Summer 2015 SILICON VALLEY UNIVERSITY CONFIDENTIAL 1 Introduction to UNIX / Linux - 7 Dr. Jerry Shiao, Silicon Valley University

SILICON VALLEY UNIVERSITY CONFIDENTIAL 2 Summer 2015 Introduction UNIX/Linux Course File Sharing  On a Computer System, collaboration on Software Project need to share files and directores related to the project.  File Sharing Methods Duplicate (Copies) of Common Files. Common Login for Team members. Common Group for separate Team member Logins. File Sharing Via Links.  Hard Links  Soft / Symbolic Links Redirection and Piping  Standard files, stdin, stdout, stderr.  Redirection of standard files.  Pipes in UNIX : Pipes and Filters perform complicated tasks that cannot be performed with a single command.  Combining Pipes and Redirection operators.  C shell and error redirection.  FIFOs: Unrelated or independent process communication.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 3 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing  Duplicate Shared Files Problem: Working of file simultaneously. Files are inconsistent.  Team Common Login Problem: Separate personal and team accounts. High Overhead.Administrator must maintain new accounts per project.  Access Permissions on Shared Files Problem: If User group not restrictive and access cannot always guarantee to be project members.  Team New Group Problem: User group must ONLY be members of the team.  File Sharing Via Links Link establish connection between file to be shared and directory entries of users who want to have access to this file. Link provides different access paths to files to be shared. Hard Links: Pointer to the inode of a file. Soft / Symbolic Links: Pointer to the path of a file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 4 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links  Hard link creates a file entry (new file name) in directory with the same inode index as the linked file.  ln [ options ] existing-file new-file  ln [ options ] existing-file-list directory  - f : Force creation of link.  - n : Do not create the link if “new-file” exists.  - s : Create symbolic link to “existing-file”.  NOTE: : Create a hard link to “existing-file”. : The user MUST have execute and write permission for all the directories in the path leading to the file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 5 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Review Inode a) Logical structure of current directory. File, Chapter3, is created in current directory. b) Contents of current directory. Directory entry contains pair. c) Relationship among a directory entry, Inode Table, and physical file contents. -Inode # index into the Inode Table. -Inode Table entry contains the physical location of file on disk.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 6 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links Current Directory  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 1 sau users :23 Chapter3  $ ln Chapter3 Chapter3.hard  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 2 sau users :23 Chapter rw-r--r-- 2 sau users :23 Chapter3.hard  $ rm Chapter3  $ ls -il total rw-r--r-x 1 sau users :47 Chapter rw-r--r-- 1 sau users :47 Chapter rw-r--r-- 1 sau users :23 Chapter3.hard  $ Create hard link. The inode index for linked file and file are the same. Link count is 2. Remove only removes the entry in the directory. File is physically still on disk. Link count is 1.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 7 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Link Count  Number of different directory entries pointing to inode of the object (i.e. directory or file).  File: Number of hard links to that file. $ ls -il memo rw-r--r-x 1 sau users :16 memo6  Directory: Number of directory entries connecting local directory to Linux file system. $ ls -a... students students_2 students.gz $ ls -ld drwxr-xr-x 2 student1 student :06. $ mkdir test_dir $ ls -a... students students_2 students.gz test_dir $ ls -ld drwxr-xr-x 3 student1 student :06.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 8 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Create Hard Link a) Logical structure of current directory. Hard link, Chapter3.hard, is linked to file, Chapter3 in current directory. b) Contents of current directory. Directory entry contains pair for Chapter3.hard. The inode# is same as file Chapter3. c) Relationship among a directory entry, Inode Table, and physical file contents. -Inode # index into the Inode Table for both Chapter3 and Chapter3.hard.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 9 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links Different Directory  $ ls -il memos/memo rw-r--r-x 1 sau users :16 memos/memo6  $ ln memos/memo6 memo6.hard  $ ls -il memos/memo rw-r--r-x 2 sau users :16 memos/memo6  $ ls -il memo6.hard rw-r--r-x 2 sau users :16 memo6.hard  $  $ rm memos/memo6  $ ls -il memos/memo6 ls: cannot access memos/memo6: No such file or directory  $ ls -il memo6.hard rw-r--r-x 1 sau users :16 memo6.hard  $ Create hard link. The inode index for linked file and file are the same. Link count is 2. Remove only removes the entry in the directory. File is physically still on disk. Link count is 1.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 10 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Link Different Directory

