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CSE 391

Shell commands

More Redirection

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Slides created by Josh Ervin and Hunter Schafer. �Based off slides made by Marty Stepp, Jessica Miller, Ruth Anderson, Brett Wortzman, and Zorah Fung

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AGENDA

  • Logistics, Roadmap
  • Combining Commands
  • More input/output redirection
  • cut, reading log files

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ROADMAP

  • Introduction to the command line
  • Input/output redirection, pipes
  • More input/output redirection, tee, xargs
  • Git: Fundamentals
  • Git: Branches and rebasing
  • Regular expressions
  • More regular expressions, sed
  • Users and permissions
  • Bash scripting
  • Industry applications

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PIPES

command1 | command2

  • Execute command1 and send its standard output as standard input to command2.
  • This is essentially shorthand for the following sequence of commands:

command1 > filename

command2 < filename

rm filename

  • This is one of the most powerful aspects of unix - being able to chain together simple commands to achieve complex behavior!

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COMBINING COMMANDS

command1 ; command2

  • Execute command1, then execute command2.

command1 && command2

  • Execute command1, and if it succeeds, then execute command2.

command1 || command2

  • Execute command1, and if it fails, then execute command2.

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What would happen after running the following command: ls *.java | javac

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Solution: This won’t work because javac does not read from stdin! Piping makes the stdout of the last program become stdin of the next.

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XARGS

  • xargs is a program that converts standard input to command line arguments (i.e. parameters).
  • For example, to compile all java files in the current directory we could use the following:
    • $ ls *.java | xargs javac

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FIND

  • find is a program for searching your filesystem for certain files.
  • For example, to list all java files in the current directory and all subdirectories, recursively, we would run the following
    • $ find -name “*.java”
  • This is commonly used with xargs. For instance, to compile all Java files in the current directory and all subdirectories recursively
    • $ find -name “*.java” | xargs javac
  • Note that find has a plethora of options and flags, but we will most commonly use find with the -name and -type flags

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COMMAND SUBSTITUTION

$(command)

  • Another powerful tool is command substitution. It executes the given command and places that string literally into the given context.
  • For example, to compile all Java files in the current directory and subdirectories recursively, we can run the following
    • $ javac $(find -name “*.java”)

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What is the command to remove all files listed in the file toRemove.txt?

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toRemove.txt

CompilerErrors.java

beans.txt

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xargs rm < toRemove.txt

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STDERR REDIRECTION

  • We’ve learned that we can redirect standard error using the 2> operator.
  • Sometimes, however, we want standard error and standard out to go to the same location. We can do that with the following syntax:
    • $ command > out.txt 2>&1
  • To understand this command, this reads as “redirect standard out to out.txt, redirect standard error to the same place as standard out”

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TEE

  • Sometimes, we want to redirect the output of a command to both a file and to the console. Do do this, we can pipe the output of a command to tee
    • $ command | tee file.txt
  • To redirect both standard output and standard error to a file, and to the console, we use the following
    • $ command 2>&1 | tee file.txt

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Suppose we want to run the Java program Mystery. What would be the command to output both standard error and standard output to mystery_out.txt and print both to the console?

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java Mystery.java 2>&1 | tee mystery.txt

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CUT

cut -d<DELIMITER> -f<FIELD>

  • cut is a simple program to split lines based on a given delimiter.
  • For example, to split the string “a,b,c,d,e” on commas and get the second entry, we would use the following:
    • $ echo “a,b,c,d,e” | cut -d, -f2
    • Note: the echo program simply prints the given string to standard out

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LOGS

  • A common exercise in daily software development and operations is looking at log files - basically a status report of what is going on inside the program.
  • We can look at the logs for all the CSE course websites by reading the file: /cse/web/courses/logs/common_log
  • For example, to actively watch the log file and only look for access to our own course website, we could use the following

$ tail -f /cse/web/courses/logs/common_log | grep “391”

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