Intro to Bash Scripting

• We want to be lazy - good to reuse things when possible

• Script - used to automate execution of tasks

• Bash script - script written with features available in Bash command line shell

• Bash Shell Script - plain text file that contains a sequence of bash shell commands

  • command to have .sh for shell script - convention, not required
  • e.g.

  #!/bin/bash 
  
  # File: basic.sh
  # Scrip thhat prints date, current user, current directory
  date
  whoami
  pwd
  

  • begin with #!, followed by path to where bash shell exists (shebang) - determines where shell is located, can specify an alternative location/alternate path
  • to run script, full path (absolute/relative) can be given to script, e.g. ./[basic.sh](<http://basic.sh>) - this only works if basic.sh has permission to run script, so you can do chmod a+x basic.sh to give execute permission for current user
  • alternatively, to execute a script that does not require execute permission, we have bash <filename> - filename is name of script
  • Bash scripts have the ability to use shell variables, command line arguments, and read input from stdin, as well as conditionals, loops, functions, etc.

Shell Variables

• Bash shell allows users to create variables that can store information as strings, with the same naming rules as valid C variable names
• e.g. to create a variable, we can simply write the following:

x=1 # string value 1
fullname="James Huang" # note - no whitespace between variable name, equals symbols, and valued assigned, if variable value has whitespace, use quotes

• Space requirement is needed for shell to distinguish between variable assignment and attempt to execute a command with = symbol to command and assigned value a second argument
• Variables can be accessed after creation by prefixing variable name with $
• good practice to put variable name in brace brackets, e.g.

word=hard
echo ${word} # hard
echo ${word}er # harder - word is substituted in

• e.g.

dir=~/cs136 # create variable named dir, assign to string which is path to file called cs136 - no output
echo $dir # echos directory variable
cd ${dir} # enter directory variable
ls ${dir} # list files in directory
echo ${dir}/a1 # concat characters

• Global Shell Variable
  • shell already has some global variables set up
  • by convention, environment/global variables are declared in uppercase
  • e.g. printenv - list currently defined environment variables on system
  • e.g. $PATH - concatenation of directories separated by colons - used by unix command line shell to search for programs, with the shell sequentially searching directories in $PATH variable - first matching program is executed, command not found otherwise

Command Line Arguments

• Scripts can be given input via arguments supplied when the script is executed and input made available on the standard input stream