SILICON VALLEY UNIVERSITY CONFIDENTIAL 11 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links Different Directory  $ ls -il memos/memo rw-r--r-x 1 sau users :16 memos/memo6  $ ln memos/memo6 memo6.hard  $ ls -il memos/memo rw-r--r-x 2 sau users :16 memos/memo6  $ ls -il memo6.hard rw-r--r-x 2 sau users :16 memo6.hard  $  $ rm memos/memo6  $ ls -il memos/memo6 ls: cannot access memos/memo6: No such file or directory  $ ls -il memo6.hard rw-r--r-x 1 sau users :16 memo6.hard  $ Create hard link. The inode index for linked file and file are the same. Link count is 2. Remove only removes the entry in the directory. File is physically still on disk. Link count is 1.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 12 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links Placed In Another User Structure /home/user1/class4/demo1 Structure of /home/user2/dir1/demo1 /home/user1 $ su … # pwd /home/user1/class4 # ln demo1 /home/user2/dir1/demo1 # exit $ ls -i demo demo1 /home/user2 $ pwd /home/user2/dir1 $ ls -i demo demo1 1) Creating Hard Link between two users. 2) Create Hard Link in /home/user2 pointing to file in /home/user1. 3) Target file must have Read/Write permission to Group.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 13 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links To Another User  User Directory Permissions $ ls -lt total 40 drwx student1 student :44 student1 drwx-----x 23 student2 student :39 student2 $ ls -lt test -rw-rw-r-- 1 student1 student :51 test $ ln test /home/student2/dir1 $ ls -lt test -rw-rw-r-- 2 student1 student :51 test /home/student1 1) Create Hard Link in /home/student2 /dir pointing to file in /home/student11. 2) Target directory, dir1, must have WriteExecute permission to Other. 3) Link count for files test is 2. [student2]$ ls -ld dir1 drwxrwx-wx 3 student2 student :50 dir1 [student2]$ ls -lt dir1 total 16 -rw-rw-r-- 2 student1 student :51 test /home/student2 1) Link count for file test is 2.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 14 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Hard Links Limitations  Cannot use Hard Links between files on different file systems. # ln /usr/bin/zip my_zip ln: creating hard link `my_zip' => `/usr/bin/zip': Invalid cross-device link  Moving a hard linked file to different file system causes physical copy of file to be made. $ ls -il memo6.hard rw-r--r-x 2 sau users :25 memo6.hard $ ls -il memos/memo rw-r--r-x 2 sau users :25 memos/memo6 $ mv memo6.hard /tmp $ ls -il /tmp/memo6.hard rw-r--r-x 1 sau users :25 /tmp/memo6.hard  Editors saves edited file in new files, and remove previous file. The editors, vi and emacs, are safe.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 15 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Soft Links  Does not have any of the Hard Link issues.  Flexible: Link files across File Systems. Link to directory.  Soft link creates a new file entry (new file name and new inode index) in directory.  ln [ options ] existing-file new-file  ln [ options ] existing-file-list directory  - f : Force creation of link.  - n : Do not create the link if “new-file” exists.  - s : Required. Create symbolic link to “existing-file”.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 16 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Soft Links Current Directory  $ ls -il Chapter rw-r--r-x 1 sau users :47 Chapter3  $ ln -s Chapter3 Chapter3.soft  $ ls -il Chapter3 Chapter3.soft rw-r--r-x 1 sau users :47 Chapter lrwxrwxrwx 1 sau users :12 Chapter3.soft -> Chapter3  $ Create soft link. 1)The inode index for linked file and file are different. These are different files. 2)Original file is file type “-”, an ordinary file. Link file is type “l”, a link file. 3)Link count 1. 4)File sizes are different. Link file is 8 bytes, the length of “Chapter3”, the pathname is placed in the link file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 17 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Soft Links Create soft link. 1)The inode index for linked file and file are different. These are different files. 2)Original file is file type “-”, an ordinary file. Link file is type “l”, a link file. 3)Link count 1. 4)File sizes are different. Link file is 8 bytes, the length of “Chapter3”, the pathname is placed in the link file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 18 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Soft Links Placed In Another User Structure /home/user1/class4/demo1 Structure /home/user2/dir1/demo1 /home/user2 $ ls -ld /home/user2 drwxr-xr-x 31 user2 users … /home/user2 $ ls -ld /home/user2/dir1 drwxrwxr-x 2 user2 users … /home/user2/dir1 … $ ls -il demo lrwxrwxrwx 1 user1 users …demo1 -> /home/user1/class4/demo1 /home/user1 $ pwd /home/user1/class4 $ ln -sf /home/user1/class4/demo1 /home/user2/dir1 $ ls -il demo rw-rw-r-x 1 user1 users … demo1 NOTE: Do not have to be Superuser to create software to another directory. Must have Write/Execute permission at linking directory. demo1  /home/user1/class4/demo1

