-rw------- 1 kuga kuga 0 Sep  2 17:27 foo.zPtFtG

Creating Directories

mktemp -d bar.XXXXXX

drwx------ 2 kuga kuga 4096 Sep  2 17:29 bar.RQAMzc

Using the /tmp Directory

Using the -t option will create files in the /tmp directory.

mktemp -t foo.XXXXXX

/tmp/foo.0IglAI

TEE Command

Simultaneously redirects data to standard output and a file.

tee - read from standard input and write to standard output and files

date | tee testfile

Mon Sep  2 05:36:44 PM CST 2024

cat testfile

Mon Sep  2 05:36:44 PM CST 2024

The same data is on both the screen and in the file.