Shell unknown rollup 1

1.1 Count the number of parameters

problem ： You want to know how many parameters are used when calling the script 、

Solution ： Use shell Built in variables $#

#!/usr/bin/env bash
# instance file : check_arg_count
#
# check out the numbers of the correct parameter
# use the grammer as below: if [ $# -lt 3 ]
if (( $# < 3))
then
    printf "%b" "Error. Not enough arguments.\n" >&2
    printf "%b" "usage: myscript file1 op file2\n" >&2
elif(( $# > 3))
then
    printf "%b" "Error.Too many arguments.\n" >&2
    printf "%b" "usage: myscript file1 op file2\n" >&2
else
    printf "%b" "Argument count correct.Proceeding...\n"
fi

1.2 Send output and error messages to the same file

problem ： Use redirection , We can save output or error messages to a separate file , But how to send both to the same file ？

Solution ： use shell The syntax redirects the standard error message to the same place as the standard output .

The preferred

both >& outfile

perhaps ：

both &> outfile

both The representative is ready to STDERR and STDOUT The program that generates the output .

Discuss

&>  and >& It's just take STDOUT and STDERR Send to the same place .

both 2> outfile 1>&2

1 For standard output

2 Represents standard error

1.3 Use... Correctly logger

problem ： You want to use logger Tools enable scripts to send syslog news , But by default ,logger Not providing enough useful information .

Solution ：

logger -t "${0##*/}[$$]" 'Your message here'

If not used logger Of -t Options , Even if it's not a fatal mistake By mistake , At least you have to accept the warning .t representative “ label ”（tag）, Follow the instructions in the man page said , It will “ Use the specified label to mark each line of recorded content ”. In other words, say , If not -t Options , It's hard for you to know where these log messages come from .

${0##*/}[$$] Labels may look like garbled code , But view syslog logging You will often see it . The label is just The basic name of the script and In square brackets The process of ID（$$） nothing more . Compare using and not using -t Option logger.

1.4 typeset

typeset Declare variables and assign attributes , Equate to declare

1.5 Find out if the command ran successfully

problem ： You need to know if the command runs successfully

Solution ： Write a command or shell Script programmers follow established conventions , that shell Variable $? Will be set to not... When the command fails 0 value .

$ somecommand
# it works...
$ echo $?
0
$ badcommand
# it fails...
$ echo $?
1
$

Discuss ： shell Variable $? The exit status of the command is saved in , Its value range is 0·255

Writing shell Script time , Good practice is ： If everything goes well , When the script exits return 0; If there is an error during operation , Then return to non 0 value . We recommend using only 0~127 As return value , because shell use 128+N Represents the received signal N“ kill die ”. in addition , If the value used is greater than 255 Or less than 0, Value wrapping will occur . Sure use exit sentence （ Such as exit 1 or exit 0） Return to exit state . But to Be careful , There is only one chance to read the command exit status

however $? The real use of this variable is In terms of scripting . If you watch the command run on the screen , You can usually see this command If there is something wrong . But in the script , The command may run unattended .

bash One of the characteristics of , The scripting language is identical to the commands you type at the terminal window prompt identical . When writing scripts , This makes checking grammar and logic much easier .

Exit status is mostly used for scripts and if In the sentence , Depending on the success of the command being executed Different ways of handling . The following example is simple , But we will discuss this later topic of conversation .

somecommand
...
if (( $? )) ; then echo failed ; else echo OK; fi

(( )) Evaluate an arithmetic expression

We do not recommend using negative numbers .shell Negative numbers are acceptable , No errors reported , But it didn't Not as you wish .

$ bash -c 'exit -2' ; echo $?
254
$ bash -c 'exit -200' ; echo $?
56