15. Quick Reference
This is a quick reference guide to the meaning of some of the less easily
guessed commands and codes of shell scripts. By their nature, they are also quite difficult to
find using search engines. These examples include process management, shell scripts arguments, and shell script test conditions.
| Command | Description | Example |
|---|---|---|
| & | Run the previous command in the background | ls & |
| && | Logical AND | if [ "$foo" -ge "0" ] && [ "$foo" -le "9"] |
| || | Logical OR | if [ "$foo" -lt "0" ] || [ "$foo" -gt "9" ] (not in Bourne shell) |
| ^ | Start of line | grep "^foo" |
| $ | End of line | grep "foo$" |
| = | String equality (cf. -eq) | if [ "$foo" = "bar" ] |
| ! | Logical NOT | if [ "$foo" != "bar" ] |
| $$ | PID of current shell | echo "my PID = $$" |
| $! | PID of last background command | ls & echo "PID of ls = $!" |
| $? | exit status of last command | ls ; echo "ls returned code $?" |
| $0 | Name of current command (as called) | echo "I am $0" |
| $1 | Name of current command's first parameter | echo "My first argument is $1" |
| $9 | Name of current command's ninth parameter | echo "My ninth argument is $9" |
| $@ | All of current command's parameters (preserving whitespace and quoting) | echo "My arguments are $@" |
| $* | All of current command's parameters (not preserving whitespace and quoting) | echo "My arguments are $*" |
| -eq | Numeric Equality | if [ "$foo" -eq "9" ] |
| -ne | Numeric Inquality | if [ "$foo" -ne "9" ] |
| -lt | Less Than | if [ "$foo" -lt "9" ] |
| -le | Less Than or Equal | if [ "$foo" -le "9" ] |
| -gt | Greater Than | if [ "$foo" -gt "9" ] |
| -ge | Greater Than or Equal | if [ "$foo" -ge "9" ] |
| -z | String is zero length | if [ -z "$foo" ] |
| -n | String is not zero length | if [ -n "$foo" ] |
| -nt | Newer Than | if [ "$file1" -nt "$file2" ] |
| -d | Is a Directory | if [ -d /bin ] |
| -f | Is a File | if [ -f /bin/ls ] |
| -r | Is a readable file | if [ -r /bin/ls ] |
| -w | Is a writable file | if [ -w /bin/ls ] |
| -x | Is an executable file | if [ -x /bin/ls ] |
| parenthesis: ( ... ) | Function definition | function myfunc() { echo hello } |
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Books and eBooks
I have written some books, all of which are available in Paperback and eBook formats. Links below are to Amazon.com.
 Shell Scripting Tutorial is this tutorial, in 88-page Paperback and eBook formats. Convenient to read on the go, and to keep by your desk as an ever-present companion. |  Shell Scripting: Expert Recipes for Linux, Bash and more my 564-page book on Shell Scripting. The first half explains the features of the shell; the second half has real-world shell scripts, organised by topic, with detailed discussion of each script. |  How to Create an RPM is an introduction to building RPMs. Aimed at helping people to create RPMs quickly and easily in real-world situations. |  How to Build a LAMP Server is a detailed walkthrough of installing and configuring a LAMP Server. |
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