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 |