Streams

CLI Output with stdout

stdout (standard outputput, standard out) is where "output" goes. In NodeJS console.log(), which "logs" a statement to the console, goes to the stdout.

ubuntu@primary:~$ echo "This was written and will go to stdout"
This was written and will go to stdout

Piping with 1> to overwrite

from stdout

stdout contents, like the string above, can be redirected, or piped.
1> will redirecto stdout to a file.

ubuntu@primary:~$ echo "this string here" 1> stdout-pipe.txt

ubuntu@primary:~$ cat stdout-pipe.txt
this string here

from cat

cat can take a file content an concatenate it to stdout.
> can take the file content, from cat, and pipe the content to another file -

ubuntu@primary:~$ cat stdout-pipe.txt 1> qwer.txt

ubuntu@primary:~$ cat qwer.txt
this string here

The this string here was cloned from stdout-pipe.txt into qwer.txt.

Replaces only

1> replaces/overwriters contents of a file and does not add to an existing file:

# write to zxcv.txt
ubuntu@primary:~$ echo "poiuy poiuy" 1> zxcv.txt
ubuntu@primary:~$ cat zxcv.txt 
poiuy poiuy

# run same command on same file with different contents
ubuntu@primary:~$ echo "lkjhg lkjhg" 1> zxcv.txt
ubuntu@primary:~$ cat zxcv.txt 
lkjhg lkjhg

Piping with 1>> to add

1>> will append to a file rather than overwriting contents like 1>.

# write to poiu.txt
ubuntu@primary:~$ echo "new string of text here" 1>> poiu.txt
ubuntu@primary:~$ cat poiu.txt 
new string of text here

# run same command on same file with different contents
ubuntu@primary:~$ echo "second string of text here" 1>> poiu.txt
ubuntu@primary:~$ cat poiu.txt 
new string of text here
second string of text here

Error output with stderr

2> can deal with stderr in the same way that 1> deals with stdout.
stderr is where error info goes.

# this produces an error
ubuntu@primary:~$ cat fake-file.txt
cat: fake-file.txt: No such file or directory
# the above line is the error output describing the error

# pipe the err message to an errors.txt file
ubuntu@primary:~$ cat fake-file.txt 2> errors.txt

# prove it
ubuntu@primary:~$ cat errors.txt 
cat: fake-file.txt: No such file or directory

Piping errors with 2>> to add

# get the date
ubuntu@primary:~$ date
Mon May 30 05:00:12 EDT 1492

# store the date in a var "D"
ubuntu@primary:~$ D=$(date)

# use that, and concat an err to the error text file
ubuntu@primary:~$ cat "$D: fake-file.txt" 2>> errors.txt

# see the output
ubuntu@primary:~$ cat errors.txt 
cat: fake-file.txt: No such file or directory
cat: 'Mon May 30 09:31:24 EDT 2022: fake-file.txt': No such file or directory

Multiple outputs with Multiple Commands

1> for stdout and 2> for stderr can be combined in a single command:

ubuntu@primary:~$ ls -lsah `> stdout.txt 2> stderr.txt

Directing contents to a program with stdin

< can be used to pipe contents TO a program.
This is sort-of like a reverse from the previous commands.
The previous commands can take output from one command, something like cat file.txt, and pipe to an output.

# a trivial example
# passing errors.txt to the "cat" command
ubuntu@primary:~$ cat < errors.txt

# another, grepping for a specific string
ubuntu@primary:~$ grep "Mon May 30" < errors.txt
cat: 'Mon May 30 09:31:24 EDT 2022: fake-file.txt': No such file or directory

A complex example:

grep "Mon May 30" < errors.txt < ls.txt 1> ls2.txt 2> /dev/null

Pipes

Pipes, compared to streams, pass programs to one another, not just data.

pass cat to grep

cat ls.txt | grep "errors.txt"

cat the ls.txt file
pipe the outputs from the cat TO grep
grep the output from the cat command

pass ps to grep

ps aux | grep "ps aux"

run ps aux: ps is "process status", listing currently running processes && some info about them
pipe the output from the ps command to the next command, grep
grep the output for the "ps aux" string

Page Tags:

linux

kernel

cli

input

output

streams

pipes