The following reference cards provide a useful summary of certain scripting concepts. The foregoing text treats these matters in more depth, as well as giving usage examples.
Table B.1. Special Shell Variables
| Variable | Meaning |
|---|---|
$0 | Filename of script |
$1 | Positional parameter #1 |
$2 - $9 | Positional parameters #2 - #9 |
${10} | Positional parameter #10 |
$# | Number of positional parameters |
"$*" | All the positional parameters (as a single word) * |
"$@" | All the positional parameters (as separate strings) |
${#*} | Number of positional parameters |
${#@} | Number of positional parameters |
$? | Return value |
$$ | Process ID (PID) of script |
$- | Flags passed to script (using set) |
$_ | Last argument of previous command |
$! | Process ID (PID) of last job run in background |
* Must be quoted,
otherwise it defaults to
$@.
Table B.2. TEST Operators: Binary Comparison
| Operator | Meaning | ----- | Operator | Meaning |
|---|---|---|---|---|
| Arithmetic Comparison | String Comparison | |||
-eq | Equal to | = | Equal to | |
== | Equal to | |||
-ne | Not equal to | != | Not equal to | |
-lt | Less than | \< | Less than (ASCII) * | |
-le | Less than or equal to | |||
-gt | Greater than | \> | Greater than (ASCII) * | |
-ge | Greater than or equal to | |||
-z | String is empty | |||
-n | String is not empty | |||
| Arithmetic Comparison | within double parentheses (( ... )) | |||
> | Greater than | |||
>= | Greater than or equal to | |||
< | Less than | |||
<= | Less than or equal to |
* If within a double-bracket [[ ... ]] test construct, then no escape \ is needed.
Table B.3. TEST Operators: Files
| Operator | Tests Whether | ----- | Operator | Tests Whether |
|---|---|---|---|---|
-e | File exists | -s | File is not zero size | |
-f | File is a regular file | |||
-d | File is a directory | -r | File has read permission | |
-h | File is a symbolic link | -w | File has write permission | |
-L | File is a symbolic link | -x | File has execute permission | |
-b | File is a block device | |||
-c | File is a character device | -g | sgid flag set | |
-p | File is a pipe | -u | suid flag set | |
-S | File is a socket | -k | “sticky bit” set | |
-t | File is associated with a terminal | |||
-N | File modified since it was last read | F1 -nt F2 | File F1 is newer than F2 * | |
-O | You own the file | F1 -ot F2 | File F1 is older than F2 * | |
-G | Group id of file same as yours | F1 -ef F2 | Files F1 and F2 are hard links to the same file * | |
! | NOT (inverts sense of above tests) |
* Binary operator (requires two operands).
Table B.4. Parameter Substitution and Expansion
| Expression | Meaning |
|---|---|
${var} | Value of var (same as
$var) |
${var-$DEFAULT} | If var not set, evaluate expression
as $DEFAULT * |
${var:-$DEFAULT} | If var not set or is empty,
evaluate expression as
$DEFAULT
* |
${var=$DEFAULT} | If var not set, evaluate expression
as $DEFAULT * |
${var:=$DEFAULT} | If var not set or is empty, evaluate expression
as $DEFAULT * |
${var+$OTHER} | If var set, evaluate expression as
$OTHER, otherwise as null string |
${var:+$OTHER} | If var set, evaluate expression as
$OTHER, otherwise as null string |
${var?$ERR_MSG} | If var not set, print
$ERR_MSG and abort script
with an exit status of 1.* |
${var:?$ERR_MSG} | If var not set, print
$ERR_MSG and abort script
with an exit status of 1.* |
${!varprefix*} | Matches all previously declared variables beginning with
varprefix |
${!varprefix@} | Matches all previously declared variables beginning with
varprefix |
* If var
is set, evaluate the expression as
$var with no side-effects.
# Note that some of the above behavior of operators has changed from earlier versions of Bash.
Table B.5. String Operations
| Expression | Meaning |
|---|---|
${#string} | Length of $string |
${string:position} | Extract substring from $string
at $position |
${string:position:length} | Extract $length
characters substring from $string
at $position [zero-indexed,
first character is at position 0] |
${string#substring} | Strip shortest match of
$substring from front of
$string |
${string##substring} | Strip longest match of
$substring from front of
$string |
${string%substring} | Strip shortest match of
$substring from back of
$string |
${string%%substring} | Strip longest match of
$substring from back of
$string |
${string/substring/replacement} | Replace first match of
$substring with
$replacement |
${string//substring/replacement} | Replace all matches of
$substring with
$replacement |
${string/#substring/replacement} | If $substring
matches front end of
$string, substitute
$replacement for
$substring |
${string/%substring/replacement} | If $substring
matches back end of
$string, substitute
$replacement for
$substring |
expr match "$string" '$substring' | Length of matching $substring*
at beginning of $string |
expr "$string" : '$substring' | Length of matching $substring*
at beginning of $string |
expr index "$string" $substring | Numerical position in $string
of first character in $substring*
that matches [0 if no match, first character counts as
position 1] |
expr substr $string $position
$length | Extract $length characters
from $string starting at
$position [0 if no match, first
character counts as position 1] |
expr match "$string"
'\($substring\)' | Extract $substring*, searching
from beginning of $string |
expr "$string" :
'\($substring\)' | Extract $substring* , searching
from beginning of $string |
expr match "$string"
'.*\($substring\)' | Extract $substring*, searching
from end of $string |
expr "$string" :
'.*\($substring\)' | Extract $substring*, searching
from end of $string |
* Where $substring is a
Regular Expression.
Table B.6. Miscellaneous Constructs
| Expression | Interpretation |
|---|---|
| Brackets | |
if [ CONDITION ] | Test construct |
if [[ CONDITION ]] | Extended test construct |
Array[1]=element1 | Array initialization |
[a-z] | Range of characters within a Regular Expression |
| Curly Brackets | |
${variable} | Parameter substitution |
${!variable} | Indirect variable reference |
{ command1; command2; . . . commandN; } | Block of code |
{string1,string2,string3,...} | Brace expansion |
{a..z} | Extended brace expansion |
{} | Text replacement, after find and xargs |
| Parentheses | |
( command1; command2 ) | Command group executed within a subshell |
Array=(element1 element2 element3) | Array initialization |
result=$(COMMAND) | Command substitution, new style |
>(COMMAND) | Process substitution |
<(COMMAND) | Process substitution |
| Double Parentheses | |
(( var = 78 )) | Integer arithmetic |
var=$(( 20 + 5 )) | Integer arithmetic, with variable assignment |
(( var++ )) | C-style variable increment |
(( var-- )) | C-style variable decrement |
(( var0 = var1<98?9:21 )) | C-style ternary operation |
| Quoting | |
"$variable" | "Weak" quoting |
'string' | 'Strong' quoting |
| Back Quotes | |
result=`COMMAND` | Command substitution, classic style |