SILICON VALLEY UNIVERSITY CONFIDENTIAL 19 Pearson Addison-Wesley Introduction UNIX/Linux Course File Sharing Soft Links Pros and Cons  Pros Establish symbolic link between files across file systems. Establish symbolic link between directories. Symbolic linked file edited by editor does not change filename.  Cons If the file that the symbolic link points to is moved from one directory to another, it can no longer be accessed via the link. UNIX has to support an additional file type (the link type) and a new file has to be created for every link. Symbolic links also result in slow file operations because, for every reference to the file, the link file has to be opened and read in order to reach the actual file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 20 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Standard Files All commands performs one or more of : Input, Output, Processing Standard files Automatically Opened on Every Command:  Standard Input (stdin) - File Descriptor = 0.  Standard Output (stdout)- File Descriptor = 1.  Standard Error (stderr)- File Descriptor = 2. Standard files can be redirected to other files using File Redirection Facilities in UNIX.  Connect several commands together to perform a complex task, that cannot be performed by single command.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 21 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Standard Files: Automatically opened by the Kernel for every command for te command to read input from and send its output and error messages to. By default – stdin is associated with keyboard – stdout is associated with display screen – stderr is associated with display screen

SILICON VALLEY UNIVERSITY CONFIDENTIAL 22 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Input Redirection  command < input-file  Input to “command” comes from “input-file” instead of stdin (i.e. keyboard). cat < tempfile stdin has been has been attached to “tempfile”. cat tempfile stdin still active NOT attached to “tempfile”. “tempfile” is file command line argument to the command.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 23 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Input Redirection ( keyboard ) Keyboard detached from stdin and input- file attached. Input-file

SILICON VALLEY UNIVERSITY CONFIDENTIAL 24 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Output Redirection  command > output-file  Send output of “command” to the “output-file” instead of stdout (i.e. monitor). cat > tempfile stdout has been has been attached to “tempfile”. cat tempfile stdin still active. stdout still active. “tempfile” is file command line argument to the command.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 25 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Output Redirection

SILICON VALLEY UNIVERSITY CONFIDENTIAL 26 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Output Redirection Network Environment rsh [ –l ] host [ command ]  Remote shell executes command on host.  rsh sort < datafile ssh [ -l ] host [ command ]  Remote login executes command on host.  ssh l root sort < datafile

