Category:UNIX Shell
The UNIX Shell is a component of terminal-based UNIX-derived systems which offers both a command-line interface for running system commands, as well as programming interface for intelligently automating tasks which use system commands.
Implementation
There are many UNIX Shells and most of them can be categorized into two families. For purposes of the Rosette Code, all examples are in Bourne-compatible syntax. The other family of shells, with a markedly different syntax, are csh and it's tcsh (Tenex C Shell) "clone." Common Bourne compatible shells include the original Bourne Shell (/bin/sh on most versions of UNIX), the GNU Bourne Again SHell (bash --- which is linked to /bin/sh on many distributions of Linux, making it their default shell), the Korn Shell (ksh), the Public Domain Korn SHell (pdksh), the Almquist SHell (ash) and the Debian Almquist SHell (dash) and the Z SHell (zsh).
The original Bourne shell went through a number of revisions in the early years of UNIX, and support for some features varies considerably. By the time the SUSv3 (Single Unix Specification, version 3) features stablized. All recent versions of the various Bourne-compatible shells should support a common set of features, which are referred to throughout Rosette Code examples as "SUSv3" features. The Korn shell (originally written by David Korn of AT&T) and it's "public domain" clone offer extensions (such as co-processes, and "associative arrays" --- called "hash arrays" by Perl, "dictionaries" by Python, "maps" by Lua, etc).
Note that even when using a common subset of supported features there are subtle implementation differences, and in some cases parsing bugs, which can affect the portability of shell script examples. For example in bash before version 2.0 the following was tolerated:
{ echo foo; echo bar } ## Bug!!!
... though this is technically a bug in the language parsing. In bash versions newer than 2.0 this was fixed and the follow is required:
{ echo foo; echo bar; } ## Note the required semicolon
... (Or the } token can be put on a separate line)
Variations of this bug probably accounts for more "breakage" during upgrades of bash and when attempting to run bash scripts under other Bourne compatible shells than any other change in the history of Bourne-compatible shells.
Another common portability issue among different Bourne-compatible shells is a subtle matter of how pipe operations are handled. In all normal UNIX shells the | (pipe) operator creates a unidirectional inter-process communications (IPC) stream between one shell process and another. Thus a command like:
echo foo | read bar
... implicitly invokes a subshell (separate process) as either the producer or the consumer (writer into or reader from) this data"pipe."
The crucial different in semantics is determined by whether a given implementation of a shell creates the subshell/sub-process to the left or the right of the pipe operator. (Conceivably a shell could even create subprocesses on both sides of the operator). To demonstrate, and even test for, the difference run the following lines of code:
unset bar; echo "foo" | read bar; echo "$bar"
... shells such as ksh and zsh spawn their subshells to the left of the pipe ... so the sub-process is writing into the pipeline. This means that the existing process is reading values; thus the local shell variable "bar" is set after the second semicolon in this example. Under shells such as bash, ash, pdksh (and even in older versions of ksh) the subshell is spawned on the right of the | operator. In those cases the read command is setting a value to a shell variable which ceases to exist after the second semicolon (which marks the end of that command, and thus the end of the completed sub-process.
To be portable such code must use command grouping:
unset bar; echo "foo" | { read bar; echo "$bar"; ...; }
... so that all of the commands after the pipe are executed within the same subshell.
Alternatively one could use an explicit shell sub-process (using the "parentheses" delimiters in lieu of the "brace" grouping operators), or one could re-structure the code using assignment and command substitution:
unset bar; bar=$(echo "foo"); echo "$bar" # some very old shells may require ` (backticks) instead of the $(...) syntax
Note that in all these examples the unset bar command is simply to avoid any confusion in the unlikely event that a variable named "bar" was present in the shell environment or local variable heap prior to our functional examples. This sort of difference, the implicit creation and scope of subshells and subproceses, and the underlying conceptual distinctions between shell and environment variables are at the root of many shell scripting portability issues and cause most of the confusion experienced by novices to UNIX shell scripting.
Language
While UNIX Shells vary in the programming languages they support, such languages carry a minimum set of features. Each language allows the programmer to execute system commands as though he were typing the commands himself, and each language allows for a header line which specifies which shell implementation is used to interpret the script.
This one tells the operating system to use the Bourne Shell:
#!/bin/sh
This line tells the operating system to use the Bourne Again SHell:
#!/bin/bash
And this one tells the operating system to use the Korn Shell:
#!/bin/ksh
Each header line consists of a hash, a bang, and the path to the interpreter binary.
Subcategories
This category has the following 5 subcategories, out of 5 total.
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- UNIX Shell User (160 P)
- Bash User (49 P)
- Korn Shell User (11 P)
Pages in category "UNIX Shell"
The following 153 pages are in this category, out of 353 total.
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P
- Palindrome dates
- Palindrome detection
- Pangram checker
- Parsing/RPN calculator algorithm
- Parsing/Shunting-yard algorithm
- Pascal's triangle
- Penney's game
- Perfect shuffle
- Permutations
- Phrase reversals
- Pick random element
- Play recorded sounds
- Population count
- Power set
- Pragmatic directives
- Primality by trial division
- Program name
- Program termination
- Pseudo-random numbers/Middle-square method
R
- Random number generator (device)
- Random number generator (included)
- Range expansion
- Range extraction
- Rate counter
- Read a configuration file
- Read a file line by line
- Read a specific line from a file
- Read entire file
- Real constants and functions
- Regular expressions
- Remove lines from a file
- Rename a file
- Rep-string
- Repeat a string
- Respond to an unknown method call
- Return multiple values
- Reverse a string
- Reverse the order of lines in a text file while preserving the contents of each line
- Reverse words in a string
- Rock-paper-scissors
- Roman numerals/Decode
- Roman numerals/Encode
- Rosetta Code/Count examples
- Rosetta Code/Run examples
- Rot-13
- RPG attributes generator
- Run-length encoding
- Runtime evaluation
- Runtime evaluation/In an environment
S
- Scope/Function names and labels
- Search a list
- Secure temporary file
- Self-describing numbers
- SHA-1
- Shell one-liner
- Short-circuit evaluation
- Show the epoch
- Sierpinski carpet
- Sieve of Eratosthenes
- Simple database
- Sleep
- Snake
- Sockets
- Sort an array of composite structures
- Sort an integer array
- Sorting algorithms/Pancake sort
- Sorting algorithms/Sleep sort
- Soundex
- Special characters
- Special variables
- Speech synthesis
- Square but not cube
- Stable marriage problem
- Stack
- Stream merge
- String case
- String comparison
- String concatenation
- String interpolation (included)
- String length
- Strip a set of characters from a string
- Strip comments from a string
- Subleq
- Substring
- Substring/Top and tail
- Sum and product of an array
- Sum multiples of 3 and 5
- Sum of squares
- Sum to 100
- Symmetric difference
- System time
T
- Take notes on the command line
- Temperature conversion
- Terminal control/Clear the screen
- Terminal control/Coloured text
- Terminal control/Cursor movement
- Terminal control/Cursor positioning
- Terminal control/Dimensions
- Terminal control/Hiding the cursor
- Terminal control/Inverse video
- Terminal control/Preserve screen
- Terminal control/Ringing the terminal bell
- Terminal control/Unicode output
- Test a function
- Text between
- The Name Game
- The Twelve Days of Christmas
- Tic-tac-toe
- Time a function
- Tokenize a string
- Topic variable
- Topological sort
- Towers of Hanoi
- Tree traversal
- Truncate a file