Script Files and Permissions Ray Lockwood Points: A script is a program made of Unix commands. chmod is the command that changes file permissions

Transcription

1 Introduction to Unix Script Files and Permissions Pg 1 Script Files and Permissions Ray Lockwood Points: A script is a program made of Unix commands. chmod is the command that changes file permissions Shell Scripts In the vernacular, a Script is: A small program written for the shell. Usually used to automate a task. The commands of which could be run one-by-one by a person. To write a script: Take the commands you would normally enter at the command line. Type them into a file. Set the file to be executable. Run the file to execute the commands. You can use your script as if it were your own custom command. Write a Script Create the jw test file (the poem Jaberwock) found at the end of this Read Me. wc stands for word count. Try this command on the Jaberwock file: lockwood@shell:~> wc jw It prints the number of lines, words, and bytes in a file, and shows the filename. Let's make a script to make an improved wc. Start your editor like this: nano wcc We're going to create the new file wcc Type the following into your editor: wc jw Type this into Nano

2 Introduction to Unix Script Files and Permissions Pg 2 Save your file and exit (Ctrl-X), which will name your file wcc. If you try to run your script now, you get this response: lockwood@linprog4.cs.fsu.edu:~> wcc wcc: Permission denied. File Permissions Let's find the problem. Look at your wcc file using ls -l (ell for long descriptions): lockwood@shell:~> ls -l wcc -rw-r lockwood CS-Faculty 107 Jul 18 22:29 wcc File permissions My username A group that I allow access User = The login name of the user who owns the file. Read Write Execute d rwx rwx rwx Group = Users can be put into groups. Instructors are in the CS-Faculty group. d=directory User (Owner) Group Others Others = Every one else (not the owner and not in ths group). Sometimes called world. Aha! The Execute permission is missing from this file. Let's give our file the execute (x) permission. Run this command: chmod u+x wcc User Set Executable Run ls -l again: lockwood@shell:~> ls -l wcc -rwxr lockwood CS-Faculty 107 Jul 18 22:29 wcc The Execute permission is granted.

3 Introduction to Unix Script Files and Permissions Pg 3 chmod is the Change-Mode command. We change the mode of the file to executable, but only for the user (the owner of the file). You use the same format for all the file permissions: chmod ug+rw wcc chmod a-x wcc chmod o-rwx wcc User and Group grant the Read and Write permissions All users deny the Execute permission Other users deny all access u = User the owner of the file (by default, the logged in user who created it). g = Group members of the users group (good for working on projects). o = Others everyone not covered by the above. a = All shorthand for ugo + = Grant the permission - = Deny the permission r = permission to Read the file w = permission to Write the file x = permission to Execute the file chmod u g o a + - r w x filename Now when we use wcc at the command line, it runs our script: Develop Our Script Add Column Headings Edit wcc and add the echo line: wc jw

4 Introduction to Unix Script Files and Permissions Pg 4 Run wcc again and we have a heading over the columns: Show the First and Last Lines head -1 shows the first line of a file, tail -2 shows the last line. (If you use tail -1 you might get a blank line). Edit wcc to add head and tail: wc jw head -1 jw tail -2 jw Run wcc again and we see the first and last lines: Accept a Command Line Parameter We can use our script on any file like a real command. The special variable $1 does this for us. Edit wcc to use $1, which passes the command line parameter: wc $1 head -1 $1 tail -2 $1

5 Introduction to Unix Script Files and Permissions Pg 5 Run wcc jw Note the command line parameter: jw Check for Valid Input If you run wcc without a parameter, it prints the heading and waits for input. Press Ctl-C to make it stop waiting. Edit wcc to add an if statement to check for input. $# is a special variable that tells the number of command line parameters. If this number is zero, then we print an error message and exit: if [ $# = 0 ]; then echo 'no input' exit fi wc $1 head -1 $1 tail -2 $1 Specify the Shell to Use The interpreter that runs the script is in the shell. Different shells run scripts differently. If we always want our script to be run by the tsch shell, then add the Hash/Bang statement (#!/bin/tcsh) as the first line of the script.

6 Introduction to Unix Script Files and Permissions Pg 6 Edit wcc one last time and add the Hash/Bang line: #!/bin/tcsh if [ $# = 0 ]; then echo 'no input' exit fi wc $1 head -1 $1 tail -2 $1 Now we have a solid script. Create the Test File Below is the poem Jaberwock. Copy-and-paste it into Nano and save it. Name it jw Did gyre and gimble in the wabe: All mimsy were the borogoves, Beware the Jabberwock, my son! The jaws that bite, the claws that catch! Beware the Jubjub bird, and shun The frumious Bandersnatch! He took his vorpal sword in hand; Long time the manxome foe he sought So rested he by the Tumtum tree And stood awhile in thought. And, as in uffish thought he stood, The Jabberwock, with eyes of flame, Came whiffling through the tulgey wood, And burbled as it came! One, two! One, two! And through and through The vorpal blade went snicker-snack! He left it dead, and with its head He went galumphing back. And hast thou slain the Jabberwock? Come to my arms, my beamish boy! O frabjous day! Callooh! Callay! He chortled in his joy. Did gyre and gimble in the wabe: All mimsy were the borogoves,