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If-then-else flow diagram A nested if–then–else flow diagram. In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs) are programming language constructs that perform different computations or actions or return different values depending on the value of a Boolean expression, called a condition.
In Unix, Linux and other Unix-like operating systems, the first two bytes in a file can be the characters "#!", which constitute a magic number (hexadecimal 23 and 21, the ASCII values of "#" and "!") often referred to as shebang, prefix the first line in a script, with the remainder of the line being a command usually limited to a max of 14 ...
A common value is /bin/sh, but some systems such as Solaris have the POSIX-compatible shell at /usr/xpg4/bin/sh. [13] In many Linux systems, /bin/sh is a hard or symbolic link to /bin/bash, the Bourne Again shell (BASH). Using bash-specific syntax while maintaining a shebang pointing to sh is also not portable. [14]
In keeping with Unix shell conventions, Bash incorporates a rich set of features. The keywords, syntax, dynamically scoped variables, and other basic features of the language are all copied from the Bourne shell, (sh). Other features, e.g., history, are copied from the C shell, (csh), and the Korn Shell, (ksh).
Here documents originate in the Unix shell, [1] and are found in the Bourne shell since 1979, and most subsequent shells. Here document-style string literals are found in various high-level languages, notably the Perl programming language (syntax inspired by Unix shell) and languages influenced by Perl, such as PHP and Ruby.
In the lower right we can see a terminal emulator running a Unix shell, in which the user can type commands as if they were sitting at a terminal. In computing , a shell is a computer program that exposes an operating system 's services to a human user or other programs.
A snippet of Python code with keywords highlighted in bold yellow font. The syntax of the Python programming language is the set of rules that defines how a Python program will be written and interpreted (by both the runtime system and by human readers). The Python language has many similarities to Perl, C, and Java. However, there are some ...
A shell script (or job) can report progress of long running tasks to the interactive user. Unix/Linux systems may offer other tools support using progress indicators from scripts or as standalone-commands, such as the program "pv". [49] These are not integrated features of the shells, however.