00. Shell programming

Create 00_OverviewShell directory. All code must reside here.

Script: executable text file

• can be executed with interpretator script_file

  • e. g. sh script

• «shebang» (#!/bin/sh at the first line)

  • chmod +x script

  • ⇒ can be executed via ./script

• run by name if it's directory is in $PATH variable

  • PATH=directory_with_script:$PATH

  • ⇒ can be executed via just script

Tasks

0. Simple script file named c1:

      1 #!/bin/sh
      2 read A
      3 echo $A
   

1. A is variable

2. $A is a substitution of A value

3. echo command is executed after substitution, so

   • if called as

     • sh c1   ABC      def  123

     • it outputs

       • ABC def 123

       without extra spaces, because echo $A expands to echo ABC      def  123, which is command echo with three command line parameters

4. How to make echo display all spaces as well?

   • Spoiler: use sh c1 "  ABC      def  123" form — it constructs one parameter instead of three with all the words and spaces in it

   • fix c1 to do this

5. Output substitution, c2 file

      1 #!/bin/sh
      2 read A
      3 Sys=`uname`
      4 echo "Hello, $A!
      5 Welcome to $Sys!"
   

   • The «`command`» (also «$(command)») construction runs command and substitute it with its' output

   • No spaces is allowed around «=» when assigning

   • All variables are strings

   • Modify c2 not to read A, but to output user login name instead (use logname or whoami command)

6. Multiline

   • echo $A squeezes all newlines as well

   • If you want to use control characters like in C, use echo -e key

   • Write a c4 script that

     • ends with command echo -e "$Msg"

     • outputs

       Hello, your_login_name!
       Welcome to Linux

     • and contains only one two extra lines:
       • one shebang

       • and one like Msg=…something…

     • make this script executable

7. Commandline parameters. This is c5 file:

      1 #!/bin/sh
      2 
      3 echo "$0"
      4 echo "$1"
      5 # ...
      6 echo "$*"
      7 echo "$#"
   

   • $1, $2 … — command line argument №1, №2 …

   • $0 — name of the script itself

   • $* — all command line arguments

   • $# — number of command line arguments so

        1 [tmpuser@sugon 00_OverviewShell]$ sh c5 A bc  cde   fff
        2 c5
        3 A
        4 A bc cde fff
        5 4
        6 
     

8. Conditional if operator.

   • Overview:

        1 if condition-conmands
        2 then
        3   true-commands
        4 else
        5   false-commands
        6 fi
     

     • if last of condition-conmands exit with success status (zero status code), true-commands are executed.

     • otherwise, if last of condition-conmands exit with non-zero exit status (an error), false-commands are executed.

     • Yes, 0 status is true, non-0 is false!

     • Yes, fi is if backwards

   • Example c6 file:

   1 #!/bin/sh
   2 if ls $*; then
   3         echo YES
   4 else
   5         echo NO $?
   6 fi

• if ls is successful, echoes YES, otherwise NO

• «$?» — is exit status of failed command

• using 2> /dev/null to redirect error messages out of terminal:

     1 $ ls c6
     2 c6
     3 $ ls c6 > o
     4 $ ls c100500
     5 ls: cannot access 'c100500': No such file or directory
     6 $ ls c100500 > oo
     7 ls: cannot access 'c100500': No such file or directory
     8 $ ls c100500 2> oo
     9 $ cat o
    10 c6
    11 $ cat oo
    12 ls: cannot access 'c100500': No such file or directory
  

• Rewrite c6 file (copy it to c7)

  • To output content of ls after «YES» message, if ls is successful

  • Not to output error message if ls is not successful

H/W

Research

What exit status the command read generates? Try it

• Important: Please consult help read (e. g. here) command instead of man read, because read is shell built-in and you got manual page on all shell builtins

Research

How to suppress «\n» output after echo (see echo)?

Task

Write a c8 script that asks user for name, and prints a welcome message either if an user has entered a name or not:

   1 $ ./c8
   2 Enter your name: Spot
   3 Hello, Spot!
   4 $ ./c8
   5 Enter your name: 
   6 Hello, tmpuser?
   7 

• Hint: In second example user had not pressed «Enter»