Shell Script

Shell Script

Need to Know

• #! /bin/bash: usually write in the first line. Mark the shell which script use
• #: comment
• echo: std out
• Do not add extra space!

Variable and Operator

• use $ to refer variables
• source XXX: load XXX variables to parent shell.
• export var=5: load var to children shell.
• unset var: log off var
• use {} to mark a variable
• $0: filename
• $1,$2,...: variables
• $*: all paras
• $@: all paras
• $#: number of paras
• Quotes:
  • ": only $,`," keeps there specific meanings
  • ': everything no specific meanings
  • `: script command will be run
  • EX:
    • log=monday
    • echo "today is $log"
    • echo 'today is $log'
    • echo "today is data"
• = and == almost the same

Expression

• $[]: mark as evaluation
• hex, oct, bin, dec: base#num
• expr: evaluation. Add space between numbers!!
• let: evaluation

Control Flow

• if:
  • if test-commands-1
  • then
    • commands
  • elif test-commands-2
  • then
    • commands
  • else
    • commands
  • fi
• case
  • case word in
  • pattern-1)
    • commands-1
    • ;;
  • pattern-2)
    • commands-2
    • ;;
  • ...
  • pattern-N)
    • commands-N
    • ;;
  • *)
    • commands-else
    • ;;
  • esac
• control: 0 -- true, else -- false!
• Condition test: has blank!
  • test expr
  • [ expr ]
  • [ -z str ]: return true if strlen(str) == 0
  • [ -n str ]: return true if strlen(str) > 0
  • digital compare: -eq, -ne, -lt, -le, -gt, -ge
  • And, Or and Not: -a (&&), -o (||), !
• loop
  • while
    • while test-commands
    • do
      • commands
    • done
  • until
    • until test-commands
    • do
      • commands
    • done
  • for: could use
    • for variable [in list]
    • do
      • commands
    • done
    • EX: for i in `seq 9`

Others

• read (XXX): read to XXX or REPLY
• exit:
• trap: EX: trap `echo "Type quit to exit"` INT
• a;b: execute a and then b

Useful Shell Commands

• cut:
  • cut -c3-6 city.txt cut 3-6th char of every line
  • cut -d" " -f2 city.txt cut 2nd word of every line
• diff
• sort: sort -k2 -r city.txt sort by 2nd keyword reversely
• uniq: sort city.txt | uniq uniq must be used for sorted results!
• tr:
  • tr "ABH" "HCA" < city.txt A->H, B->C, H->A
  • tr "ABC" "[Z*]" A->Z, B->Z, C->Z
  • tr --delete " " < city.txt delete all space
• wc: wc city.txt 5 10 64 city.txt: 5 lines, 10 words, 64 bytes
• substr: Should use expr to get value.
  • expr substr "hello world" 1 5
• seq:
  • seq 8
  • seq -1 3
  • seq 9 -3 0
• alias
  • ll='ls -l'
  • write to ~/.bashrc
  • source .bashrc enable immediately