Shell-scripting: -z og -n muligheder med hvis

Du kan finde en meget flot reference til bashs operatører her . Hvis du bruger en anden skal, skal du bare søge efter <my shell> operators, så finder du alt hvad du har brug for. I dit særlige tilfælde er du ved hjælp af:

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

For at illustrere:

$ 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

• Hvis du har problemer med at få -n til at fungere, kan det skyldes, at du følger nogle dårlige guider på nettet (f.eks. GeeksforGeeks eller TutorialsPoint ) som ikke citerer variablerne. Dette svar, og guiden der er linket her, citerer korrekt Hvis du bruger -n uden at citere, vil det fortælle dig, at det ' ikke er tomt, selv når det er! @terdon, mange tak!

For at udvide terdons svar fandt jeg, at Unix / Linux – Shell Basic Operators på Tutorials Point inkluderer også filrelaterede operatører (såvel som andre nyttige).

-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.