Question on bash redirection

Thread Tools Search this Thread
Top Forums UNIX for Beginners Questions & Answers Question on bash redirection
# 1  
Old 06-12-2019
Question on bash redirection


Can I get some explanation around this bash redirection?
From what I have read, x < y means call the shell to redirect the output of y into x.
Does this mean that this sequence of commands is executed from right to left?

diff <(sort testfile.txt) <(sort testfile2.txt)


Moderator's Comments:
Mod Comment edit by bakunin: please use CODE-tags to surround code, data and terminal output. Thank you

Last edited by bakunin; 06-12-2019 at 02:30 AM..
# 2  
Old 06-12-2019
Originally Posted by sand1234
Can I get some explanation around this bash redirection?
diff <(sort testfile.txt) <(sort testfile2.txt)

This is a so-called "process substitution", where a process (or, rather, its outout) is put into the place of a file. First, there is a diff command:

diff file1 file2

which would list the differences between these two files. Now, both "file1" and "file2" are substituted by a process:

<(sort testfile.txt)

So, the file "testfile.txt" is sorted and the result of this sorting (the output of this process) is used as input to the diff command instead of a regular file. Likewise with the second file. This mechanism is called "process substitution".

I hope this helps.

This User Gave Thanks to bakunin For This Post:
# 3  
Old 06-12-2019
x < y means: run command x and feed its input (stdin) from file y.
It is true that file y is opened for reading before command x runs.
diff <(sort testfile.txt) <(sort testfile2.txt) means: open a device ("/dev/fd/..."), run sort and redirect its output to the device, open another device, run another sort and redirect its output, run diff with the two devices as arguments (that diff treats as input filenames).
man bash
Process substitution
The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection. Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.
This User Gave Thanks to MadeInGermany For This Post:
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Input redirection within bash script

Hi, when I try to redirect input and the command is described as a string within an array redirection does not work. why? #!/bin/bash dir=("tail < ./hello.txt") tail < ./hello.txt #works ${dir} #does not work (2 Replies)
Discussion started by: heinzel
2 Replies

2. Shell Programming and Scripting

Error redirection question

Hi gurus, I have a question, need some of your inputs... I have a command like this : export LINE_COUNT=`wc -l test.dat | awk '{print $1}'` echo $LINE_COUNT --- > gives me 2 which is fine as the file has 2 lines. This works fine if the file test.dat is present but in case... (3 Replies)
Discussion started by: calredd
3 Replies

3. Shell Programming and Scripting

Redirection question

I want to redirect stderr and have the following peice of code $ cat t1.ksh #!/bin/ksh func2() { diff /tmp/jdlkwjdlkejew /tmp/djlkwejdlewdjew >$OUTPUT_FILE 2>>$ERR_FILE } func1() { let counter=0 while do print -u2 "Error: In main function" func2 let... (1 Reply)
Discussion started by: BeefStu
1 Replies

4. Shell Programming and Scripting

Question about IO redirection

for shell operators like <, >, <<, and >> do the LHS always have to be a process or device and the RHS a file or device? Is it possible for the RHS to be a process? (1 Reply)
Discussion started by: stevenswj
1 Replies

5. Shell Programming and Scripting

Redirection Question

I just wondered if I'm understanding this command line correctly cat 2>save1 0<memo | sort 2>save2 1>letter This means that stdin will read from memo and if there is an error then stderr will write to save1. Am I correct in saying that the sort command will sort the memo file and write... (2 Replies)
Discussion started by: snag49ers
2 Replies

6. Shell Programming and Scripting

BASH Problem / Question regarding redirection

Hi all, Maybe someone is able to help: Need to redirect the output of a command in realtime to a second command. Command-A executes a remote shell to another host, and outputs its results. Command-B displays a "dialog" with the outputs of Command-A. Command-A Output: Updating FileA... (2 Replies)
Discussion started by: mharald
2 Replies

7. Shell Programming and Scripting

File redirection question

Hi all, I am working with the Grinder tool (unrelated to my question) to redirect the output of a program to a file as follows: java -cp $CLASSPATH net.grinder.TCPProxy > grinder.txt This is a proxy server which pipes output to a file. When I do something on my proxy, more and more goes to... (1 Reply)
Discussion started by: Annorax
1 Replies

8. Shell Programming and Scripting

input redirection question

Hi, in my script I need to execute the following command: query $id 456 432 but it waits for a RETURN character from keyboard and therefore, it fails. I tried something like: query $id 456 432 << '\n' but, i'ts clear it is not correct. Is there any way to do this? Thxs. (0 Replies)
Discussion started by: luistid
0 Replies

9. Shell Programming and Scripting

question on redirection (<<)

I came across the following problem, where file contents are overwritten using redirection. Can somebody please explain what cat << BAR seems to be doing and say why this is a problem? Explain the contents and relation between the two filenames used before the cat command. thanks /tmp# echo... (2 Replies)
Discussion started by: newkidintown
2 Replies

10. Shell Programming and Scripting

Bash multiple output redirection

Hello all. Is there a way to redirect output to more than one file at a time? I have a method1() that writes to a logfile. In the method cksums are done on files by doing a "for i in `ls`; do cksum $i > $LOGFILE done " In the logfile I want to show the cksums under each directory like... (2 Replies)
Discussion started by: yoi2hot4ya
2 Replies
Login or Register to Ask a Question

Featured Tech Videos