LinuxScripts

Shell scripting

Assign variables

foo=bar
echo "$foo" # prints bar
echo '$foo' # prints $foo as literal string

Special characters

• $0 - Name of the script
• $1 to $9 - Arguments to the script. $1 is the first argument and so on.
• $@ - All the arguments should be quoted
• $* - All the arguments as a single string; must be quoted
• $# - Number of arguments
• $? - Return code of the previous command
• $$ - PID for the current script
• $! - PID of last job run in background
  • possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
• !! - Entire last command, including arguments. A common pattern is to execute a command only for it to fail due to missing permissions, then you can quickly execute it with sudo by doing sudo !!
• $_ - Last argument from the last command

Control flow techniques

• if
• case
• while
• for
• etc

mcd () {
    mkdir -p "$1"
    cd "$1" # also can use $_
}

Error codes

• Commands will often return output using STDOUT, errors through STDERR and a Return Code to report errors in a more script friendly manner
• exit codes to conditionally execute commands

false || echo "Oops, fail" # false returns 1 to STD:ERR
true || echo "Will not be printed" # true returns 0 to STD:ERR
true && echo "Things went well"
false && echo "Will not be printed"
false ; echo "This will always run"

Substitution

• Command substitution $( CMD )
  • for file in $(ls)
• Process substitution <( CMD )
  • diff <(ls foo) <(ls bar)

Globbing

• Wildcards ?, *
• Common substring {}

Test [[]]

• Expressions
  1. ( EXPRESSION ) -> EXPRESSION is true
  2. ! EXPRESSION -> EXPRESSION is false
  3. EXPRESSION1 && EXPRESSION2 -> both EXPRESSION1 AND EXPRESSION2 are true
  4. EXPRESSION1 || EXPRESSION2 -> either EXPRESSION1 OR EXPRESSION2 is true
• Strings
  1. -n STRING -> the length of STRING is nonzero
  2. -z STRING -> the length of STRING is zero
  3. STRING1 = STRING2 -> the strings are equal
  4. STRING1 = PATTERN -> the string matches pattern
  5. STRING1 =~ REGEX -> the string matches regex
  6. STRING1 != STRING2 -> the strings are not equal
• Integers
  1. INTEGER1 -eq INTEGER2 -> INTEGER1 is equal to INTEGER2
  2. INTEGER1 -ge INTEGER2 -> INTEGER1 is greater than or equal to INTEGER2
  3. INTEGER1 -gt INTEGER2 -> INTEGER1 is greater than INTEGER2
  4. INTEGER1 -le INTEGER2 -> INTEGER1 is less than or equal to INTEGER2
  5. INTEGER1 -lt INTEGER2 -> INTEGER1 is less than INTEGER2
  6. INTEGER1 -ne INTEGER2 -> INTEGER1 is not equal to INTEGER2
• Files
  1. FILE1 -ef FILE2 -> FILE1 and FILE2 have the same device and inode numbers
  2. FILE1 -nt FILE2 -> FILE1 is newer (modification date) than FILE2
  3. FILE1 -ot FILE2 -> FILE1 is older than FILE2
  4. -b FILE -> FILE exists and is block special
  5. -c FILE -> FILE exists and is character special
  6. -d FILE -> FILE exists and is a directory
  7. -e FILE -> FILE exists
  8. -f FILE -> FILE exists and is a regular file
  9. -g FILE -> FILE exists and is set-group-ID
  10. -G FILE -> FILE exists and is owned by the effective group ID
  11. -h FILE -> FILE exists and is a symbolic link (same as -L)
  12. -k FILE -> FILE exists and has its sticky bit set
  13. -L FILE -> FILE exists and is a symbolic link (same as -h)
  14. -N FILE -> FILE exists and has been modified since it was last read
  15. -O FILE -> FILE exists and is owned by the effective user ID
  16. -p FILE -> FILE exists and is a named pipe
  17. -r FILE -> FILE exists and read permission is granted
  18. -s FILE -> FILE exists and has a size greater than zero
  19. -S FILE -> FILE exists and is a socket
  20. -t FD -> file descriptor FD is opened on a terminal
  21. -u FILE -> FILE exists and its set-user-ID bit is set
  22. -w FILE -> FILE exists and write permission is granted
  23. -x FILE -> FILE exists and execute (or search) permission is granted