CSC 140 Introduction to IT IO Redirection CIT
- Slides: 28
CSC 140: Introduction to IT I/O Redirection CIT 140: Introduction to IT 1
Topics 1. 2. 3. 4. 5. 6. 7. Standard Files: stdin, stdout, stderr. Input Redirection Output Redirection Stderr Redirection Appending Pipes Combining Pipes and Redirection CIT 140: Introduction to IT 2
Introduction All commands perform at least one of the following: 1. Input 2. Output 3. Processing Standard files for commands 1. Standard Input (stdin) 2. Standard Output (stdout) 3. Standard Error (stderr) CIT 140: Introduction to IT 3
Standard Files and File Descriptors By default – stdin is associated with keyboard – stdout is associated with display screen – stderr is associated with display screen CIT 140: Introduction to IT 4
Input Redirection Use ‘<‘ for input redirection command < input-file Input comes from input-file instead of keyboard. Examples: cat < tempfile mail waldenj <. bashrc CIT 140: Introduction to IT 5
Input Redirection CIT 140: Introduction to IT 6
Output Redirection Use ‘>‘ for output redirection command > output-file Output sent to output-file instead of screen. Examples: cat file 1 file 2 > mergefile find / -name “*. h” >headerfiles CIT 140: Introduction to IT 7
Output Redirection CIT 140: Introduction to IT 8
Output Redirection In a network environment, use the following command to sort on the server machine the file called datafile residing on your machine: ssh server sort < datafile CIT 140: Introduction to IT 9
Combining I+O Redirection command < input-file > output-file command > output-file < input-file Input from input-file, output to output-file. Order of > and < operators does not matter. Example: cat < lab 1 > lab 2 cat takes input from lab 1 and sends output to lab 2. CIT 140: Introduction to IT 10
Redirection with File Descriptors File descriptor: a small integer that the UNIX kernel attaches with every open file standard input (sdin) — 0 standard output (stout) — 1 standard error (sderr) — 2 By making use of file descriptors, standard output and standard input can be redirected, using, 0< and 1> respectively. Example: $ grep “John” 0< tempfile CIT 140: Introduction to IT 11
Redirecting Standard Error command 2> error-file Errors sent to error-file instead of screen. Example: ls –l foo 2> error. log find / -name “*. h” 2>error. log CIT 140: Introduction to IT 12
Redirecting Standard Error CIT 140: Introduction to IT 13
Redirecting stdout and stderr Redirect stdout + stderr to same file using descriptors cat lab 1 lab 2 1> cat. output 2>cat. errors Send file descriptor 2 to fd 1 with 2>&1 cat lab 1 lab 2 lab 3 1>cat. output. errors 2>&1 cat lab 1 lab 2 lab 3 >cat. output. errors 2>&1 Send file descriptor 1 to fd 2 with 1>&2 cat lab 1 lab 2 lab 3 2>cat. output. errors 1>&2 CIT 140: Introduction to IT 14
Redirecting stdout and stderr CIT 140: Introduction to IT 15
Redirecting stdout and stderr CIT 140: Introduction to IT 16
Redirecting stdin, stdout, stderr command 0<input-file 1>output-file 2> error-file Command gets stdin from input-file sends stdout to output-file sends stderr to error-file File descriptors 0 and 1 not required as they are default values sort 0<students 1>students. sorted 2>sort. error sort 2>sort. error 0<students 1>students. sorted CIT 140: Introduction to IT 17
Appending Output By default, output and error messages overwrite the contents of the destination file. Append by using >> instead of using > Examples: cat memo letter >>stuff 2>error. log find / -name “*. h” >>files 2>>errors find / -name “*. h” >>find. output 2>&1 CIT 140: Introduction to IT 18
Noclobber Options > set -o noclobber > touch a > cat small. File >a bash: a: cannot overwrite existing file > set +o noclobber > cat small. File >a CIT 140: Introduction to IT 19
The null device > ls -l /dev/null crw-rw-rw- 1 root 1, 3 Oct 15 10: 11 /dev/null > cat small. File >/dev/null > cat /dev/null > find / -name “*. h” 2>/dev/null /usr/include/zconf. h … > ls –l small. File -rw-r--r-1 waldenj 1100 Oct 19 14: 13 small. File > cat /dev/null >small. File > ls -l small. File -rw-r--r-1 waldenj 0 Oct 29 20: 29 small. File CIT 140: Introduction to IT 20
UNIX Pipes (‘|’) Connects the stdout of one command to the stdin of another. command 1 | command 2 | … | command. N Standard output of command 1 is connected to stdin of command 2, …, stdout of command N-1 is connected to stdin of command N. Filters: a special class of UNIX commands that take input from stdin process it and send it to stdout. I/O redirection and pipes can be used in a single command. CIT 140: Introduction to IT 21
UNIX Pipes (‘|’) CIT 140: Introduction to IT 22
UNIX Pipes (‘|’) CIT 140: Introduction to IT 23
Sending stdout+stderr to a pipe. 1. Send file descriptor 2 to fd 1. 2. Use pipe as usual. Example: find / -name "*. h" 2>&1 | less CIT 140: Introduction to IT 24
Redirection and Piping combined command 1| tee file 1…file. N|command 2 Standard output of ‘command 1’ is connected to ‘stdin’ of tee, and tee sends its input to files ‘file 1’ through ‘file. N’ and as stdin on ‘command 2’ Example: cat names stuents | grep “John Doe” | tee file 1 file 2 | wc –l CIT 140: Introduction to IT 25
Redirection and Piping combined CIT 140: Introduction to IT 26
Redirection in the C Shell Input, output, and append operators(<, > , >>) same in csh. The operator for error redirection is >& in the C Shell. command >& file Redirects the stdout and stderr of command to file. Examples: ls –l foo >& error. log Csh does not have an operator for redirecting stderr alone. Use >>& operator to redirect and append stdout + stderr. CIT 140: Introduction to IT 27
Redirection in the C Shell Allows stdout and stderr of a command to be attached to stdin of another command with |& operator. command 1 |& command 2 Send stdout + stderr of command 1 to command 2. Examples: cat file 1 file 2 |& grep “John Doe” file* |& wc –l CIT 140: Introduction to IT 28
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