Shell-skript: -z och -n alternativ med if

Du kan hitta en mycket trevlig referens för bashs operatörer här . Om du använder ett annat skal, sök bara efter <my shell> operators så hittar du allt du behöver. I ditt specifika fall är du med:

-n string is not null. -z string is null, that is, has zero length 

För att illustrera:

$ foo="bar"; $ [ -n "$foo" ] && echo "foo is not null" foo is not null $ [ -z "$foo" ] && echo "foo is null" $ foo=""; $ [ -n "$foo" ] && echo "foo is not null" $ [ -z "$foo" ] && echo "foo is null" foo is null 

Kommentarer

• Om du har problem med att få -n att fungera kan det bero på att du följer några dåliga guider på nätet (till exempel GeeksforGeeks eller TutorialsPoint ) som inte citerar variablerna. Detta svar, och guiden som länkas här, citerar korrekt Om du använder -n utan att citera, kommer det att säga att det ' inte är tomt även när det är! @terdon, tack så mycket!

För att förlänga terdons svar fann jag att Unix / Linux – Shell Basic Operators on Tutorials Point inkluderar även filrelaterade operatörer (liksom andra användbara).

-b file Checks if file is a block special file; if yes, then the condition becomes true. [ -b $file ] is false. -c file Checks if file is a character special file; if yes, then the condition becomes true. [ -c $file ] is false. -d file Checks if file is a directory; if yes, then the condition becomes true. [ -d $file ] is not true. -f file Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true. [ -f $file ] is true. -g file Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true. [ -g $file ] is false. -k file Checks if file has its sticky bit set; if yes, then the condition becomes true. [ -k $file ] is false. -p file Checks if file is a named pipe; if yes, then the condition becomes true. [ -p $file ] is false. -t file Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true. [ -t $file ] is false. -u file Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true. [ -u $file ] is false. -r file Checks if file is readable; if yes, then the condition becomes true. [ -r $file ] is true. -w file Checks if file is writable; if yes, then the condition becomes true. [ -w $file ] is true. -x file Checks if file is executable; if yes, then the condition becomes true. [ -x $file ] is true. -s file Checks if file has size greater than 0; if yes, then condition becomes true. [ -s $file ] is true. -e file Checks if file exists; is true even if file is a directory but exists. [ -e $file ] is true.