Output

Standard File Descriptors#

Standard File Descriptors.

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.