doesn't natively support a 'not' operator. To invert a condition, use a ! outside the first bracket to use the shell's facility for inverting command return values. == and != are literal string comparisons [[is a bash is bash-specific, though others shells may have implemented similar constructs. Don't expect it in an old-school UNIX sh.

The above (along with many more forms of ${parameter…something_else} constructs) are discussed at greater length in the shell’s man page, bash(1). A Note on Quotes. Note that you should always quote shell variables unless you have a good reason not to, and you’re sure you know what you’re doing. By contrast, while braces can be ...

== is specific to bash (not present in sh (Bourne shell), ...). Using POSIX = is preferred for compatibility. In bash the two are equivalent, and in sh = is the only one that will work.

May 27, 2011 · Commands separate by ; are executed sequentially regardless of their completion status.. With &&, the second command is executed only if the first completes successfully (returns exit status of 0).. This is covered in the bash manpage under Lists.I would expect any Unix-like shell to support both of these operators, but I don't know specifically about the Android shell.

Jan 30, 2019 · This is defined in the shell grammar, although not terribly clearly: the and_or production (for &&/||) is defined to have a a pipeline in its body, while pipeline just contains command, which doesn't contain and_or - only the complete_command production can reach and_or, and it only exists at the top level and inside the bodies of structural constructs like …

Nov 27, 2013 · Most important difference is that > makes shell open a file or file-like object with O_WRONLY|O_CREAT|O_TRUNC flags - the file will be created or truncated if it exists, while >> opens file with O_WRONLY|O_CREAT|O_APPEND flags - file will be created or appended to if it exists. This is evident if you trace system calls, for example with

Mar 19, 2020 · What is [? [performs a test.The purpose of [is to test something (e.g. if some file exists) and to return exit status zero if the test succeeds, non-zero otherwise. [is a command.[is a command name like cat or echo.[(like echo) is a builtin in Bash, but this only means it can be run without spawning an additional process, it still behaves like a command.

Sep 27, 2015 · @adosar The reason why the shell put all components of a pipeline in a subshell is it's easier to implement by shell developers. There is no potential conflicts between commands to solve. That's also the defaut POSIX behavior. (POSIX: Additionally, each command of a multi-command pipeline is in a subshell environment) –

Oct 14, 2020 · Difference between for and do-while loop in C, C++, Java for loop: for loop provides a concise way of writing the loop structure. Unlike a while loop, a for statement consumes the initialization, condition and increment/decrement in one line thereby providing a shorter, easy to debug structure of looping.

Oct 6, 2014 · Consider two commands A and B.When you write. A | B A and B are executed in parallel, and the standard output of A is sent as the standard input of B.. When you write. A; B A is executed, then B is executed (unless set -e has been used and A terminates with a non-zero exit status, in which case the shell terminates just after A).. When you write. A && B A is executed, …