1 / 13

Shell Programming – Extra Slides

Shell Programming – Extra Slides. Counting the number of lines in a file. #!/bin/sh #countLines1 filename=$1 #Should check if arguments are given count=0 while read aline do count=`expr $count + 1` done < $filename echo "$filename has $count lines“ From the command line:

alyssa-diaz
Download Presentation

Shell Programming – Extra Slides

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Shell Programming – Extra Slides

  2. Counting the number of lines in a file #!/bin/sh #countLines1 filename=$1 #Should check if arguments are given count=0 while read aline do count=`expr $count + 1` done < $filename echo "$filename has $count lines“ From the command line: $wc –l data.txt 11 data.txt $countLines1 data.txt Data.txt has 0 lines

  3. Counting the number of lines in a file #!/bin/sh #countLines2 filename=$1 #Should check if arguments are given count=0 Cat $filename | while read aline do count=`expr $count + 1` done echo "$filename has $count lines“ From the command line: $wc –l data.txt 11 data.txt $countLines2 data.txt Data.txt has 0 lines WHY? Subshell execution takes place if input is piped into a for, while, until, if or case command or if the output is piped out.

  4. Counting the number of lines in a fileSolution #!/bin/sh #countLines3 filename=$1 #Should check if arguments are given count=0 exec < $filename #any commands that will read from stand in # will read from $filename while read aline do count=`expr $count + 1` done echo "$filename has $count lines“ From the command line: $countLines3 data.txt Data.txt has 11 lines exec < /dev/tty --- Reassign the standard input back to terminal

  5. trap • The pressing of DELETE key at the terminal, when a program is in execution, sends a signal to the executing program. • Using the trap command, the program can specify the action to be taken on receiving the signal. • Usage: trap commands signals, where commands are the actions to be taken on receiving the signals. • Some Commonly used signal numbers • 0 Exit from Shell • 1 Hangup • 2 Interrupt (eg: Delete key) • 15 Software termination (sent by kill, for example)

  6. Example Example 1: #!/bin/sh i=1 JUNK=junkfile trap ‘rm $JUNK$$;exit’ 2 while [ $i -le 100 ] Do # remove the file when interrupt is received echo $i >> $JUNK$$ i=`expr $i + 1` done

  7. Examples Example 2: trap 'echo Caught SIGINT - exiting; exit' 2 X=0 while : #loop forever do echo "X=$X" X=`expr ${X} + 1` sleep 1 Done Example 3 #!/bin/sh trap 'echo `pwd` $$ >>./errdir' 2 3 15 while (true) do echo 'Hi' done

  8. Example #!/bin/sh #What does this program do? trap 'increment' 2 increment() { echo "Caught SIGINT ..." X=`expr ${X} + 500` if [ "${X}" -gt "2000" ] then echo "Okay, I'll quit ..." exit 1 fi } ### main script X=0 while : do echo "X=$X" X=`expr ${X} + 1` sleep 1 done

  9. Input and Output Redirection • We know that Unix shell allows us to redirect the input and output of programs using redirection (> and <) and piping(|). • When the Unix kernel starts any process, for example, grep, ls and so on, it sets up several places, called open files, for that process t read from and write to. Each of these files is given a number to identify with, called a file descriptor. • A file descriptor (also known as a file handle) is a non-negative digit that points at a file.  • The file descriptors for stdin, stdout, and stderr are 0, 1, and 2, respectively.  Any of these may be redirected to a file or to each other.  • In other words, it is quite possible to send the output from stdin and stderr to the same file.  This is quite useful when a user would rather check a script's results after it has completed processing.  • By default, the file that is opened for stdin, stdout and sterr is /dev/tty (your terminal). • But, when the shell starts a process, you can tell the shell what file to connect to any of those file descriptors. • For example in the following command, grep aPattern somefile > output, the file descriptor 1 is connected to the file, output.

  10. More On I/O • What if you want to send the standard out to screen and capture the standard error in a pipe or a file? • It is easy to redirect any file descriptor to any file. Eg: command 2>errorFile command 2> file – redirects the standard error from any command cd JUNK 2>>out #the directory JUNK does not exist cat out sh: JUNK: not found.

  11. More On I/O • What if you want to send the standard out to screen and capture the standard error in a pipe or a file? • It is easy to redirect any file descriptor to any file. Eg: command 2>errorFile command 2> file – redirects the standard error from any command cd JUNK 2>>out #the directory JUNK does not exist cat out sh: JUNK: not found. • Let us take a look at a few cases: • Sending both standard output and errors to the pipe or backquotes. command 2>&1 |… or var=`command 2>&1` This means that send standard error (with file descriptor 2) to the same place standard output is going (down the pipe or backquotes)

  12. More On I/O • Sending stderr go down a pipe and stdout to the screen. command 2>&1 1>&2|… Will Not Work We should use file descriptors 3 to 9,as holding places, to accomplish this. command 3>&2 2>&1 1>&3 |… or var=` command 3>&2 2>&1 1>&3`

  13. More On I/O Example: For the following grep command, assume that afile contains one matching line for “unix” and bfile does not exist. var=`grep “unix” afile bfile` echo $var #What is the output? var=`grep “unix” afile bfile 3>&2` echo $var #What is the output? var=`grep “unix” afile bfile 3>&2 2>&1` echo $var #What is the output? var=`grep “unix” afile bfile 3>&2 2>&1 1>&3` echo $var #What is the output?

More Related