C2110 UNIX and programming Lesson 3 / Module 2 -1C2110
UNIX and programming
Petr Kulhanek
kulhanek@chemi.muni.cz
National Center for Biomolecular Research, Faculty of Science
Masaryk University, Kamenice 5, CZ-62500 Brno
Lesson 3 / Module 2
PS / 2020 Distance form of teaching: Rev2
C2110 UNIX and programming Lesson 3 / Module 2 -2Streams,
Redirects, Pipes
C2110 UNIX and programming Lesson 3 / Module 2 -3Process
Communication
Process
Process can communicate with the environment in a number of ways:
• GUI (Graphical User Interface = using the appropriate API)
• signals, shared memory, MPI (Message Passing Interface), etc.
• standard currents
One option is to read input data from standard input current, output data into standard
output or error stream.
C2110 UNIX and programming Lesson 3 / Module 2 -4Standard
Streams
process
(command, program)standard input stream
(keyboard)
standard output stream
(terminal screen)
standard error stream
(terminal screen)
Input-output streams serve the process for communication with its surroundings. Every
process opens up three standard streams:
C2110 UNIX and programming Lesson 3 / Module 2 -5-
Redirection
process
(command, program)standard input stream
(keyboard)
standard output stream
(terminal screen)
standard error stream
(terminal screen)
Input-output streams can be redirected to use files instead of the keyboard or screen.
C2110 UNIX and programming Lesson 3 / Module 2 -6Redirection
and Ending Input
Standard input redirection of program my_command from file input.txt.
$ my_command < input.txt
Standard input redirection of program my_command from script file.
.......
./my_command << EOF
first line text
second line text
third line text
EOF
......
mark indicating the end of the entry
(selected by user)
end of input, mark can not be
surrounded by spaces
text forming the read input
This method of redirection is especially useful in scripts, but it also works in the command line. The
advantage is the expansion of variables in the read text.
Terminal (useful keyboard shortcuts):
Ctrl + D closes the input stream of the running process
C2110 UNIX and programming Lesson 3 / Module 2 -7Output
Redirection
Standard output redirection of program my_command to a file output.txt. (File output.txt
is created. If it already exists, its original content is deleted.)
$ my_command > output.txt
Standard output redirection of program my_command to a file output.txt. (File output.txt is
created. If it already exists, output of the program my_command is connected to its end.)
$ my_command >> output.txt
Similar rules apply to standard error output, in this case the following operators are used:
$ my_command 2> errors.txt
$ my_command 2 >> errors.txt
C2110 UNIX and programming Lesson 3 / Module 2 -8Joining
Output Streams
Standard output and standard program error output of my_command program can be
redirected at the same time to a file output.txt.
$ my_command &> output.txt
$ my_command &>> output.txt
Alternative solutions for &>>: First, it is necessary to redirect standard output and then
join standard error output with standard output.
$ my_command >> output.txt 2>&1
$ my_command 2>&1 >> output.txt does not work
order is important!
works in new versions bash
C2110 UNIX and programming Lesson 3 / Module 2 -9-
Pipes
Pipes serve to combine the standard output of one process with the standard input of
another process.
process 1 process 2
input
output | input
output
error output
error output
$ command_1 | command_2
Usage:
C2110 UNIX and programming Lesson 3 / Module 2 -10Pipes
and Error Stream
The transmission of the standard error output via the pipe can be performed after its
connection with the standard output.
process 1 process 2
input
output | input
output
error output error output
$ command_1 2>&1 | command_2
Use:
2>&1
C2110 UNIX and programming Lesson 3 / Module 2 -11Commands
for Exercise
Examples of use:
$ cat file1.txt file2.txt
$ paste file1.txt file2.txt
$ wc file.txt
$ head -15 file.txt
$ tail -6 file.txt
concatenates the content of the files file1.txt and file2.txt and prints the result on the screen
concatenates the contents of file1.txt and file2.txt (side by side) and prints the result on the screen
lists the number of lines, words, and characters in file.txt
prints the first 15 lines of file.txt
prints the last 6 lines of file.txt
cat joins the content of several files into one (one after the other), or lists the contents
of one file
paste joins the content of multiple files into one (side by side)
wc file information (number of lines, words and characters)
head prints the inital part of a file
tail prints the final part of the file
C2110 UNIX and programming Lesson 3 / Module 2 -12Commands
for Exercise...
Command tr is used for transformation or deletion of characters from standard input. The
result is sent to standard output.
Examples:
$ cat file.txt | tr --delete "qwe"
$ cat file.txt | tr --part "[:space:]"
$ echo $PATH | tr ":" "\n"
from the contents of file.txt, removes the characters "q", "w" and "e"
from the contents of the file file.txt removes all whitespace
in the text sent by the echo command, the characters ":"
will be replaced by a newline character "\n "
C2110 UNIX and programming Lesson 3 / Module 2 -13Exercise
1
1. Find all files with the extension .f90 which are in the directory
/home/kulhanek/Documents/C2110/Lesson03. Save list of files to a file
~/Procesy/list.txt
2. How many lines does the file list.txt contain?
3. Write the first two lines from the file list.txt, first onto the screen and then to the file
two_lines.txt
4. Write only the third line of the file list.txt
5. In the directory /proc, find all files that begin with letters cpu. Remove the
unauthorized access information from the listing by redirecting of the error current to
/dev/null
6. List the directory names contained in the variable PATH, each on one line.