pipes and redirections

a STDIN, STDOUT could be a shell/keyboard/network!

3 std streams in linux:

• STDIN —> descriptor-0 (the command reads its input from)
• STDOUT —> descriptor-1 (the command writes its normal output to)
• STDERR —> descriptor-2 (the command writes its error to)

Output redirection (> and >>)

echo "hello world" 1> random.txt # 1> stdout redirect
cat random.txt                                                               hello world

ls fggfgdsgf 2> random.txt # 2> stderr redirect
cat random.txt
ls: fggfgdsgf: No such file or directory

”>” overwrites the file all the time! use ”>>” to append lines to the file without overwriting

ls jjkjklj 2>> random.txt  # appends to the file                             
cat random.txt                                                               
ls: fggfgdsgf: No such file or directory
ls: jjkjklj: No such file or directory

”<” is used to input something from a file, for ex sending the contents of a file in an email

mail -s "cli email" aniket < random.txt

the pipe “|” command in linux is a very useful command that can be used to hook up the output of one command into the input of another one! ex- listing all processes with pagination.

p1 | p2 | p3 # p1 o/p -> p2 -> p2 o/p -> p3 -> p3 o/p -> STDOUT

ps aux | less

Note: date | echo “date is :” will not pass the output of the date command to the echo command because date is a system default command passing the output to STDIN while echo only reads the STDOUT

commands which read STDIN- cat, grep, sed, awk, sort, wc commands which read STDOUT- echo, ls, date, pwd

Links:

linux shell

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