SILICON VALLEY UNIVERSITY CONFIDENTIAL 27 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Combine Input and Output Redirection  command output-file  The “command” takes input from “input-file” instead of stdin (i.e. keyboard) and sends its output to the “output-file” instead of stdout (i.e. monitor). cat lab2 stdout has been has been attached to “lab2”. stdin has been attached to “lab1”. The cat command takes its input from the lab1 file and sends its output to the lab2 file.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 28 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor With I/O Redirection  File Descriptor is an integer that the UNIX kernel attaches with every open file to stdin, stdout, and stderr. Standard Input (stdin) = 0 Standard Output (stdout) = 1 Standard Error (stderr) = 2  File Descriptors can be used for I/O Redirection 0< : Redirect Standard Input 1> : Redirect Standard Output 2> : Redirect Standard Error >& : Redirect one File Descriptor to another File Descriptor  2>&1 : Redirects File Descriptor 2 (stderr) to File Descriptor 1 (stdout).  1>&2 : Redirects File Descriptor 1 (stdout) to File Descriptor 2 (stderr).  Using File Descriptors: “cat > outfile” equivalent to “cat 1> outfile”. “grep John 0< tempfile” equivalent to “grep John < tempfile”.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 29 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor With I/O Redirection  command 2> error-file  Error messages generated by “command” sent to stderr are redirected to “error-file”. $ cat memo 1> letter 2>&1 $ cat letter cat: memo: No such file or directory $ rm letter $ cat memo > letter 2>&1 $ cat letter cat: memo: No such file or directory Redirects stderr to what stdout was (“letter”) when the argument was encountered. NOTE: If want to redirect stderr, have to use the syntax “2>”.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 30 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor With I/O Redirection Redirect stdout and stderr to the same file by using the descriptors with > symbol  $ cat lab1 lab2 lab3 1> cat.output 2>cat.output Using string 2>&1 to tell the command shell to make descriptor 2 a duplicate of descriptor 1, resulting in error messages going to the same place the output is goes to  $cat lab1 lab2 lab3 1> cat.output.errors 2>&1 Using string 1>&2 to tell the command shell to make descriptor 1 a duplicate of descriptor 2, resulting in output going to the same place as the error messages  $ lab1 lab2 lab3 2> cat.output.errors 1>&2 The evaluation for command line content for file redirection is from left to right

SILICON VALLEY UNIVERSITY CONFIDENTIAL 31 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor With Standard Error Redirection command 2> error-file

SILICON VALLEY UNIVERSITY CONFIDENTIAL 32 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor Redirect Stdout and Stderr  cat lab1 lab2 lab3 1> cat.output.errors 2>&1  cat lab1 lab2 lab3 2> cat.output.errors 1>&2

SILICON VALLEY UNIVERSITY CONFIDENTIAL 33 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor Redirect Stdout and Stderr Redirection Evaluated Left to Right  cat lab1 lab2 lab3 2>&1 1> cat.output.errors Evaluation of command line for file redirection is left to right. One notation is dependent on notation to the left. Evaluation of cat command: 1) Stderr (2) is made a duplicate of stdout (1). 2) Stdout (1) has NOT been redirected. Stderr (2) becomes duplicate of stdout (1) default (console). 3) Stdout (1) is redirected to file “cat.output.errors”. 4) Output  cat.output.errors. Errors  console

SILICON VALLEY UNIVERSITY CONFIDENTIAL 34 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor Redirect Stdout and Stderr Redirection Evaluated Left to Right  cat lab1 lab2 lab3 2>&1 1> cat.output.errors File Descriptor 2, duplicates what File Descriptor 1 is currently pointing to, stdout. File Descriptor 1, duplicates file cat.output.errors. File Descriptor 2  stdout. File Descriptor 1  cat.output.errors.  cat lab1 lab2 lab3 2> cat.output.error 1>&2 File Descriptor 2, duplicates file cat.output.errors. File Descriptor 1, duplicates File Descriptor 2, which is currently pointing to file cat.output.errors. File Descriptor 2  cat.output.error. File Descriptor 1  cat.output.error.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 35 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping cat lab1 lab2 lab3 2>&1 1> cat.output.errors (a)and (b) Stdout and Stderr does not change. (c) Stdout to “cat.output.errors”. (d) Command Semantics

