Conditional Expressions in BASH

• Bash uses the command test to do logical tests.
  • test expression
  • [ expression ]
  • In either case, it sets the exit code
    • But this is reverse from C.
    • 0 indicates true
    • not 0 indicates false
  • It is important to have a space before and after the [ and ]
  • Logical
    • Empty expression is false.
    • ! is not.
    • -a is logical and, -o is logical or
  • String Operations
    • -n string : true if the length of a string is not 0
    • But you can just give the string as well for this.
    • -z string : true if the length of a string is 0
    • = works for strings,
    • as does !=
  • Integers
    • Welcome to fortran: -eq, -ge, -gt, -le, -lt, -ne
  • But the real strength is with files
    • file1 -ef file2 : same device and inode numbers.
    • file1 -nt file2 : file1 is newer than file 2
    • file1 -ot file2 : file1 is older than file 2
  • File Types
    • -b file : file exists and is block special
    • -c file : file exists and is character special
    • -d file : file exists and is a directory
    • -e file : file exists
    • -f file : file exists and is a regular file
    • -h file : file exists and is a symbolic link
    • -p file : file exists and is a pipe
    • -S file : file exists and is a socket
  • File info
    • -s file : file exists and has a size greater than 0
    • -r file : file exists and program can read
    • -w file : file exists and program can write
    • -x file : file exists and program can execute
  • There are others.