Bash operators

You have a shell script and wonder what these operators do? Then this article is for you.

* and ?

You may already know these used for so called "shell globbing" (pattern matching). * replaces an arbitrary amount of characters (including none), while ? replaces exactly one character.

These wildcards are actually bash operators! Imagine, we have the files a1, a2:

cp a*

now is a valid bash command which overrides one of these files with the other one.

已經改好了 請 smart update 後安裝 oxim其實不安裝 qt-immodule 或 gtk-immodule 也可以但是預設的設定檔環境變數有設GTK_IM_MODULE=oxim 和 QT_IM_MODULE=oxim若真不需要將 /etc/X11/xim.d/oxim 中將上面兩個變數改成*_IM_MODULE=xim

$(())

The operator $(()) in the bash shell is replaced by the arithmetic result of the expression enclosed in the parentheses.

Examples:

# echo $((2*2))
4

$$

The operator $$ delivers the id of the currently-running process.

Example:

# kill -9 $$

kills the current process

$!

The operator $! delivers the id of the process that has most recently sent to the background.

Example:

# xosview & sleep 5; kill $!

shows xosview for 5 seconds and stops it

> and >>

> and >> can be used to redirect the output (only stdout, not stderr, for that see below) of a command to a file. The difference between them is, that >> appends to the given files, while > will truncate it.

Examples:

ls -alh > files-in-directory.txt
ls -alh /otherdir >> files-in-directory.txt  # Append list of files in other directory

2> or 2>>

stderr can be redirected by using 2> or 2>>.

But sometimes, you need stderr to be included in stdout for certain purpose, e.g. when another program is parsing/logging only stdout and you need errors to be in there, too. In these cases, you can redirect stderr to stdout with "2>&1". (Of course, it is also possible to redirect stdout to stderr this way, if you need to silence a program on stdout for some purpose).

Examples:

cat notexisting 2> test  # Writes "File or directory not found." to test
cat notexisting 2>&1  # Outputs "File or directory not found." to stdout

|

| is used to pipe the stdout of one programm to the stdin of another one.

tail -n 100 foo | sort -r  # Reverse sort the last 100 lines of the file foo

&&

&& evaluates the binary result of the command left of it and ands it with the binary result right of it if it is not already false, where false!=0. In other words, if you write

command1 && command2

command2 will only be executed if command1 returned success.

See also

• bash operator
• shell Programming