Special Characters#
What makes a character special? If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character. Along with commands and keywords, special characters are building blocks of Bash scripts.
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 |
Operator Precedence#
In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones.
| Operator | Meaning | Comments |
|---|---|---|
| HIGHEST PRECEDENCE | ||
| var++ var– | post-increment, post-decrement | C-style operators |
| ++var –var | pre-increment, pre-decrement | |
| ! ~ | negation | logical / bitwise, inverts sense of following operator |
| ** | exponentiation | arithmetic operation |
| * / % | multiplication, division, modulo | arithmetic operation |
| + - | addition, subtraction | arithmetic operation |
| « » | left, right | shift bitwise |
| -z -n | unary comparison | string is/is-not null |
| -e -f -t -x, etc. | unary comparison | file-test |
| < -lt > -gt <= -le >= -ge | compound comparison | string and integer |
| -nt -ot -ef | compound comparison | file-test |
| == -eq != -ne | equality / inequality | test operators, string and integer |
| & | AND | bitwise |
| ^ | XOR | exclusive OR, bitwise |
| | | OR | bitwise |
| && -a | AND | logical, compound comparison |
| | -o | OR | logical, compound comparison |
| ?: | trinary operator | C-style |
| = | assignment | (do not confuse with equality test) |
| *= /= %= += -= «= »= &= | combination assignment | times-equal, divide-equal, |
| , | comma | links a sequence of operations |
| LOWEST PRECEDENCE |
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