command-line-pipes.md

layout	main
title	Command line Pipes

Command line pipes

Write input from one command to a file or passing the output to another command as an input

Redirection

Write to a file >

• Writes the output to a file
• If the file exists it will overwrite its content
• If file does not exist it will create it and add content to it

Note:

• there is a command to prevent overwriting files: set -o noclobber
• there is >| operator to ignore the noclobber command

Append to a file >>

• Does exactly like the >
• but instead it appends the output to the existing content of the file

Redirecting standard output and error

• By default > is equivalent to 1>
• The symbol 1> means redirect standard output
• If we are interested in errors produced by command we use 2>
• example: ls non-existing-folder/ 2> out.txt
• If we are interested in saving both output and error we use 2>&1 which means redirect standard error to the same place of standard output
• If you are not interested in anything you can use /dev/null
• example: ls /dir1 /dir2 > /dev/null 2>&1

Read from file <

Nothing interesting in my mind now

Pipelines

Unnamed pipelines |

• Simply you can take the output from some command and give to another
• example: cut -f 1 -d ',' f.csv | sort

Named pipelines using mkfifo

• Create a pipe to use from anywhere
• example: mkfifo sefrwahed ls > sefrwahed wc -l < sefrwahed