I've been a bit quiet on Stack Overflow lately, but just managed to reach 150,000 reputation!
For me, Stack Overflow has not simply been a quest for reputation, but more about learning and helping fellow programmers in need.
I've been a bit quiet on Stack Overflow lately, but just managed to reach 150,000 reputation!
For me, Stack Overflow has not simply been a quest for reputation, but more about learning and helping fellow programmers in need.
This post shows the difference between the three standard input redirection operators: <
, <<
and <<<
.
1. <
<
allows you to pass data contained in a file to the standard input of a command. The format is:
command < file
Examples:
# replace comma with new-lines: tr ',' '\n' < file # read a file line-by-line and do something with each line: while IFS= read -r line; do # do something with each line ssh "$line" who -b done < hosts.txt
2. <<
<<
allows you to pass multi-line text (called a here-document) to a command until a specific "token" word is reached. The format is:
command << TOKEN line 1 line 2 TOKEN
For example:
# query a database sqsh -S server -D db << END select * from table where foo='bar' go END
3. <<<
<<<
allows you to pass a string of text (called a here-string) to a command. If you find yourself echo-ing text and piping it to a command, you should use a here-string instead. (See my previous post on Useless Use of Echo.) The format is:
command <<< text
Examples:
tr ',' '\n' <<< "foo,bar,baz" grep "foo" <<< "foo,bar,baz" mailx -s subject $USER <<< "email body"