bash variables & substitution

read and write

x=foo # assign a (string) value echo x # write a literal x to STDOUT echo $x # write the value of the variable x to STDOUT echo ${x} # same as above, { } used to remove ambiguity x='\s\t\u' echo $x echo -e $x # enable escapes (\t is Tab) echo -n $x # no newline read x # read a line from STDIN, assign to a variable foo bar # type these characters, then Enter echo $x read x foo bar # type these characters (3 leading, 3 interior spaces), then Enter echo $x echo "$x" # preserves interior spaces IFS='' read x # set internal field separator to null foo bar # type these characters (3 leading, 3 interior spaces), then Enter echo "$x" # preserves leading (and trailing) spaces read y z # two variables foo # type these characters (1 word), then Enter echo $y # 1st word of input echo $z # null read y z foo bar # type these characters (2 words), then Enter echo $y echo $z read y z foo bar baz # type this input (3 words), then Enter echo $y # 1st word of input echo $z # all remaining input read x a\b\c # type these 5 characters, then Enter echo $x # \ characters interpreted as escapes read -r x # disable escapes a\b\c # type these 5 characters, then Enter echo $x # \ characters preserved read -N2 x # number of characters, then Enter a\b\c # type these 5 characters, read terminates after 2 characters ^C # erase extra characters typed after read terminated echo $x # 2 characters

arithmetic substutution

typeset -i a=8 b=3 # set two integer variables echo $(( a + b )) # add, $ not required inside $(( )) echo $(( $a - $b )) # subtract echo $(( $a * $b )) # multiply echo $(( $a / $b )) # divide (integer) echo $(( $a % $b )) # modulus (remainder) echo $(( $a ** $b )) # power echo $(( 1000 ** 3 )) # OK echo $(( 1000 ** 7 )) # oops, integer overflow with no warning echo $(( -a )) # negation (no space after -) (( ++a )) # increment (no space after ++) echo $a (( --a )) # decrement (no space after --) echo $a (( a += b )) # same as (( a = a + b )) echo $a (( a -= b )) # same as (( a = a - b )) echo $a echo $(( 1 + 2 * 3 )) # precedence echo $(( ( 1 + 2 ) * 3 )) # grouping

arrays

A=(foo bar baz qux) # define an array, assign 4 elements echo ${A[0]} # first element, {} required here, zero-based index echo ${A[-1]} # last element (-2 is next to last, etc) echo ${A[*]} # all elements (single value) echo ${A[@]} # all elements (separate values) echo ${#A[@]} # length (number of elements) echo ${!A[@]} # all indexes (useful for iterating over an array) A[2]=zzz # reassign third element echo ${A[@]} A+=(aaa) # append element echo ${A[@]} n=0 echo ${A[n]} # variable OK as index echo ${A[n+1]} # arithmetic expression OK as index echo ${A[@]:1} # :i gives from index i to last echo ${A[@]:1:2} # :i:j gives from index i count j declare -A AA=([this]=foo [that]=bar [whatever]=baz) # associative array echo "${AA[that]}" # retrieve value with string index AA[this]=qux # set value with string index echo "${AA[@]}" # not necessarily ordered Index=whatever # string variable echo "${AA[$Index]}" # retrieve value with index in variable

command substitution

cat /etc/passwd | wc -l # count lines z=$( cat /etc/passwd | wc -l ) # assign command output to a variable echo $z cat /etc/passwd | wc # count lines, words, and characters z=$( cat /etc/passwd | wc ) echo $z # entire output is one value z=($( cat /etc/passwd | wc )) # array echo ${z[2]} # array element

indirection

X=foo Y=X echo $X echo $Y echo ${!Y} # expands word after ! then expands expression again (braces required)

execute a script

L=foo # local variable export G=bar # global variable cat > script1 # create a script (use an editor if you prefer) #!/bin/bash # the previous shebang line defines bash as the script interpreter echo $L echo $G Ctrl+d cat script1 bash script1 # execute in a subshell, does not show local variable source script1 # execute in current shell, does show local variable . script1 # same as above chmod u+x script1 # set execute permission ./script1 # execute directly in a subshell rm script1 # clean up