Lauri's notes

${var}            # value of var (same as $var)
${var-DEFAULT}    # replace by $DEFAULT when $var is not defined (or "DEFAULT" when $DEFAULT is not defined).
${var=DEFAULT}    # create $var with the value $DEFAULT when $var is not defined (or "DEFAULT" when $DEFAULT is not defined).
${var+DEFAULT}    # replace by $DEFAULT when $var is defined. Otherwise leave empty.
${var?ERR_MSG}    # if var not set, print $ERR_MSG and abort script with an exit status of 1.

# The `:` rules affect both unassigned and empty variables;
# the non-`:` rules affect only unassigned variables.

${!var*} ${!var@} # matches all previously declared variables beginning with var

# foo=/tmp/my.dir/filename.tar.gz

${foo#*.}         # remove shortest match from front -> dir/filename.tar.gz
${foo##*/}        # remove longest match from front -> filename.tar.gz (like basename)
${foo#??}         # remove two first chars
${foo%??}         # remove two last chars
${foo%${foo#??}}  # get two first chars
${foo#${foo%??}}  # get two last chars
${foo%/*}         # remove shortest match from end -> /tmp/my.dir (like dirname)
${foo%%.*}        # remove longest match from end -> /tmp/my

${#str}           # length of str
${str:pos}        # extract substring from $str at $pos
${str:pos:len}    # extract $len characters substring from $str at $pos [zero-indexed, first character is at position 0]

${str/old/new}    # Replace first match of "old" with "new".
${str//old/new}   # Replace all matches of "old" with "new".
${str/#old/new}   # If "old" matches front end of $str, replace "old" with "new".
${str/%old/new}   # If "old" matches back end of $str, replace "old" with "new".

# Special variables

$0 - $9, ${10}    # positional arguments ($0 refers to the name of the script itself).
$#                # the number of positional arguments.
$*                # a single string of positional arguments "$1 $2 .. $n" separated by IFS variable, starting at $1.
$@                # a sequence of positional arguments ("$1", "$2", ... "$n").
$?                # the exit status of the last command executed. When a command
                  # completes successfully, it returns the exit status
                  # of 0 (zero), otherwise it returns a non-zero exit
                  # status.
$$                # the process number of this shell - useful for
                  # including in filenames, to make them unique.
$!                # the process id of the last command run in
                  # the background.
$-                # the current options supplied to this invocation
                  # of the shell.
$_                # last argument of previous command.
$IFS              # internal field separator character.
$RANDOM           # a random integer <= 200 $((RANDOM%=200)). random number between 100 and 300 $((RANDOM%200+100)).

# arithmetic binary operators
-eq  equal                         -ne  not equal
-lt  less than                     -le  less than or equal
-gt  greater than                  -ge  greater than or equal

# string operators
-n   not empty                     -z   is empty
=    equal to                      !=   not equal to
\<   less than                     \>   greater than (ASCII)

# Files
-e  file exists.                   -r  file is readable.
-f  file is a regular file.        -w  file is writable.
-d  file is a directory.           -x  file is executable.
-h  file is a symbolic link.       -s  file is not zero size.
-b  file is a block device.        -u  SUID (set user ID) bit is set.
-c  file is a character device.    -g  SGID bit is set.
-p  file is a named pipe (FIFO).   -k  sticky bit is set.
-S  file is a socket.              -O  file is owned by you.
-t  file refers to a terminal.     -G  file is owned by your group.

-N          file modified since it was last read.
F1 -nt F2   file F1 is newer than F2, or F1 exists and F2 does not.
F1 -ot F2   file F1 is older than F2, or F2 exists and F1 does not.
F1 -ef F2   files F1 and F2 are hard links to the same file

-o OPT      True if shell option "OPT" is enabled.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

Operation	Effect
[ ! EXPR ]	True if EXPR is false.
[ ( EXPR ) ]	Returns the value of EXPR. This may be used to override the normal precedence of operators.
[ EXPR1 -a EXPR2 ]	True if both EXPR1 and EXPR2 are true.
[ EXPR1 -o EXPR2 ]	True if either EXPR1 or EXPR2 is true.

touch 'a,comma' 'a|pipe' 'a space' $'a\nnewline'
ls | cat
# a,comma
# a
# newline
# a|pipe
# a space

# Iterate over files in current directory
for f in *; do
	test -f "$f" || continue

	echo "file: $f"
done


# Recursively iterate over files
find . -type f -exec echo "file: {}" \;


# BAD! Do not do this!
for f in $(ls); do ... done
for f in $(find . -maxdepth 1); do ... done
ls | while read f; do ... done


# The -print0 feature is typically found on GNU and BSD systems.
# For find implementations lacking it, it can be emulated
find . -type f -exec printf '%s\0' {} \; | xargs -0 rm

# push to remote
tar czf - /path/directory_to_get | ssh user@host "cat > /path/data.tgz"
tar czf - /path/directory_to_get | ssh user@host tar xzf - -C /path/
cat ~/.ssh/id_rsa.pub | ssh user@host "cat - >> ~/.ssh/authorized_keys"

# pull from remote
ssh user@host "tar czf - /path/directory_to_get" | cat > /path/data.tgz
ssh user@host "tar czf - /path/directory_to_get" | tar xzvf - -C /path/

# get from internet
wget -qO- http://www.rooden.ee/pub/id_rsa.pub >> ~/.ssh/authorized_keys
curl -Ls http://www.rooden.ee/pub/id_rsa.pub >> ~/.ssh/authorized_keys

# Extract tarball from internet without local saving
wget -qO - "http://www.tarball.com/tarball.gz" | tar zxvf -

# File system permissions
find /www -type d -print0 | xargs -0 chmod 0755
find /www -type f -print0 | xargs -0 chmod 0644

# Remove broken symlinks
find /home -xtype l -delete

# Find links pointing to file
find -L . -samefile /link/target


### Getting file size
# POSIX
size=$(wc -c -- "$file")
# GNU
size=$(stat -c %s -- "$file")


find . -type f -name .dropbox -exec rm {} +
find . -name *.conf -print0 | xargs -0 grep -l -Z mem_limit | xargs -0 -i cp {} {}.bak
ls -1 -b | grep \.avi | while read FILE; do mkdir "${FILE%%.avi}"; mv "$FILE" "${FILE%%.avi}"; done