SILICON VALLEY UNIVERSITY CONFIDENTIAL 36 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping File Descriptor With Redirect Stdin, Stdout, Stderr command 0 output-file 2> error-file Input to ‘command’ comes from ‘input-file’ instead of keyboard, output of ‘command’ goes to ‘output-file’ instead of the display screen and error messages generated by ‘command’ are sent to ‘error-file’ The file descriptors 0 and 1 are not required as they are the default values $ sort 0 students.sorted 2> sort.error $ sort 2>sort.error 0 students.sorted

SILICON VALLEY UNIVERSITY CONFIDENTIAL 37 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping I/O Redirection Without Overwriting Contents “ > “ Overwrites contents of destination file. “ >> ” Replace “ > “ to Append stdout and stderr at end of file. “ >> “ default File Descriptor is 1 (stdout). “ 2>> “ Append errors to a file. ls -l 1>> output.dat 2>> error.log Evaluation: Output appended to output.dat. Error appended to error.log cat memo letter >> stuff 2> error.log Evaluation: Output appended to stuff. Error overwrites error.log

SILICON VALLEY UNIVERSITY CONFIDENTIAL 38 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping I/O Redirection: File Descriptors [student1]$ exec 4<>test_r4 [student1]$ cat >&4 Hello test_r4 Hello test_r4 Hello exit [student1]$ ls -lt test_r4 -rw-rw-r-- 1 student1 student :21 test_r4 [student1]$ cat test_r4 Hello test_r4 Hello test_r4 Hello exit [student1]$ exec: Execute in place of the current shell (instead of creating a new process).

SILICON VALLEY UNIVERSITY CONFIDENTIAL 39 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping UNIX Pipes ( “ | “ ) Allows stdout of a command to be connected to stdin of next command. Pipe implemented in main memory. command1 | command2 | … | commandN Standard output of ‘command1’ is connected to stdin of command2,…,stdout of ‘command N-1’ is connected to stdin of ‘command N’ ls -l | more Using piping. ls -l > temp more < temp No piping. rm temp

SILICON VALLEY UNIVERSITY CONFIDENTIAL 40 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping UNIX Pipes ( “ | “ ) The semantics of a pipe with N commands. The semantics of the “ls – l | more” command. The semantics of the “grep ‘John’ < Students | lpr –Pspr” command.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 41 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping UNIX Pipes ( “ | “ ) The semantics of the “ls – l | more” command. diff datafile - command

SILICON VALLEY UNIVERSITY CONFIDENTIAL 42 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Redirect stdout to files or another command. Limitation Redirect and Pipe:  Cannot use redirect operators and pipes to redirect stdout of a command to a file and connect it to stdin of another command. Command1 | tee file1 … fileN | command2  Standard output of “command1” is connected to stdin of tee, and tee sends its input to file “file1” through “filen” and as stdin of “command2”.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 43 Pearson Addison-Wesley Introduction UNIX/Linux Course Redirection and Piping Redirect stdout to files or another command. cat names students | grep “John Doe” | tee file1 file2 | wc –l

SILICON VALLEY UNIVERSITY CONFIDENTIAL 44 Pearson Addison-Wesley Introduction UNIX/Linux Course C Shell Error Redirection “>&” C Shell Limitation:  File Descriptors cannot be used with ( “ ”, “>>” ).  Error Redirection is “>&”, but for both stdout and stderr.  To attach stdout and stderr to different files, parenthesize the command to have the stdout redirection in the subshell.  % (find ~ -name foo –print > foo.paths) >& error.log Subshell inherits standard files of the parent shell (stdout and stderr redirected to error.log). Subshell redirects stdout to foo.paths. Stderr from parent shell still redirected to error.log. Subshell Parent Shell executes before SubShell

