Bash

• An sh-compatible shell that incorporates useful features from the Korn shell (ksh) and the C shell (csh)

Resources

• Bash Shell: Take Control of PS1, PS2, PS3, PS4 and PROMPT_COMMAND

  Your interaction with Linux Bash shell will become very pleasant, if you use PS1, PS2, PS3, PS4, and PROMPT_COMMAND effectively. PS stands for prompt statement. This article will give you a jumpstart on the Linux command prompt environment variables using simple examples.

Shortcuts

Shortcut	Function
Ctrl+L	Clear screen
Ctrl+R	Start or continue a search through

Scripting

• Bash scripting titbits:
• if ! command >/dev/null 2>&1; then ... fi:
  • If command (run without output) returns an exit code greater than 0, then …

if statements

Notes

• Basic syntax:

  mynum=200
  
  if [ $mynum -eq 200 ]
  then
      echo "The condition is true."
  else
    echo "The condition is false."
  fi

• Checks within single brackets
  • Between variables/literals
    • Comparing integers
      • -eq (equal to)
      • -ne (not equal to)
      • -gt (greater than)
      • -ge (greater than or equal)
      • -lt (less than)
      • -le (less than or equal)
    • Comparing strings
      • = (equal to)
      • != (not equal to)
    • Comparing files
      • -nt (newer than)
      • -ot (older than)
  • Before the variable/literal
    • -f (regular file exists)
    • -d (directory exists)
    • -n (string has non-zero length)
    • -z (string has length zero)
  • Combining checks
    • && (and)
    • || (or)
• Checks without brackets
  • command -v COMMAND (command exists)
  • COMMAND (command exits with return code 0)
• ! reverses the following condition