SILICON VALLEY UNIVERSITY CONFIDENTIAL 45 Pearson Addison-Wesley Introduction UNIX/Linux Course C Shell stdout and stderr Append “>>&”  command >& file  Redirect and append stdout and stderr to “file”. stdout and stderr Redirect “|&”  command1 |& command2  Redirect stdout and stderr of “command1” to “command2”, i.e., pipe stdout and stderr of “command1” to “command2”. Pipe and Redirect “|” and “|&” Single Command  cat file1 file2 |& grep “John Doe” | sort |& wc –l  stdout of grep command (grep BOTH stdin and stderr) is attached to the stdin of the sort command.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 46 Pearson Addison-Wesley Introduction UNIX/Linux Course C Shell Parent Shell ( … ) >& error.log Sub Shell (find ~ -name foo –print > foo.paths) … (find ~ -name foo –print > foo.paths) >& error.log

SILICON VALLEY UNIVERSITY CONFIDENTIAL 47 Pearson Addison-Wesley Introduction UNIX/Linux Course C Shell C Shell Special Built-In Variable  set noclobber : Protect from output redirection  unset noclobber  cat file1 > file2 noclobber set: Error if file2 exists: Creates file2 if not exists.  cat file1 >> file2 noclobber set: Error if file2 not exists. file2 exists, append. noclobber clear:  >!, >>!, >>&! Operators override effect of noclobber. cat file1 >! file2 Overwrites existing file2. cat file1 >>! file2 Creates file2, if not exists.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 48 Pearson Addison-Wesley Introduction UNIX/Linux Course Table I/O and Error Redirection

SILICON VALLEY UNIVERSITY CONFIDENTIAL 49 Pearson Addison-Wesley Introduction UNIX/Linux Course Table I/O and Error Redirection Bourne ShellKorn Shell C Shell

SILICON VALLEY UNIVERSITY CONFIDENTIAL 50 Pearson Addison-Wesley Introduction UNIX/Linux Course FIFOs FIFOs (Named Pipes) used for communicating between two processes on the system.  Pipes can only be used for communication between commands connected via a pipeline. FIFOs created on disk with a name. FIFOs has to be created and opened before processes.  Pipe is in main memory buffer and has no name. mknod() or mkfifo() to create a FIFO.  mkfifo [ option ] file-list  Creates FIFOs with pathnames given in “file-list”.  - m : Set access permission to “mode” (octal value).

SILICON VALLEY UNIVERSITY CONFIDENTIAL 51 Pearson Addison-Wesley Introduction UNIX/Linux Course FIFOs $ mkfifo myfifo1 myfifo2 $ ls -lt myfifo1 myfifo2 prw-r--r-- 1 sau users :26 myfifo1 prw-r--r-- 1 sau users :26 myfifo2 $ cat myfifo1 | grep "d$" & [1] $ cat myfifo2 | wc -l & [2] $ ps -e | tee myfifo1 myfifo2 | wc -l 2 ? 00:00:00 kthreadd 15 ? 00:00:00 kacpid ? 00:00:00 nmbd 131 [1]- Done cat myfifo1 | grep "d$" [2]+ Done cat myfifo2 | wc -l Create FIFOs. File type is “p” for FIFO. Run command as background process. Block on waiting for FIFO myfifo1. Run command as background process. Block on waiting for FIFO myfifo2. Writes to FIFO files, myfifo1 and myfifo2. The blocked commands performs the actions and outputs written to stdout.

SILICON VALLEY UNIVERSITY CONFIDENTIAL 52 Pearson Addison-Wesley Introduction UNIX/Linux Course FIFOs Semantics for two commands communicate via FIFO. cmd1 < myfifo1 & cmd2 infile | tee myfifo1 | cmd3